ALCOHOLS AND PHENOLS CLASS XII
Introduction Alcohols are substituted hydrocarbon molecules containing one or more -OH group bound to a saturated sp 3 hybridized carbon atom. The more common alcohols are derived from saturated hydrocarbons or alkanes that contain hydroxyl groups linked to alkyl radicals. They may be regarded as being derived from them by replacing one or more hydrogen atoms by hydroxyl groups, even though they are not prepared in this way. The hydroxyl group is the functional group that characterizes the alcohols. However, alcohols differ from bases such as NaOH or KOH in that they do not furnish hydroxide ion (OH-) in water nor do they have other properties of bases. This is because the alcohols are covalent compounds while the inorganic bases are ionic compounds. Phenols are an important class of aromatic compounds that are hydroxyl derivatives of aromatic hydrocarbons. The hydroxyl group is directly linked (and not as a part of a side chain) to the carbon atoms of the benzene rings. Interestingly, it was the first compound that was to be specifically employed as an antiseptic (Lister, 1867) under the name of carbolic acid. In solution, phenol acts as an antiseptic (0.2% solution) or a disinfectant (1% solution). Ethers are hydrocarbon molecules with -C-O-C- as the functional group. Here, oxygen is bound to two organic (alkyl) groups i.e., R -O-R' where R and R' may be same or different alkyl or aryl groups or one of them may be an alkyl group and the other aryl group. They can be straight or branched chains, cyclic rings, saturated and unsaturated and aromatic compounds. All of these molecules are widely represented in nature and are important in industry and as pharmaceuticals. For instance, ethanol is used as a solvent, as a fuel (can be mixed with petrol), used to make 'ethyl esters' and is the 'potent' chemical present in alcoholic drinks. Ordinary spirit used for polishing
wood contains ethanol the hydroxyl compound. Most of the biomolecules of nature like cellulose (used to make paper and cloth), lactose in milk, glucose, sucrose and fructose found in vegetables and fruits contain large amounts of -OH groups. Their use is so widespread that many have common names. For example:
Some of the large variety of plastic materials that we see around is derived from phenolic resins and plastics. Ethers are widely used as solvents both in the laboratory and industry because of their almost inert nature and good dissolving power. Life would have been different without these compounds.
Classification Alcohols are compounds of the general formula ROH, where R is any alkyl or substituted alkyl group. The group may be open-chain or cyclic; it may contain a double bond, a halogen, an aromatic ring, or additional hydroxyl groups. Based on these possibilities all alcohols are divided into two broad categories called aliphatic alcohols and aromatic alcohols. Aliphatic Alcohols The aliphatic alcohols are a homologous series of compounds containing one or more hydroxyl groups [-OH] attached to an alkyl radical. The aliphatic alcohols can be regarded as derivatives of alkanes in which one or more hydrogen atoms have been replaced by hydroxyl groups [-OH]. The general formula of saturated aliphatic alcohols is C nH2n+1OH, where n=1,2,3,
etc. The saturated carbon chain is often designated by the symbol R, so that ROH can represent any alcohol in the homologous series. Methanol
and
ethanol
are
the
first
two
members
of
the
Methanol:
series. CH3-OH
Ethanol: The next two members of the series are propanol and butanol, whose names also
end
in
...ol,
which
means
Propanol:
the
molecule
is
an
alcohol.
CH3-CH2-CH2-OH
Butanol: Other examples of aliphatic alcohols with a closed ring cyclclic structure are,
Compounds of this type with one hydroxyl group per molecule are known as monohydric alcohols. Aromatic Alcohols Any of the compounds containing the hydroxyl group in a side chain to a benzene ring are aromatic alcohols. An aromatic alcohol where a methanol is bonded to a benzene is benzyl alcohol.
Mono,
Di,
Tri
or
Polyhydric
Compounds
Aliphatic and aromatic alcohols and phenols are classified as monohydric, dihydric, trihydric and polyhydric compounds according to the number of hydroxyl groups in their molecules. Monohydric alcohols have one hydroxyl group per molecule, dihydric alcohols have two hydroxyl groups and polyhydric alcohols contain three or many hydroxyl groups in their structures as shown below:
Monohydric alcohols are classified according to the hybridization of the carbon atom i.e., sp3 or sp2 to which the hydroxyl group is attached. Compounds Containing sp 3 C-OH Bond The sp3 hybridized monohydric alcohols are classified into three categories as primary (1), secondary (2) and tertiary (3) in the same manner as alkyl halides.
This classification is important because the type of reaction an alcohol undergoes depends on what is attached to its carbon. If the carbon to which the -OH group attached also binds to one carbon atom, the alcohol is designated primary (1). If two carbons are attached to that carbon, the alcohol is secondary (2) and if three carbons are attached the alcohol is tertiary (3). This terminology refers to alkyl substitution of the carbon atom bearing the hydroxyl group (colored blue).
Allylic Alcohols When the hydroxyl group is attached next to the double bonded carbon atom (C=C) then such compounds are called allylic alcohols. The carbon atom to which the hydroxyl group is attached is sp 3 hybridized in such compounds.
Allylic alcohols can also be primary, secondary and tertiary as follows:
Benzylic Alcohols In aromatic compounds when the hydroxyl group is attached to a carbon atom bonded to a benzene ring benzylic alcohols result. Similar to the allylic alcohols, the carbon atom to which the hydroxyl group is attached is sp 3 hybridized in such compounds.
Compounds Containing sp 2 C-OH Bond Vinylic Alcohols Sometimes the hydroxyl group is directly attached to double bonded carbon atoms (C=C). In such compounds the carbon atom to which the -OH group is attached is sp2 hybridized and they have special names like vinylic alcohols. The replacement of hydroxyl group for these types of hydrogen atoms results in alkyl alcohols having the corresponding common names.
Aryl Alcohols or Phenols Aryl alcohols or phenols are hydoxyl derivatives of aromatic hydrocarbons, which are derived by replacing hydrogen atom attached to sp 2 hybridized carbon atom(s) of the benzene ring by a hydroxyl group. For example,
Ethers Like alcohols ethers are divided into two broad categories called aliphatic ethers and aromatic ethers. Aliphatic Ethers Aliphatic ethers are those in which R and R' are both alkyl groups. For example,
Ethers in which the two alkyl groups R and R' are identical they are simple ethers or symmetrical ethers e.g., CH3.O.CH3. Aromatic Ethers
Aromatic ethers are those in which either one or both R and R' are aryl groups. For example,
Aromatic ethers are further subdivided into phenolic ethers and diaryl ethers. Phenolic Ethers Ethers in which one of the groups is aryl while the other group is alkyl are called phenolic ethers or alkyl aryl ethers. Examples are:
Diaryl Ethers In diaryl ethers both the groups are aryl groups. For example: C6H5-O-C6H5 Diphenyl ether
Both aliphatic and aromatic ethers are also classified as symmetrical and unsymmetrical ethers. Symmetrical Ethers
Many simple ethers are symmetrical, in that the two alkyl substituents are the same CH3.O.CH3. These are named as 'dialkyl ethers'. Examples are: CH 3CH2-O-CH2-CH3, diethyl ether (sometimes referred to as ether), and CH 3-OCH2-CH2-O-CH3, ethylene glycol dimethyl ether (glyme). Unsymmetrical Ethers If the ethers are different they are mixed ethers e.g., CH 3.O.CH2H5 and are called unsymmetrical ethers.
Nomenclature Alcohols Since the vast majority of alcohols have their -OH groups attached to alkanes, their names are most easily generated by using those alkanes as their base. Alcohols have the highest priority of the groups so if an -OH is present it's an alcohol. Common System The common name for alcohols is generated simply by taking the name of the alkane chain and adding alcohol to the end. This is how common names are usually generated. The isopropyl alcohol molecule provides an example. In CH3-CHCl-CH3 if one replaces the Cl in isopropyl chloride with -OH isopropyl alcohol is obtained. For monofunctional alcohols, this common system consists of naming the alkyl group followed by the word alcohol.
One should also know the common names of a few popular alcohols:
IUPAC System Alcohols are given IUPAC names based on the alcohol being derivatives of the parent alkane. This is common for the carbon-carbon double and triple bonds which have the respective suffixes ene and yne. In the IUPAC system of nomenclature, functional groups are normally designated in one of two ways: the presence of the function may be indicated by a characteristic suffix and a location number. Alcohols are usually named by the first procedure and are designated by an ol suffix, as in ethanol, CH 3CH2OH (note that a locator number is not needed on a two-carbon chain). On longer chains the location of the hydroxyl group determines chain numbering.
A stepwise process for naming alcohols is as follows: Find the longest carbon chain which includes the carbon to which the OH group(s) is attached. Select this as parent chain containing the hydoxyl group. Name the molecule as if it were an alkane, replacing -e with -ol.
With complex alcohols containing multiple functional groups, the longest chain will be the hydroxy-containing chain with the largest number of principle functional groups such as alcohol, alkene etc., but not halide, sulfide, ether etc. Number the longest chain from the hydroxyl group end. Start counting from the end closest to the -OH group.
