CHEMISTRY
CARBOXYLIC ACIDS & ACID DERIVATIVES Introduction : Compounds containing the carboxyl group are distinctly acidic and are called carboxylic acids.
O R–C–O–H Carboxylic acid There have general formula CnH2nO2 Carboxylic acid derivatives are compounds with functional groups that can be converted to carboxylic acids by a simple acidic or basic hydrolysis. The most important acid derivatives are esters, amides, nitriles, acid halides and anhydrides.
O
O
O
O
R–C–O–C–R anhydride
R–C–X acid halide
O
R – C – O – R' R – C – NH2 ester amide RCO R' RCONH
R–C N nitrile
Esters and amides are particularly common in nature. For example, isoamyl acetate found in ripe bananas and geranyl acetate is found in the oil of roses, geraniums and many other flowers. N, N-diethyl-metatoluamide (DEET) is one of the best insect repellents known and penicillin G is one of the antibiotics that revolutionized modern medicine. O
O
O – C – CH3
O – C – CH3 isoamyl acetate (banana oil)
geranyl acetate (geranium oil)
O O H3C
PhCH2 – C – NH
S
C N(CH2CH3)2
N, N-diethyl-meta-toluamide
N O
CH3 CH3 COOH
Penicillin G
"manishkumarphysics.in"
1
CHEMISTRY IUPAC Nomenclature of Acid and Acid derivatives:Table- 1
Compound
IUPAC Name
O Methanoic acid
(1) H – C – OH O
Ethanoic acid
(2) CH3 – C – OH
O 2-Cyclohexylpropanoic acid
(3) CH3 – CH – C – OH O
O
(4) CH3CCH – C – OH
3-Oxo-2-propylbutanoic acid
CH2CH2CH3 NH2
O
(5) CH2 – CH2 – CH2 – C – OH Ph
4-Aminobutanoic acid
O
(6) CH3CH2CH – CH2 – C – OH CH3
3-Phenylpentanoic acid
O
(7) CH3 – CH – CH 2 – C – OH
3-Methylbutanoic acid
O Ethanoylfluoride
(8) CH3 – C – F O
Propanoylchloride
(9) CH3 – CH2 – C – Cl O
Br
(10) CH3 – CH – CH2 – C – Br
3-Bromobutanoylbromide
O (11)
Cyclopentanecarbonyl chloride
– C – Cl O
O
(12) CH3 – C – O – C – CH 3 O
Ethanoic anhydride
O
(13) CF3 – C – O – C – CF3
Trifluoroethanoic anhydride
O O
(14)
1,2-Benzenedicarboxylic anhydride
O
"manishkumarphysics.in"
2
CHEMISTRY O
O
CH3 – C – O – C – H
Ethanoic methanoic anhydride
O
O
CH3CH2 – C – O – C – CF3
–C N
Trifluoroethanoic propanoic anhydride
Cyclopropane carbonitrile
–
CN CH3 – CH2 – CH – CH2 – COOH
3-Cyanopentanoic acid
O C – OCH2CH3
Ethyl o-cyanobenzoate
CN O C – NH2 2-Formylcyclohexane carboxamide C–H O OH CH3 – CH2 – CH – C N
2-Hydroxybutane nitrile
Dicarboxylic acids If the substituent is a second carboxyl group, we have a dicarboxylic acid. For example :
HOOCCH2COOH Malonic acid Propanedioic acid
HOOCCH2CH2COOH Succinic acid Butanedioic acid
HOOCCH2CH2CHCOOH | Br Bromogluta ric acid 2 Bromopen tan edioic acid
CH3 | HOOCCH2CCH2COOH | CH3
HOOCCH2CH2CH2CH2COOH Adipic acid Hexanedioic acid
, Dimethyglu taric acid 3,3 Dimethyl pen tan edioic acid
HOOCCHCH2CHCOOH | | Cl Cl , Dichloroglutaric acid 2,4 Dichloro pen tan edioic acid
"manishkumarphysics.in"
3
CHEMISTRY
Physical properties of acids and acid derivatives : (1) First three members are colourless pungent smelling liquid. The next three members also colurless oily liquid with unpleasant smell. Higher member (> 7) are colourless waxy solids. (2) Boiling points: The boiling point of carboxylic acids are higher than that of alcohols, ketones or aldehydes of similar molecular weight. O
O CH 3CH2CH2OH 1-propanol bp 97ºC
CH3 – C – OH acetic acid, bp 118ºC
CH3CH3CH Propionaldehyde bp 49ºC
The high boiling points of carboxylic acids is the result of formation of a stable hydrogen-bonded dimer. O ----- H – O R–C
C–R O – H ----- O
hydrogen bonded acid dimer
Carboxylic acids have higher boiling points than corresponding molucular mass alcohols because of (i) –OH bond in carboxylic acid is more polar than alcohol due to the presence of (ii) Carboxylic acid molecules are held together by two H-bonds.
group.