Place the -OH location prior to that chain name using numbers (if required to remove ambiguity).
When more than one -OH functionality is present i.e., polyhydric alcohols retain the 'e' of alkane and use the endings '-diol', '-triol,' etc., for two or three hydroxy groups in the molecule.
Number all the substituents by position on the chain. The positions of the substituent alkyl (or other) groups are denoted by using the lowest possible numbers for the associated carbon atoms in the main chain. If there is more than one 'type' of substituent e.g., methyl.... and ethyl.. etc., they are written out in alphabetical order in composing the final name (di, tri are ignored in using this rule). Many functional groups have a characteristic suffix designator, and only one such suffix (other than "ene" and "yne") may be used in a name. When the hydroxyl functional group is present together with a function of higher nomenclature priority, it must be cited and located by the prefix hydroxy along with an appropriate number. For example, lactic acid has the IUPAC name 2-hydroxypropanoic acid. Cycloalcohols (Cycloalkanols) are named on the basis of the number of carbon atoms in the ring (minimum 3) and the prefix 'cyclo' and the suffix 'ol'. The OH attached to the carbon is numbered 1. The prefix alkane name e.g. 'prop' has an 'a' added but leaves out the end 'ne' if more than one OH group e.g., in mono-hydroxy alcohols it is propan... and in diols/triols etc., it is propane. Other examples of IUPAC nomenclature are shown below, together with the common names often used for some of the simpler compounds. Common Name
IUPAC Name
methanol
methanol
ethanol
ethanol
n-propanol
1-propanol
isopropanol
2-propanol
Structure
n-butanol
1-butanol
sec-butanol
2-butanol
isobutanol
2-methyl-1-propanol
tert-butanol
2-methyl-2-propanol
cyclohexanol
cyclohexanol
octanol or caprylic
2-octanol
alcohol ethylene glycol
ethane-1,2-diol
glycerol
propane-1,2,3-triol
menthol
5-methyl-2-isopropyl cyclohexanol
D-glucose
2,3,4,5,6pentahydroxyhexanal
Phenols The general formula for monohydric phenols is ArOH, Ar = phenyl, substituted phenyl, or any other aryl group. When the -OH is attached directly to a benzene ring the molecule is called a phenol.
Common
System
All substituted phenols are named as derivatives of phenol. The substituted compounds are named in terms of ortho, meta and para in the common system.
IUPAC System In the IUPAC system all substituted phenols are named as derivatives of phenol. The position of the substituent with respect to hydroxyl group is indicated by Arabic numerals with the carbon carrying the OH group being numbered 1 e.g., ortho (1,2- disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted).
If there is a 'higher ranking' functional group in the molecule, the substituent OH is called by the prefix 'hydroxy'. Example:
In the IUPAC name the di, tri and poly hydroxy phenols are named as hydroxyl derivatives of benzene.
Ethers Ethers are compounds having two alkyl or aryl groups bonded to an oxygen atom, as in the formula R 1-O-R2. Common System In simple ethers containing no other functional groups, name the two alky or aryl groups linked to the oxygen atom in alphabetical order and add the word ether, e.g., CH 3CH2-O-CH2CH2CH3 (Ethyl propyl ether)
In case of symmetrical ether the prefix 'di' is used before the name of the alkyl or aryl group.
IUPAC System The ether functional group does not have a characteristic IUPAC nomenclature suffix, so it is necessary to designate it as a substituent. The ether functional group is named as an alkoxy substituent on the parent compound. Ethers are named on the basis of the longest carbon chain with the O-R or alkoxy group. The common alkoxy substituents are given names derived from their alkyl component, e.g. methoxy CH 3O- or ethoxy CH 3CH2Oetc., treated as a substituent group. Alkyl Group
Name
Alkoxy Group
Name
CH3-
Methyl
CH3O-
Methoxy
CH3CH2-
Ethyl
CH3CH2O-
Ethoxy
(CH3)2CH-
Isopropyl
(CH3)2CHO-
Isopropoxy
(CH3)3C-
tert-Butyl
(CH3)3CO-
tert-Butoxy
C6H5-
Phenyl
C6H5O-
Phenoxy
The parent compound is the longest carbon chain if both are alkyl groups; but with any primary functional group (alkene, alcohol, aldehyde, acid etc.) ethers become substituent groups on that primary chain, for example:
CH3CH=CHCH2OCH2CH3 is 1-Ethoxy-2-butene.
Structure of Functional Groups In alcohols, phenols and ethers the carbon-oxygen bond is the most powerful bond that makes their chemistry different. A carbon-oxygen bond is a covalent bond (σ) between sp3 hybridized carbon and oxygen atoms. Here, oxygen with its 6 valence electrons prefers to share two electrons by bonding with carbon, leaving the remaining 4 non-bonding electrons as 2 lone pairs. Alcohols In alcohols the simplest representatives of the C-O bond can be thought of organic derivatives of water. They also contain hydroxyl (OH) group as the functional group attached to an alkyl group. Bond Length The bond lengths for C-O bonds and O-H bonds are in the range of 143 and 96 picometer respectively, which is shorter than that of C-C or C-H bonds. The shortening of C-O and O-H is due to increasing electronegativity of O (Electronegativity C vs O = 2.55 : 3.44).
Bond Poarity An alcohol molecule is dipolar in nature with the oxygen carrying a partial negative charge (δ-) and carbon and hydrogen each carrying a partial positive charge (δ+). For example in the case of methanol, the charge separation is as follows:
This is supported by the face that the methanol has a dipole moment of 1.71 D. Bond Strength The C-O bond strength is larger compared to C-N or C-C, for example, it is 91 kcal/mol (298 K) in methanol (87 in methyl amine and 88 in ethane). Bond Angle All the C-C-H, H-C-H, C-O-H, C-C-O or H-C-O bond angles are approximately 109o in all the non-cyclic alcohols or alkanols. This is slightly less than the tetrahedral angle (109 28 ) due to the repulsion between the two lone ′
electron pairs of oxygen. As the CO bond is strongly polarized towards oxygen many alcohols are water soluble.
Phenols In phenols the hydroxyl (OH) functional group attached to an aryl group.
Phenol had a lower dipole moment of 1.54 D than methnaol. Shortened single bonds are found in phenols (136 pm). This is due to partial double bond character of the sp 2 hybridized carbon of the benzene ring, and conjugation of unshared electron pair of oxygen with the aromatic ring.
Ethers In ethers, the oxygen atom forms a bridge between two alkyl groups by forming two single bonds each with the alkyl C atoms just like the one in water where the oxygen forms a bridge between the two hydrogen atoms. The structure of diethyl ether (Ethoxy ethane) is shown below.
Two of the four sp 3 orbitals of the oxygen atom overlap with sp 3 orbitals of the alkyl groups to form two sp 3-sp3 (σ) bonds with the C-O bond length of 141 pm almost equal to that in alcohols. The remaining two sp 3 orbitals of oxygen contain a lone pair of electrons each. The four electron pairs, i.e., the two bond pairs and two lone pairs of electrons
on
oxygen
are
arranged
approximately
in
a
tetrahedral
arrangement. The bond angle of 110 o is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (-R) groups.
Alcohols and Phenols As mentioned earlier, alcohols refer to a class of chemical compounds consisting of atoms of carbon, hydrogen and oxygen. A variety of alcohols exist and they contain at least one hydroxyl group (OH-) as the functional group in them. The oxygen atom of the hydroxyl group is bonded to the carbon atom. Although alcohols are mostly soluble in water they do not get ionized by water i.e., no hydroxide ion (OH-) is formed. Hence, alcohols have the characteristic property of the alcoholic group. For example, in alcohols like ethanol and phenol, -OH is the functional group.
Preparation of Alcohols Hydration of Alkenes Alkenes:: In an aqueous acid solution, alkenes add water across the double ( π) bond thereby saturating the molecule producing an alcohol. The reaction is region selective since only the more highly substituted alcohol is produced (Markovnikov's rule).
In accordance with Markovnikov's rule reaction with water and sulphuric acid as catalyst gives the predicted product. In these reactions molecular rearrangement often occurs which indicates that the reaction involves a carbocation intermediate. The reason why Markovnikov's Rule always results in the more substituted alcohol can c an be understood if one looks at the reaction mechanism proposed for this reaction. In the first step the hydrogen ion from the acid catalysts attaches itself to the alkene at the double bond to the carbon which will result in the most stable carbocation.
In the next step the carbocation attaches itself to an oxygen atom of a water molecule with the aid of a lone pair making the oxygen atom positively charged oxonium ion.
In the final step, a water molecule pulls a hydrogen ion off the oxygen atom to produce the final alcohol.
The obvious synthetic problem with this reaction is that it may undergo molecular rearrangement thereby producing more than one alcohol. In addition it always produces the more substituted alcohol. Hydroboration-oxidation Alkenes react with diborane (B2H6) to form trialkylboranes, which upon subsequent treatment with alkaline H2O2 gives alcohol.
The principal adding reagent in this reaction is borane (BH 3), but borane is much too reactive in air to be stored in a bottle. Therefore diborane is used in a tetrahydrofuran solvent (THF) where it cleaves and the resulting borane molecules are stabilized by complexing with the oxygen atom in the cyclic ether, THF. Borane adds across the double bond of the alkene, with one of the hydrogens attaching to the more substituted sp 2 carbon and the boron attaching to the least substituted sp2 carbon.
This is because boron being a larger atom needs more room and is less sterically hindered on the least substituted carbon. As long as boron has hydrogen attached to it, this reaction will occur again with another alkene molecule until the boron is attached to three alkene molecules producing the organoborane an addition product. Even the organoboranes will react with air, so they must be kept in a suitable solvent like THF. Organoboranes are useful in themselves. They can be used to produce alkanes from alkenes by reduction of the organoborane with acetic acid under reflux conditions or alcohol by oxidation with hydrogen peroxide in presence of aqueous NaOH.
This replaces boron with a hydrogen atom on each of the three organic groups attached to the boron thus producing three molecules of alcohol for every one molecule of the organoborane. Oxymercuration-reduction Alkenes react with mercuric acetate (CH 3COO)2Hg to form adducts which upon reduction with NaBH4 in basic medium gives alcohol.
This reaction occurs in 3 stages. In the first part mercury (II) acetate, Hg(oAc)2 forms a bridged species with the mercury atom, bridging the two sp2 carbons.
Then in the second step a water molecule attaches itself to the least substituted of the sp2 carbons in the bridged ring intermediate with the aid of a lone pair on the oxygen atom of the water molecule. This produces the oxonium intermediate and breaks the bridged structure. In a third step an acetate ion pulls a hydrogen ion off the oxygen to produce the hydroxy organo mercury product. Now a strong reducing agent sodium borohydride in basic solution is added to the mercury compound when the mercury group is replaced by a hydrogen atom to produce the final alcohol product.
Like the hydration reaction, the oxymercuration-reduction reaction is region selective and follows Markovnikov's rule, producing only the th e more substituted alcohol. However, no molecular rearrangement is possible in this reaction indicating that no carbocation is produced. This is a distinct advantage over acid catalyzed hydration giving less product mixing. Hydrolysis of Haloalkanes Haloalkanes on treatment with aqueous solution of KOH or moist silver oxide (Ag2O/H2O) give alcohols.
Reduction of Acyl (Carbonyl) Compounds An acyl compound is one that has a carbon doubled bonded to an oxygen (C=O) in its molecule. This group is also referred to as a carbonyl group. There are several categories of acyl compounds: Aldehydes Ketones Carboxylic Acids Esters Acyl compounds may undergo reduction with a suitable reducing agent. The reducing agent undergoes oxidation in the process.
Organic Reducing Agents There are four reducing agents that will reduce one or more of the acyl compounds. Lithium Aluminum Hydride, LiAlH4 (LAH) Sodium Borohydride, NaBH4 (NBH) H2 /Cu + CuCr2O4 at 5000psi, 175 oC H2 /Pt Lithium aluminum Hydride is stronger than sodium borohydride and will reduce any of the acyl compounds to products shown above. Indeed, lithium aluminum hydride is so reactive that it must be prepared in a totally anhydrous environment as it reacts explosively with water. Sodium Borohydride is a weaker reducing agent that is less effective and is capable of reducing only the aldehydes and ketones; it will not reduce carboxylic acid or ester functions. The use of hydrogen gas in the presence of Cu and copper (II) chromate is called 'hydrogenolysis' and is a common way of reducing esters in small scale laboratory reductions. This is also called high pressure hydrogenation and copper and copper (II) chromate serves as catalysts. The fourth method uses hydrogen gas in the presence of a noble metal such as platinum to reduce aldehydes, ketones, and carboxylic acids.
Aldehydes
These can be reduced to primary alcohols in presence of platinum or palladium metals or sodium borohydride.
Ketones These can be reduced to secondary alcohols in presence of sodium borohydride.
Carboxylic Carboxylic
Acids
These acids are reduced to primary alcohols via aldehydes by lithium aluminum hydride.
Esters Esters are reduced to two alcohols, one from the acid fragment of the ester and one from the alcohol fragment by hydrogenolysis.
Reaction with Organometallic Reagents Organometallic reagents are organic compounds that are bonded to a metal. There are a number of important organometallic reagents. Of these, two are important in the synthesis of alcohols i.e., organomagnesium (Grignard's reagent), R-+MgX and organolithium, R-Li+. Grignard's reagent react with aldehydes, ketones and esters to form addition products (adducts).
Upon decomposition with water in presence of dilute HCl or H2SO4, the adducts give alcohol.
When a Grignard's reagent reacts with formaldehyde a primary alcohol is produced. Indeed, this is the only way to prepare a primary alcohol via an organometallic reagent.
Grignard's reagent reacts with all other aldehydes to produce secondary alcohols.
Grignard's reagent reacts with ketones to produce tertiary alcohols.
Hydrolysis of Esters Alcohols are prepared from naturally occurring esters by hydrolysis with aqueous alkalis. This reaction is called saponification reaction.
Reaction of Primary Amines Primary aromatic amine reacts with nitrous acid to form a diazonium salt, which liberates nitrogen to form an unstable carbonium ion. These carbonium
ions react in aqueous medium to form an alcohol.
Preparation
of
Phenols
Phenols are prepared by the reaction of benzene derivatives with different reagents. Alkali Treatment of Haloarenes Chlorobenzene is converted into phenol when it is heated with 6-8% aqueous NaOH at temperatures above 623 K under 300 atm. pressure. This reaction is also called as Dow's process.
Hydrolysis of Aromatic Diazonium Salts Aromatic diazonium salts on treatment with water or preferably with dilute acid form phenols. The reaction is usually carried out by adding the solution of a diazonium salt to a large volume of boiling dilute sulphuric acid. The acidic conditions minimize the side reaction of the coupling of the phenol with diazonium salt.
This is one of the most convenient methods of preparing phenols. Reaction with Grignard's Reagent When a solution of an aryl halide (bromo benzene) in dry ether is treated with magnesium, an aryl magnesium halide (phenylmagnesium bromide) is formed.
When oxygen is bubbled through an ethereal solution of phenylmagnesium bromide, it forms an addition product.
The treatment of the adduct with dilute mineral acid gives phenol.
Alkali Fusion of Benzene Sulfonic Acid This is one of the oldest methods useful for the industrial synthesis of phenol. When benzene sulphonic acid is fused with NaOH at 300-350 oC, it forms sodium benzene sulphonate as an intermediate. The reaction is followed by an acidification reaction to neutralize the phenoxide and give phenol. This reaction occurs via the addition-elimination mechanism with SO32- functioning as the leaving group.
Acidic Oxidation of Cumene Oxidation of cumene (2-phenylpropane or isopropylbnezene) at the benzylic position gives hydroperoxide which upon subsequent hydrolysis with aqueous acid cleaves to give phenol and acetone.
Low-cost reagents and important products make this process attractive. Decarboxylation of Salicyclic Acid Phenol is obtained by the decarboxylation of sodium salt of salicyclic acid with soda lye (CaO + NaOH) followed by acidification with dilute HCl.
Physical
Properties
The physical properties of an alcohol are best understood if one thinks of it as a composite of an alkane and water. As such it contains a lipophilic, alkane-like group and a hydrophilic, water-like hydroxyl group. The hydroxyl -OH group gives the alcohol its characteristic physical properties, while the alkyl group which modifies these properties. Boiling Point: Alcohols have a very high boiling point when compared to the related alkanes. Boiling point increases with increasing molecular weight. The boiling point of the diols is quite high, hence their use as an additive to car radiators.
Melting Point: Melting point increases with increasing molecular weight, expectedly, because of the of van der Waal attraction. Symmetric branching increases melting point. Specific Gravity: Specific gravity or density increases with increasing molecular weight. Alkyl
alcohols
are
less
dense
than
water,
cyclic
alcohols
are
approximately the same as water, and aromatic alcohols and diols are greater than water. Water Solubility: Lower weight alcohols, up to C 4, are infinitely soluble in water. Solubility
decreases
with
increasing
molecular
weight
hydrophobicity of alkyl groups dominates the nature of the molecule. Most diols are infinitely soluble in water.
Name
M.P. oC B.P. oC S.G. 20 oC Solub g/100
Methyl
-97
0.793
I
Ethyl
-115
0.789
I
n-Propyl
-126
0.804
I
n-Pentyl
-78.5
0.817
2.3
n-Octyl
-15
0.825
0.05
n-Decyl
6
0.829
----
as
Isopropyl
-86
0.789
I
Isobutyl
-108
0.802
10.0
tert-Butyl
25.5
0.789
I
Cyclohexanol
24
0.962
---
Allyl
-129
0.855
I
Benzyl
-15
1.046
4
Ethylene Glycol
-16
1.113
I
1.040
I
1.261
I
Propylene Glycol Glycerol
18
Acidity of Alcohols: Alcohols : Alcohols are weakly acidic, as might be expected due to their similarity to water. They can also function as weak Lewis bases, becoming protonated to give oxonium ions:
pKa values: CH3CH2OH = 16.0; H 2O = 15.7; CH 3OH = 15.5; HCl = -7.0. Hydrogen Bonding Alcohols and phenols differ significantly from hydrocarbons (alkanes, alkenes, alkynes, and aromatics) and alkyl halides in their melting points and their boiling points. Hence, for short chain alcohols and for phenols one finds much higher boiling points than one would expect on the basis of their sizes. The most dramatic differences occur with the smallest molecules: methane (MW = 16), chloromethane (MW = 50), and methanol (MW = 32) with boiling points of -161.4 C, -23.7 C, and 64.5 C, respectively. Thus in alcohols alcoh ols
boiling points are not related to size. They obviously require a different explanation. Compound
M W
Boiling Pt (C)
CH3CH2OH
46
78
100% Soluble
CH3CH2CH3
44
-42
Insoluble
CH3CH2CH3OH
60
97
100% Soluble
CH3CH2CH2CH3
58
0
88
138
86
69
CH3CH2CH2CH2CH2
Sol. in Water (g/100mL)
Insoluble Insoluble
2.3 g/100mL
OH CH3CH2CH2CH2CH3C insoluble
H2
The high boiling points of alcohols can be explained by the existence of hydrogen bonds (H-bonds). This is the same explanation for the incredibly high boiling point of water (water has about 4 H-bonds per molecule, vs. about 2 for alcohols). Hydrogen bonds involve a very strong polar attraction combined with very small atomic radii, resulting in a partial covalent bond being formed. Although this bond is significantly weaker than the covalent bonds common in organic molecules molecul es (i.e. 400 kJ/mol for O-H bonds vs. about 20 kJ/mol for an H-bond), but is much stronger than the weak van der Waals forces holding alkanes together (about 0.4 - 0.8 kJ/mol).
The -OH group is highly polar and is capable of hydrogen bonding with other alcohol molecules and also to other neutral molecules and to anions. This property is particularly important in defining boiling point and aqueous solubility. The increased intermolecular bonding increases the energy needed to boil by decreasing the vapor pressure. As expected the polar nature increases with the diols. Chemical
Reactions
The functional group of the alcohols is the hydroxyl group, -OH. Unlike the alkyl halides, this group has two reactive covalent bonds, the C-O bond and the O-H bond. Accordingly, alcohols and phenols will undergo two distinct types of reactions. Reactions involving the cleavage of the O-H bond with substitution or removal of hydrogen as a proton. Reactions involving the cleavage of the C-O bond with substitution or removal of OH group. Nature of the -OH Group The electronegativity of oxygen is substantially greater than that of carbon and hydrogen. Consequently, the covalent bonds of this functional group are polarized so that oxygen is electron rich and both carbon and hydrogen are electrophilic. As a result, there is a low electron density on H atom of -OH group alcohol, and with H+ character being more, alcohols are acidic (pKa ~ 16). Acidity of Alcohols Nevertheless, alcohols are weaker acids than water (pK a ~ 14) because the electron releasing inductive effect of the alkyl groups increases the electron density around the oxygen atom. As a result the electrons of the O-H bond
are not sufficiently attracted towards the oxygen atom and so the release of proton becomes difficult. On comparing the acidity of primary, secondary and tertiary alcohols the electron releasing inductive effect of the alkyl groups is maximum in tertiary alcohol and minimum in primary alcohols. Thus, primary alcohols are the strongest acids while tertiary alcohols are the weakest. The acidic strength of alcohols follows the order: Primary > Secondary > Tertiary In addition, the presence of lone pairs in the alcohol O atom makes it a region of high electron density. Alcohol oxygen atoms are Lewis bases and allow the ability to alcohols to react as either bases or nucleophiles at the oxygen atom. Indeed, the dipolar nature of the O-H bond is such that alcohols are much stronger acids than alkanes - by roughly 1030 times, and nearly that much stronger than ethers which are oxygen substituted alkanes that do not have an O-H group. The most reactive site in an alcohol molecule is the hydroxyl group, despite the fact that the O-H bond strength is significantly greater than that of the C-C, C-H and C-O bonds, demonstrating again the difference between thermodynamic and chemical stability. Removal of the proton generates the anion - alkoxide ion. Alkoxides are important bases in organic chemistry. Acidity of Phenols In some respects, the chemical behavior of phenols is different from that of the alcohols, so it is sensible to treat them as a similar but characteristically distinct group.
In contrast, with phenols the substitution of the hydroxyl hydrogen atom is even more facile, and makes them roughly a million times more acidic than equivalent alcohols.
This
phenolic
acidity
is
further
enhanced
by
electron-withdrawing
substituents ortho and para to the hydroxyl group, as displayed.
The alcohol cyclohexanol is shown for reference at the top left. It is noteworthy that the influence of a nitro substituent is over ten times stronger in the para-location than it is in meta, despite the fact that the latter position is closer to the hydroxyl group. Furthermore additional nitro groups have an additive influence if they are positioned in ortho or para locations. The trinitro compound shown at the lower right is a very strong acid called picric acid. Why is phenol a much stronger acid than cyclohexanol? To answer this
question one must evaluate the manner in which an oxygen substituent interacts with the benzene ring. In electrophilic aromatic substitution reactions, an oxygen substituent enhances the reactivity of the ring and favors electrophile attack at ortho and para sites. It is proposed that resonance delocalization of an oxygen non-bonded electron pair into the pielectron system of the aromatic ring was responsible for this substituent effect.
The resonance stabilization in these two cases is very different. The contributing structures to the phenol hybrid all suffer charge separation, resulting in very modest stabilization of this compound. On the other hand, the phenolate anion is already charged, and the canonical contributors act to disperse the charge, resulting in a substantial stabilization of this species. The conjugate bases of simple alcohols are not stabilized by charge delocalization as in phenols, so the acidity of these compounds is similar to that of water. An energy diagram showing the effect of resonance on cyclohexanol and phenol acidities is shown below. Since the resonance stabilization of the
phenolate conjugate base is much greater than the stabilization of phenol itself, the acidity of phenol relative to cyclohexanol is increased. However, phenols are weaker acids than carboxylic acids (pKa = 5) or even carbonic acids (pKa = 7).
The phenolate negative charge is delocalized on the ortho and para carbons of the benzene ring and comes from the influence of electron-withdrawing substituents at those sites. Electron withdrawing substituents (e.g., NO 2) can make phenols as acidic as many carboxylic acids.
Electron donating substituents like -NH2, -OR, -R etc., which destabilize the phenoxide ion tend to decrease the acid strength of phenols. The table given below summarizes some of these facts. Compound Phenol
pKa 10.0
Compound
pKa
o-Methoxyphenol
10.0 p-Methoxyphenol
10.2
o-Methylphenol
10.3 p-Methylphenol
10.3
o-Chlorophenol
8.6
p-Chlorophenol
9.4
o-Nitrophenol
7.2
p-Nitrophenol
7.2
m-Nitrophenol
8.4
The -OH group is a poor leaving group and needs to be converted to a better leaving group before substitution can occur. Reactions Involving the Cleavage of the O-H Bond Both alcohols and phenols readily undergo cleavage of the O-H Bond. The ease of cleavage follows the sequence: Phenols > primary alcohols > secondary alcohols > tertiary alcohols Electrophilic Substitution at Oxygen Substitution of the Hydroxyl Hydrogen Because of its enhanced acidity, the hydrogen atom on the hydroxyl group is rather easily replaced by other substituents. A simple example is the facile reaction of simple alcohols with alkali metals like sodium or potassium similar to water to give the alkoxide:
The mechanism by which many substitution reactions of this kind take place is straightforward. The oxygen atom of an alcohol is nucleophilic and is therefore prone to attack by electrophiles. The resulting 'onium' intermediate then loses a proton to a base, giving the substitution product. If a strong electrophile is not present, the nucleophilicity of the oxygen may be
enhanced by conversion to its conjugate base (an alkoxide). This powerful nucleophile then attacks the weak electrophile. These two variations of the substitution mechanism are illustrated in the following diagram.
Like the alcohols, the phenolic hydroxyl hydrogen is rather easily replaced by alkali metals.
However, phenol reacts easily with alkalis itself to form phenoxides or phenates.
Alcohols do not react with NaOH solution and so this test distinguishes phenols from alcohols.
Reaction with Alkyl Halides Alkyl substitution of the hydroxyl group leads to ethers. Alkoxide salts react with alkyl halides to give ethers by the Williamson ether synthesis reaction.
This reaction provides examples of both strong electrophilic substitution, and weak electrophilic substitution. The weak electrophilic substitution is a SN2 reaction and is generally used only with primary alkyl halide reactants because the strong alkoxide base leads to E2 elimination of secondary and tertiary alkyl halides.
Primary alkyl halides are most reactive (E 2 competes with more hindered substrates). Thus unsymmetrical ethers are best prepared with the less hindered member being the alkyl halide, and the more hindered member being the alkoxide. For example, if we use a tertiary alkoxide ion and a methyl halide the SN2 reaction will predominate:
However, if a primary (or any other) alkoxide ion attacks a tertiary halide the
reaction is blocked by steric hindrance and thus the E 2 elimination is preferred:
The reaction of phenols with alkyl halides is carried out by first dissolving phenols in NaOH solution to form the phenoxide and then heating it with the alkyl halide to from the ether.
The phenate ion is an excellent nucleophile. Reaction with Carboxylic Acids Alcohols react with carboxylic acids in the presence of a few drops of concentrated H 2SO4 or dry HCl acid as catalyst, to form esters.
This reaction is called Fischer esterification reaction and is usually slow and exothermic. Phenols do not easily undergo the esterification reaction with carboxylic acids because the reaction is endothermic.
Reaction with Acid Chlorides and Anhydrides When alcohols or phenols are treated with acid chlorides or anhydrides in the presence of a base such as pyridine or dimethylaniline, the H-atom of the OH group is replaced by the acyl (RCO-) group to form esters.
Phenols react with acid chlorides or acetic anhydride to give phenyl acetate.
Similar to acylation, the introduction of acetyl (CH 3CO) group in alcohols or phenols is known as acetylation. The reaction of alcohols and phenols with benzoyl chloride in presence of
aqueous NaOH is called Schotten Baumann reaction.
Reaction with Grignard's reagent Both alcohols and phenols react with Grignard's reagent to form hydrocarbons.
Reactions Involving the Cleavage of the C-O Bond A step toward improving the reactivity of alcohols in breaking the C-O bond would be to modify the -OH functional group in a way that improves its stability as a leaving anion. One such modification is to conduct the substitution reaction in strong acid so that -OH is converted to -OH 2(+). Since the hydronium ion (H3O(+)) is a much stronger acid than water, its conjugate base (H 2O) is a better leaving group than hydroxide ion. The only problem with this strategy is that many nucleophiles, including cyanide, are deactivated by protonation in strong acid, effectively removing the nucleophilic co-reactant needed for the substitution. The strong acids HCl, HBr and HI are not subject to this difficulty because their conjugate bases are good nucleophiles and are even weaker bases than alcohols.
Nucleophilic
substitution
of
1
-alcohols
proceeds
by
a
SN2
mechanism, whereas 3 -alcohols react by a SN1 mechanism. Reactions of 2 alcohols may occur by both mechanisms and often produce some rearranged products.
Action of Halogen Acids Alcohols when treated with halogen acids (HX) get converted into haloalkanes. This reaction does not occur with phenols. The nature of alcohol and the halogen acid both influence the rate of the reaction. Primary and secondary alcohols form chloroalkanes when hydrochloric acid gas is passed through alcohol in the presence of anhydrous zinc chloride (Groove's process).
ZnCl2 is a Lewis acid and it readily coordinates with the oxygen atom of the alcohols. As a result, the C-O bond weakens and breaks to form carbocation, which reacts with chloride ion to form chloroalkanes. Thus, anhydrous ZnCl2 helps in the cleavage of the C-O bond. Tertiary alcohols are very reactive and therefore, they react readily with concentrated HCl even in the absence of zinc chloride.
The three classes of alcohols show difference in reactivity with HCl. In fact this is one of the methods called Lucas test that is used to distinguish them from one another. For example, primary alcohols on reacting with Lucas reagent (conc. HCl and ZnCl2) does not produce turbidity at room temperature while in case of tertiary alcohols, turbidity is produced
immediately since they form the halides easily. Similarly, when an alcohol is heated with hydrobromic acid (48%) bromo alkanes are obtained. HBr can also be generated in situ (during the reaction) by the action of concentrated H 2SO4 on KBr or NaBr.
On heating alcohols with constant boiling hydroiodic acid (57%) generated in situ by the action of phosphoric acid on potassium iodide, iodoalkanes are obtained.
Secondary and tertiary alcohols cannot be used to prepare respective bromides and iodides unlike alkyl chlorides. This is because secondary and tertiary alcohols undergo dehydration on heating with concentrated H2SO4, to form alkenes. The order of reactivity of halogen acids on alcohols is in accordance with the bond
dissociation
energies
of
H-X
bonds:
HI > HBr > HCl Reactivity
of
alcohols
towards
this
reaction
is:
Tertiary > Secondary > Primary The cleavage of C-O bond becomes easy and reactivity increases when the polarity of C-OH increases with the number of electron releasing groups on the -carbon atom of the alcohol.
Action of Phosphorus Halides Phosphorus halides react with alcohols to form haloalkanes. The action of phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl 3) on alcohols brings about the cleavage of C-O bond.
Likewise, bromoalkanes and iodoalkanes are generated by the action of phosphorus tribromide (PBr3) and phosphorus tri-iodide (PI 3) respectively on alcohols. As PBr3 and PI3, are not very stable compounds, they are prepared in situ by the action of red phosphorus on Br 2, or I 2, as follows:
Treatment of phenols with PCl5 gives only a small amount of chlorobenzene, the major product being tiphenyl phosphate. Action of Thionyl Chloride By refluxing alcohols with thionyl chloride in the presence of pyridine, chloro alkanes can also be prepared from alcohols.
Phenols do not react with thionyl chloride. Action of Ammonia When a mixture of the vapours of an alcohol and ammonia are passed over heated alumina (Al2O3) at 633 K, a mixture of primary, secondary and tertiary amines is produced.
The reaction of ammonia on phenol give aniline under severe conditions only, as phenol is less reactive towards nucleophilic displacement reactions.
Reaction with Zinc Dust Phenols but not alcohols on distillation with zinc dust give the corresponding aromatic hydrocarbon. For example,
Reactions Involving the Alcohol Molecule as a Whole There are two types of reactions that alcohols are prone to and engage in; one is essentially the reverse of addition of water to an alkene. This is called dehydration (the loss of water from an alcohol). The other is called oxidation. This reaction results in loss of hydrogen and the conversion of the C-O single bond to a double bond. Dehydration of Alcohols In the case of dehydration either water is removed as the reaction progresses or the equilibrium already favors alkene production. The dehydration of alcohols as a method of synthesizing alkenes with the products usually predicted by Zaitsev's rule - the more highly substituted alkene will be the major product. This reaction is acid catalyzed, commonly being carried out in sulphuric acid or heated alumina requiring higher temperatures. Tertiary and secondary alcohols react under mild conditions, such as with phosphoric acid at lower temperatures.
Different products are obtained under different reaction conditions and temperature e.g., ethers are obtained when excess alcohol is used.
The mechanism of the dehydration reaction is an E 1 elimination reaction:
Alcohols act as bases due o the presence of lone pair of electrons on the oxygen atom. Therefore they react with strong mineral acid to form the oxonium salts in the first step. In the 2nd slow step the C-O bond weakens due to the presence of positive charge on the highly electronegative oxygen. As a result, the protonated alcohol readily eliminates a molecule of water to form ethyl carbocation.
In step 3 the ethyl carbocation being a reactive chemical species looses a proton to form the ethene molecule. The
relative
ease
of
dehydration
is
as
follows:
Tertiary > Secondary > Primary This path is essentially the reverse of the water addition path. For interior OH groups, the final location of the C=C bond depends on the branching of the alkane. For example:
In general, the double bond will locate on the carbons with the most branching, although frequently there will be some of the other product present. Oxidation of Alcohols Simple 1 and 2-alcohols in the gaseous state lose hydrogen when exposed to a hot copper surface. This catalytic dehydrogenation reaction produces aldehydes and ketones. In the case of tertiary alcohols alkenes are produced.
Since the carbon atom bonded to the oxygen is oxidized, such alcohol to carbonyl conversions are generally referred to as oxidation reactions. Gas phase dehydrogenations of this kind are important in chemical manufacturing.
However, when alcohol oxidations are carried out in solution, the hydroxyl hydrogen is replaced by an atom or group that is readily eliminated together with the alpha-hydrogen. The most important reactions of alcohols are their oxidation to aldehydes, ketones, and carboxylic acids (carbonyl compounds) and may be viewed as follows:
The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon. In order for each oxidation step to occur, there must be H on the carbinol carbon. Primary alcohols are oxidized to aldehydes, which in turn may be oxidized to acids, both containing the same number of carbon atoms as the original alcohol. Secondary alcohols are oxidized to ketones with the same number of carbon atoms as the original alcohol. Tertiary alcohols are not normally oxidized since it would be necessary to break a C-C bond (C-C bonds are very strong and stable, thus resistant to reaction without destabilizing structural relations or high energy).
While secondary alcohols can be oxidized to ketones, only under drastic conditions further can oxidation occur, when for instance, carboxylic acids with lesser number of carbon atoms than the original alcohol is formed.
Tertiary alcohols cannot be oxidized easily (no carbinol bond C-H) and only under strong acid oxidizing agents they convert to a mixture of ketone and an acid each containing lesser number of carbon atoms than the original alcohol.
Common oxidizing agents consisting of chromate and manganate species are summarized below.
Primary alcohols can be oxidized to aldehydes or further to carboxylic acids. In aqueous media, the carboxylic acid is usually the major product. However, they can be oxidized to aldehydes by the careful selection of an oxidizing agent like PCC or PDC in dichloromethane. The best reagent for laboratory scale
reactions
is
pyridinium
chlorochromate,
C5H6NCrO3Cl
(PCC),
in
dichloromethane solvent:
Other oxidizing agents, such as sodium dichromate (Na2Cr2O7) in aqueous acid or chromium trioxide (CrO 3), continue the oxidation process to the carboxylic acid product. The intermediate aldehydes are not generally isolated because they are oxidized to acids too rapidly:
Secondary alcohols are readily oxidized to ketones:
In each case a chromate ester of the alcohol substrate is believed to be an intermediate, which undergoes an E2 like elimination to the carbonyl product. The oxidation state of carbon increases by 2; while the chromium is reduced and decreases by 3. Since chromate reagents are a dark orange-red color (VI oxidation state) and chromium III compounds are normally green, the progress of these oxidations is easily observed. Indeed, this is the chemical transformation on which the Breathalizer test is based.
Jones' reagent: a solution of sodium dichromate in aqueous sulphuric acid Oxidation of Phenols Generally phenols are more easily oxidized than simple alcohols. Oxidation can be achieved by reaction with silver oxide (Ag 2O) or chromic acid (Na2Cr2O7), or other oxidizing agents. Particularly important are the oxidation of 1,2- and 1,4-benzenediol (pyrocatechol and hydroquinone, respectively) and their derivatives.
These types of systems are important in biological redox-systems such as coenzyme Q and vitamin K. Here's a closer look at the two one electron transfers that are believed to take place when hydroquinone is oxidized to benzoquinone.
Loss of a proton and an electron generates a phenoxy radical.
Loss of a second proton and a second electron completes the oxidation. Electrophilic Reactions Phenols are potentially very reactive towards electrophilic aromatic
substitution. This is because the hydroxy group -OH, is a strongly activating, ortho-/para- directing substituent. Substitution typically occurs para to the hydroxyl group unless the para position is blocked, then ortho substitution occurs.
The strong activation often means that milder reaction conditions than those used for benzene itself can be used. Phenols are so activated that polysubstitution and oxidation can be a problem at times. Halogenation Phenols react with halogens in the presence of less polar solvents like CS2, CHCl3 and CCl 4 to give ortho and para isomers of monohalophenols.
Phenols when treated with chlorine or bromine water give polyhalogen derivatives in which all the H-atoms present at the ortho- and para- positions with respect to OH group are replaced by chlorine and bromine atoms.
Nitration With dilute HNO3 at 293 K phenol give mixtures of ortho and para isomers of mononitrophenols.
The yield of mononitrophenols is quite low due to the partial oxidation of the ring by HNO3. However, if phenol is first treated with nitrous acid, nirosophenol is formed which on further oxidation with dilute nitric acid yields good yeild of mononitrophenol. The ortho and para isomers can be separated by steam distillation. onitrophenol undergoes intramolecular hydrogen bonding while p-nitrophenol undergoes intermolecular hydrogen bonding. While o-nitrophenol is steam volatile and p-nitrophenol is less volatile due to the association of molecules and so can be separated by distillation.
With concentrated nitric acid and in the presence of sulphuric acid, phenol gives 2,4,6-trinitrophenol (picric acid). The yield is low due to oxidation of phenol.
Sulphonation When phenol is treated with concentrated H2SO4 sulphonation occurs. A mixture of ortho- and para- isomers are obtained. If the sulphonation is carried at low temperatures (293 K), the ortho isomer predominates and at high temperatures (2373 K) the para isomer is the chief product.
Alkylation and Acylation
The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is carried by treating phenol with alkyl chloride or acyl chloride in the presence of a catalyst like anhydrous aluminium chloride. For example,
Carboxylation nucleophilic
of
Phenols
phenolate
salt
(Kolbe-Schmitt with
carbon
reaction) Heating reaction) dioxide
under
the high
pressure/temperature results in regioselective ortho-substitution.
On further acidification the o-and p-isomers of o-hydroxybenzoic acid are formed. This process is also known as the Kolbe-Schmitt synthesis.
However, o-hydroxybenzoic acid is the main product and is commonly known as salicyclic acid. This is the starting material for preparing 2-acetoxybenzoic acid (aspirin) which is widely used as analgesic and antipyretic. The acetylation of salicylic acid produces aspirin.
Reimer-Tiemann Reaction On treating phenol with chloroform in the presence of aqueous sodium or potassium hydroxide at 340 K followed by hydrolysis of the resulting product, gives 2-hydroxybenzaldehyde (salicylaldehyde) as the major product and 4hydroxybenzaldehyde as a minor product.
If instead of chloroform, CCl4 is used salicyclic acid is formed as the major product.
The Reimer-Tiemann reaction occurs by the generation of dichlorocarbene (:CCl2) electrophile which attacks the ortho- para- electron rich centres to substitute the hydrogen atom at these positions.
Some Commercially Important Alcohols Methanol (Wood Alcohol) Methanol or Methyl Alcohol can be obtained by the destructive distillation of wood, hence its popular name of wood alcohol. Commercially, methanol is prepared by the hydrogenation of carbon monoxide in the presence of a metal oxide catalyst such as zinc oxide and chromic oxide under high temperature and pressure.
Methane is also used for producing methanol.
Properties Methanol is a clear, colourless, organic liquid with a pleasant odour. It is inflammable and has a high calorific value and hence it is used as a fuel.
It burns with a good smokeless flame.
Methanol is used mostly for industrial purposes. It is an important solvent for many substances used in dyes, drugs etc. When mixed with water, the mixture does not freeze even at sub-zero temperatures. Hence, in cold countries, it is used as antifreeze for automobile radiator. It is used as a starting material for making other organic compounds e.g., when
methanol
is
oxidized
by
acidified
potassium
permanganate
or
potassium dichromate, formaldehyde is formed.
Formaldehyde on further oxidation gives formic acid.
Methanol is highly poisonous. If consumed it causes blindness due to destruction of the cells of the optic nerve. If consumed in large amount, it can cause death. Spurious alcohol or illicit liquor is often made by improper distillation or by using methylated spirit. It is cheap and is mostly made available to the lower strata of our society. Spurious alcohol contains higher percentage of methyl alcohol which is poisonous. Consumption of such liquor may cause blindness, serious health problems and even death. Sometimes, to give the consumer gets a pronounced feeling of 'intoxication', other chemicals are mixed with ethyl alcohol. Such mixtures can prove to be exceedingly poisonous and can cause severe damages to the body organs, nervous system resulting in
death. Ethanol Ethanol or ethyl alcohol has been used for thousands of years especially in the form of wine. Usually the term 'alcohol' refers to ethanol. The raw materials used for the preparation of alcohol are plant products that contain some form of starch or sugar. Grapes, barley, rice, potatoes, apples etc., are examples of such materials. The process of converting the starch to sugar is called fermentation. Today, most of the alcohol produced in the world is by the fermentation of molasses, a brownish syrupy liquid obtained in the sugar industry. It contains a large amount of sugar, which cannot be further crystallized. Yeast is added to these molasses, and it is allowed to ferment for usually three weeks. The enzyme maltase present in yeast converts the sucrose of molasses to glucose.
Yeast also contains an enzyme zymase, which converts glucose to ethanol and carbon dioxide.
The alcohol so formed is separated by fractional distillation to obtain rectified spirit. The product-rectified spirit contains about 96% ethanol and 4% water. Pure alcohol (100% alcohol) called 'absolute alcohol' is obtained from this product by distillation with benzene (C6H6).
Ethanol is also be obtainable from petroleum products. During the cracking of petroleum, large quantities of ethene (ethylene) are produced. This can be hydrated using sulphuric acid as a catalyst to produce ethanol.
Water molecule adds to an alkene molecule across the double bond in the presence of dilute acids and a catalyst. For example, ethane gives ethanol when a mixture of ethene and steam is passed over phosphoric acid and silica under a pressure 65 atm, and at 300 C
Properties Ethanol is colourless and has a pleasant odour. Its boiling point is 78 oC and its freezing point is -114 oC. It is soluble in water and almost all the organic solvents. It is highly intoxicating in nature. It is combustible and burns with a blue flame. Ethanol
is
used
for
many
industrial
applications
because
of
some
characteristic reactions. The products formed are starting materials for carrying out many organic reactions both at industry and laboratory. For example, When a piece of sodium is dropped in ethyl alcohol, bubbles of hydrogen gas are observed.
Ethanol reacts with phosphorus trichloride to form ethyl chloride.
When treated with concentrated H2SO4 at 170 oC, ethyl alcohol undergoes dehydration to form ethane.
At lower temperature of 140 oC, and when present in excess, ethyl alcohol forms a pleasant smelling substance called diethyl ether.
Alcohols on oxidation give aldehydes. The aldehydes on further oxidation give carboxylic acids.
Based on these reactions important uses of alcohol are as follows: Ethyl Alcohol is used as a solvent for many organic solutes, especially which are insoluble in water. It is used in the preparation of perfumes. It is used in the manufacturing of 'power alcohol', which is 80% mixture of petrol (gasoline) and 20% absolute alcohol. It can be used to generate power as a motor fuel and help to save gasoline. As absolute
alcohol does not mix with petrol, benzene, ether or tetralin is added into this mixture. Ethyl Alcohol is used in making tinctures and medical syrups. It is used as a solvent for paints, varnishes, dyes etc. It is used in the production of many organic compounds. It is used in alcoholic beverages. The chemical usage and common usage of the term 'alcohol' differ. Chemically, the term alcohol refers to a group of organic compounds having OH group in their composition; the common man reference to alcohol relates to ethyl alcohol or ethanol. Ethyl alcohol has a variety, of uses as seen above, especially as a solvent. But by far the greatest use of ethyl alcohol is in the form of alcoholic beverages, such as wine, beer, rum, brandy, whisky etc. Alcohol consumed in sizeable amounts is detrimental and affects the nervous system In small quantities it may serve as a source of energy, but consumption of alcohol is a habit forming activity.. If consumed over a period of time, alcohol can ruin one's health especially the liver, which gets affected by cirrhosis. . The person loses his or her sense of balance and mental ability when consumed in excess and this type of consumption often ruins family life and can have tragic consequences. Alcoholic drinks are heavily taxed by the government, so as to discourage people from over consumption. Certain state have banned alcoholic drinks altogether, inspite of the taxes being a sizable revenue earning for state government coffers. Alcohol used for industrial and surgical purposes is not taxed heavily. But in order to prevent unscrupelous people fromm buying and
consuming industrial and surgical alcohol, it is mandatory that ethyl alcohol be mixed with a certain percentage of highly poisonous methyl alcohol or methanol. This renders the ethyl alcohol unfit for human consumption. This mixture is called 'Methylated Spirit'. If chemicals like copper sulphate or pyridine are added to ethyl alcohol it is called 'denatured alcohol'. Both these mixtures are prepared so as to prevent people from drinking alcohol heavily.
Ethers Ethers can be prepared by the following general methods. Preparation of Ethers Williamson Ether Synthesis Ethers are usually prepared from alcohols or their conjugate bases. One important procedure is the Williamson Ether Synthesis mentioned earlier. This reaction proceeds by an SN2 reaction of an alkoxide nucleophile with an alkyl halide. The substitution reaction permits the convenient synthesis of both symmetrical and non-symmetrical ethers. This type of reaction happens in two steps. The alcohol firstly reacts with sodium (or some other substance that removes a proton from the alcohol (-OH) group. Then a haloalkane is added to a solution of the resulting anion, when the alkyl nucleophile from the haloalkane attaches to it. Na + ion is a spectator ion forming a halide salt.
For preparation of ethers with different R groups bound to oxygen (non-symmetrical ethers), all one has to do is use an alcohol with one organic group and an organic
chloride with a different group. Reactions of a secondary alkyl alkoxide and a primary aryl halide and vice versa are shown as two examples of this procedure.
In this case unsymmetrical ether benzyl isopropyl ether is obtained. As there are two different combinations, two different types of reactants are possible, one usually better than the other. Since alkoxide anions are strong bases, the possibility of a competing E2 elimination also occurs here. Bearing in mind the factors that favor substitution over elimination, a primary alkyl halide is a preferred reactant as it gives a better and cleaner yield of benzyl isopropyl ether than does other reaction which generates considerable elimination product. Best yields of unsymmetrical ethers are obtained when the alkyl halides are primary and the alkoxides are tertiary. Aryl halides are much less reactive than alkyl halide and so are usually prepared by treating sodium phenoxide with alkyl halides. Alkoxy Mercuration A second general ether synthesis involves the oxymercuration reaction. The alcohol reactant is used as the solvent, and a trifluoroacetate mercury (II) salt is used as nucleophile in preference to the acetate). Alkenes readily react with mercury (II) trifluoroacetate acetate in the presence of an alcohol to give alkoxy mercurial compounds, which on reduction with NaBH4 in basic
medium gives ethers in high yield. Reactions of butene and cyclopentene as examples of this two-step procedure are given below.
Note that this reaction gives ether in accordance with Markovnikov's addition rule and does not involve a carbocation and competing elimination reaction. Dehydration of Alcohols Symmetrical ethers (R-O-R) can be prepared by gently heating an alcohol in the presence of an acid catalyst like concentrated sulphuric acid. This process is very similar to that used to dehydrate an alcohol to an alkene. The primary difference is that dehydration uses somewhat higher temperatures.
The yield of ether depends upon the nature of the alcohol whether it is 1, 2, or 3 . The formation of ethers from these alcohols is best achieved when alcohol is used in excess. However, the order of dehydration of alcohols leading to the formation of ethers follows the sequence, Primary > Secondary > Tertiary Acid-catalyzed dehydration of small 1 alcohols constitutes a specialized method of preparing symmetrical ethers. As shown in the following two equations, the success of
this procedure depends on the temperature. At 110 C to 130 C an S N2 reaction of the alcohol conjugate acid leads to an ether product. At higher temperatures (> 150 C) an E2 elimination takes place.
The dehydration of an alcohol to either an alkene or ether shows the reactions to proceed over a relatively narrow temperature range (20 for ethanol). Such a narrow range of conditions suggests that when these reactions are carried out one may obtain more than one product and such is frequently the case. Catalytic Dehydration Dehydration of alcohols can also be achieved by passing the vapours of an alcohol over alumina (Al2O3) at 623 K (350 C). For example,
The order of the ease of dehydration of alcohols is, Tertiary > Secondary > Primary Reaction of Alkyl Halides with Dry Silver Oxide Haloalkanes can also be converted into ethers by heating with dry silver oxide.
Reaction With Grignard's Reagent Higher ethers can be produced by the action of -halo ethers with Grignard's reagent.
Reaction With Diazomethane Ethers can be prepared by the reaction of diazomethane with alcohols in the presence of tetrafluoroboric acid (HBF 4) as catalyst.
Physical Properties Physical State Higher Ethers are gases at ordinary temperature while the other lower members are colourless liquid with a characteristic 'ether smell'. Dipole Nature The C-O bonds in ethers are slightly polar because of greater electronegativity of oxygen than carbon atom. Since the two C-O bonds in ethers are inclined to each other at an angle of 110o the two dipoles do not cancel each other and thus they a dipole moment of 1.15-1.3 D. Boiling Points Ethers are isomeric with monohydric alcohols but their boiling points are much lower than the isomeric alcohols. This is because unlike alcohols ethers do not form hydrogen
bonds and exit as associated molecules. Further the weak polarity of the C-O bonds do not affect the boiling points of ethers which are comparable with those of alkanes of same molecular masses. Diethyl ether (MW 74): 308 K; butyl alcohol (MW 74): 351 K; pentane (MW 72) 309 K Solubility Ethers have water solubilities intermediate between alkanes and alcohols. Because ethers have no O-H bonds, they cannot participate in hydrogen bonding to the same extent that alcohols do. Nevertheless, the oxygen in the ether can form a hydrogen bond to the hydrogen in water. The presence of only single site on the ether for a limited kind of hydrogen bonding interaction means that ethers generally have significantly smaller solubilities in water than do alcohols. Still they have higher solubilities than any hydrocarbon. As their molecular mass increases, the solubility of ethers in water decreases due to corresponding increase in the hydrocarbon portion of the molecule. Ethers are fairly soluble in common organic solvents like alcohol, benzene, chloroform, acetone etc. Chemical Reactions Ethers are generally unreactive and make good as solvents. They do not react with halogens, nucleophiles or mild acids or bases. They are widely used as solvents for a variety of organic compounds and reactions, suggesting that they are relatively unreactive
themselves.
With
the
exception
of
the
alkanes,
cycloalkanes
and
fluorocarbons, ethers are probably the least reactive, common class of organic compounds. The inert nature of the ethers relative to the alcohols is undoubtedly due to the absence of the reactive O-H bond. Reactions of Ethereal Oxygen
The ethereal O atom is a region of high electron density due to the presence of lone pairs. Hence, ether oxygen atoms are Lewis bases undergoing reactions involving the formation of co-ordinate bonds. Formation of Oxonium Salts The oxygen atom of ether has two lone pairs of electrons. As a result they behave as Lewis bases and thus dissolve in cold concentrated inorganic acids to form stable oxonium salts. For example,
Formation of Coordination Complexes Ethers form coordinate bonds with Lewis acids such as BF3, AlCl3, FeCl3, Grignard's reagent etc., to produce complexes called as etherates.
The etherate complexes from Grignard's reagent dissolve in ether and that is the reason Grignard's reagent are usually prepared in ethers. Reactions Involving the Cleavage of C-O bond Like an alcohol -OH group, the -OR group is a poor leaving group and needs to be converted to a better leaving group before substitution can occur. The most important reaction of ethers is their cleavage by strong acids. Alkyl ethers are cleaved by the strong acids HI or HBr in a nucleophilic substitution reaction similar to that of alcohols. Aqueous HI is the acid most commonly used, but HBr also works
sometimes.
Protonation of the ethereal oxygen creates a good leaving group, a neutral alcohol molecule. The halide ion, bromide or iodide are both good nucleophiles. Depending on the structure of the alkyl groups, the reaction can be S N1 or SN2. Reaction with Halogen Acids The cleavage of the C-O bond by strong halo acids is the most common reaction of ethers. This occurs by SN1 or E 1 mechanisms for 3-alkyl groups or by an S N2 mechanism for 1-alkyl groups. The conjugate acid of the ether is an intermediate in all these reactions as shown below.
Ethers are readily cleaved by hydroiodic acid at 373 K to form an alcohol and an alkyl halide.
However, if excess of acid is used the alcohol first formed reacts further with the halogen acid to form the corresponding alkyl halide.
The reaction is a typical nucleophilic substitution, taking place via a S N1 or SN2 mechanism. Primary and secondary alkyl ethers are attacked by iodide ion to give an S N2 mechanism with acid catalysis:
Firstly, ether being Lewis bases undergoes protonation to form oxonium salts as the intermediate. Protonation of the alcoholic oxygen takes place to make a better leaving group. This step is very fast and reversible. In the second step the formed protonated ether undergoes nucleophilic attack by the halide ion. The halide ion functions to displace the good leaving group, neutral alcohol molecule, by cleaving the C-O bond. This results in the formation of an alkyl bromide and an alcohol. Note that the iodide attacks the less hindered alkyl group, so in the unsymmetrical ethers alkyl halide is always formed from the smaller alkyl group. Protonation of tertiary ethers in acid lead to spontaneous cleavage to give a carbocation intermediate, resulting in either an SN1 or E1 mechanism for cleavage:
There is a similarity to the analogous alcohol reactions when 'water' is lost, except an alkyl group has been substituted for the H of water to give an alcohol leaving group. The order of halogen acids follow the sequence: HI > HBr > HCl In case of alky aryl ethers the products are always phenol and alkyl halide and never an aryl halide and alcohol.
. This occurs because the phenol formed in this reaction does not react further, since SN2, SN1 and E1 reactions do not take place on aromatic rings. Reaction with Sulphuric Acid Ethers undergo hydrolysis reaction to from alcohols when heated with dilute sulphuric acid under pressure.
When heated with concentrated sulphuric acid alcohol and alkyl hydrogen sulphates are formed.
Reaction with Acid Chlorides and Anhydrides When ethers are treated with acid chlorides or anhydrides in the presence of anhydrous ZnCl2 or AlCl3 esters and alkyl halide are formed.
Reaction with Phosphorus Pentachloride The action of phosphorus pentachloride (PCl5) on ethers brings about the cleavage of CO bond leading to the formation of alkyl halide.
Reactions Involving the Alkyl Group Action of Air and Light Ethers in which oxygen is bonded to 1 - and 2 -alkyl groups are subject to peroxide formation in the presence of air (gaseous oxygen) and light.
This reaction presents an additional hazard to the use of these flammable solvents, since peroxides decompose explosively when heated or struck. The mechanism of peroxide formation is believed to be free radical in nature due to the presence of two
unpaired electrons molecular oxygen. Action of Halogens Ethers react with chlorine or bromine to give substitution products. The extent of halogenation depends on the reaction condition. For example,
Electrophilic Substitution Reactions Like phenols, the aromatic ethers having the alkoxy group undergo electrophilic substitution reactions of the benzene ring at ortho- and parap- positions.
Halogenation Anisole reacts with halogens in the presence of less polar solvents like CS 2, CHCl3 and CCl4 to give ortho and para isomers of monohaloanisoles.
Nitration With concentrated HNO3 /H2SO4 the nitrating mixture at 293 K anisole give mixtures of ortho and para isomers of mononitroanisoles.
Friedel-Craft's Alkylation and Acylation The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is carried by treating anisole with alkyl chloride or acyl chloride in the presence of a catalyst like anhydrous aluminium chloride. For example,
Using Ethers as Protective Groups Due to their relative lack of chemical reactivity, ethers have proven to be useful protective groups for alcohols and phenols. By converting a hydroxyl function to ether, its acidity and ease of oxidation (in the case of 1 and 2-alcohols) can be suppressed to such a degree that normally incompatible reactions, such as those employing Grignard reagents, may be carried out.
Summary Alcohols contain -OH functional group attached to a saturated sp 3 hybridized carbon atom like alkane (R-OH), while phenols have a -OH group attached to the carbon atoms of an aromatic ring (Ph-OH). Ethers are organic molecules with -C-O-C- as the functional group. Alcohols are divided into two broad categories - aliphatic alcohols and aromatic alcohols. Aliphatic and aromatic alcohols and phenols are classified as monohydric, dihydric, trihydric and polyhydric according to the number of hydroxyl groups in their molecules. Alcohols containing sp3 C-OH bond are classified into three categories as primary (1), secondary (2) and tertiary (3). These can be allylic or benzylic alcohols
(attached to next to double bonded carbon atom). Alcohols containing sp2 C-OH bond are named as vinylic alcohols (directly attached to double bonded carbon atoms) or aryl alcohols or phenols that are hydoxyl derivatives of aromatic hydrocarbons. Ethers are of two broad categories - aliphatic or aromatic ethers which can be symmetrical and unsymmetrical ethers. Alcohols are named using the -ol ending. On longer chains the location of the hydroxyl group determines chain numbering. Phenols are named using the word 'pheno' at the end. All substituted phenols are named as derivatives of phenol with the position of the substituent with respect to-OH group indicated by Arabic numerals. The ether functional group is named as an alkoxy substituent on the parent compound. Alcohols and phenols are dipolar in nature with the oxygen carrying a partial negative charge (δ-) that allows them to participating in hydrogen bonding. Ethers have a tetrahedral geometry with two sp 3 orbitals of oxygen contain a lone pair of electrons each. Alcohols are prepared by - hydration of alkenes (i) in presence of an acid and (ii) hydroboration-oxidation reaction (iii) oxymercuration-reduction - hydrolysis of haloalkanes - reduction of carbonyl compounds by (i) catalytic reduction and (ii) the action of Grignard's reagents - hydrolysis of esters and - reactions of primary amines.
Phenols may be prepared by - alkali treatment of haloarenes - alkali fusion of benzene sulfonic acid - hydrolysis of diazonium salts - decarboxylation of salicyclic acid - acidic oxidation of cumene and - reaction of Grignard's reagents. Alcohols, phenols and ethers are soluble in water because of the ability to form intermolecular hydrogen bonds with water. Solubility of alcohols in water decreases as the size of the organic group increases. Alcohols and phenols have higher boiling and melting points due to hydrogen bonding. Ethers do not hydrogen bond, and are more like alkane in their properties. Alcohols and phenols are weak acids. Alcohols are similar to water in acidity; phenols are more acidic than alcohols or water. Electron withdrawing groups in phenol increase its acidic strength and electron releasing groups decrease it. Alcohols and phenols undergo reactions involving the cleavage of the O-H Bond, the C-O bond and the whole alcohol molecule. Alcohols undergo nucleophilic substitution with hydrogen halides, carboxylic acids, acid chlorides and acid anhydrides and Grignard's reagent. The cleavage of C-O bond in alcohols and phenols takes place by halogen acids, phosphorous halides, thionyl chloride, ammonia and zinc dust (phenol only). Dehydration of alcohols gives alkenes predicted by Zaitsev's rule. On oxidation, primary alcohols yield aldehydes with mild oxidizing agents and form carboxylic acids with strong oxidizing agents while secondary alcohols yield ketones. Tertiary alcohols are resistant to oxidation.
The presence of -OH group in phenols activates the aromatic ring towards electrophilic substitution like sulphonation, nitration, halogenation, and Friedel-Craft reaction at ortho and para positions. In presence of sodium hydroxide, phenol generates phenoxide ion which is even more reactive than phenol. Phenol undergoes Reimer-Tiemann reaction to yield salicylaldehyde and Kolbe's reaction to yield hydroxyl acids. Methanol and ethanol are the two commercially important alcohols used both in the industry and in the laboratory. Ethers are prepared by - Williamson synthesis - alkoxy Mercuration - dehydration of alcohols - reaction of alkyl halides with dry silver oxide - reaction of Grignard's Reagent and - reaction with diazomethane. The boiling points of ethers resemble those of alkanes while their solubility is comparable to those of alcohols due to the polar nature of the C-O bond. Ethers undergo reactions involving the reactions of ethereal oxygen, the cleavage of C-O bond and the alkyl group. Reaction of ethereal oxygen leads to the formation of oxonium salts and complexes. The C-O bond in ethers can be cleaved by hydrogen halides, sulphuric acid, phosphorous pentachloride, acid chlorides and anhydrides.
Ethers are subject to peroxide formation in the presence of air and light. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the incoming group to ortho and para positions. Ethers are useful protective groups for alcohols and phenols.