Esters and acid chlorides have boiling points near those of the unbranched alkanes with similar molecular weights. Nitriles also have higher boiling points than esters and acid chlorides of similar molecular weight. This effect results from a strong dipolar association between adjacent cyano groups. δ– δ δ + δ– (dipolar association of nitriles) :N C–R R – C N: These acid derivatives contain highly polar carbonyl groups, but the polarity of the carbonyl group has only a small effect on the boiling points.
+
× ×
300
× × Examples (MW 55 – 60)
1° amides
bp(ºC)
O
200 222
O CH3–C–OH CH3CH2CH2OH CH3CH2–CN O
118 97 97
Boiling point (ºC)
CH3–C–NH2
×
N-methyl 2°amides
CH3CH2CH2CH3
×
×
×
×
×
× ×
×
× ×
× acids
32 0 0
×
×
× nitriles
methyl esters
×
×
×
×
× 100
×
×
N,N-dimethyl 3° amides×
1° aocohol
H–C–OCH3
×
×
×
×
×
×
acid chlorides
×
–100 n-alkanes 20
60
100
140
180
Molecular weight
"manishkumarphysics.in"
4
CHEMISTRY (3) Melting points : Acids containing more than 8 carbon atoms are generally solids, unless they contain double bonds. The presence of double bonds (especially cis double bond) in a long chain impedes the formation of a stable crystal lattice resulting in a lower boiling point.
H
HH
CH3 – (CH2)16 – C – OH Stearic acid, mp 70ºC CH3(CH2)4
H O
O
C=C
C=C CH2
(CH2)7 – C – OH
linoleic acid mp –5ºC Melting point of carboxylic acids : There is no regular pattern in melting point of carboxylic acid (up to 10 carbon atoms) having even number of C atoms are higher than neighbouring members having odd number of C atoms because carboxylic acid and methyl group in even members lie in opposite side of zig-zag carbon chain hence they fit better into crystal lattice resulting in higher melting points.Vice-versa is observed in case of carboxylic acid having odd no. of carbon atoms.
Amides have surprisingly high boiling points and melting points compared with other compounds of similar molecular weight. Primary and secondary amides participate in strong hydrogen bonding. .. :O: –
:O: C R
C
R'
.. N
R
R'
R'
+ N R'
– O
H + C=N
H
R
H .. . O–
N–H +
– H. . . O
C
+ C=N R
R H
Hydrogen bonding R'
–
R C
R' + N+
+ +N
–O
– O
C
R'
R'
R
Intermolecular attraction Strong hydrogen bonding between molecules of primary and secondary amides also results in unusually high melting points.
O
CH3
H–C–N CH3 dimethylformamide (DMF) mp –61ºC
O
O
H
CH3 – C – N CH3 N-methylacetamide m.p. 28ºC
H
CH3CH2CN H Propionamide mp 79ºC
(4) Solubility: Carboxylic acids form hydrogen bonds with water and the lower molecular - weight carboxylic acids ( upto 4 carbon atoms) are miscible with water. Acid derivatives (esters, acid chlorides, anhydrides, nitriles and amides) are soluble in common organic solvents such as alcohols, ethers, chlorinated alkanes and aromatic hydrocarbons. Acid chlorides and anhydrides cannot be used in nucleophilic solvents such as H2O and alcohols, because they react with these solvents.
"manishkumarphysics.in"
5
CHEMISTRY Physical Properties of Carboxylic Acids Table -2 IUPAC name
Common Name
Formula
mp (ºC)
bp (ºC)
Solubility (g/100 g H2O)
Methanoic
formic
HCOOH
8
101
(miscible)
Ethanoic
acetic
CH3COOH
17
118
Propanoic
propionic
CH3CH2COOH
–21
141
2-Propenoic
acrylic
H2C=CH–COOH
14
141
Butanoic
butyric
CH3(CH2)2COOH
–6
163
2-Methylpropanoic
isobutyric
(CH3)2CHCOOH
–46
155
23
Trans-2-butenoic
crotonic
CH3–CH=CH–COOH
71
185
8.6
Pentanoic
valeric
CH3(CH2)3COOH
3-Methylbutanoic
isovaleric
(CH3)2CHCH2COOH
2,2-Dimethylpropanoic
pivalic
(CH3)2C–COOH
Hexanoic
caproic
Octanoic Decanoic
–34
186
–29
177
5
3.7
35
164
2.5
CH3(CH2)4COOH
–4
206
1.0
caprylic
CH3(CH2)6COOH
16
240
0.7
capric
CH3(CH2)8COOH
31
269
0.2
Physical Properties of Acid Derivatives Table -3 Compound
Name
mp (ºC)
bp (ºC)
Water
17
118
Solubility CH3COCl
Ethanoylchloride
(CH3CO)2O
Ethanoic anhydride
CH3COOH
Ethanoic acid
CH3CONH2
Ethanamide
222
O CH3–C–OCH2CH3
Ethyl acetate
– 83
77
10%
Dimethylformamide (DMF)
– 61
153
miscible
Dimethylacetamide (DMA)
– 20
165
miscible
Acetonitrile
– 45
82
miscible
O H–C–N(CH 3) 2 O CH 3–C–N(CH 3)2
CH3–C N
Methods of preparation of carboxylic acids 1.
Synthesis of carboxylic acids by the carboxylation of grignard reagents O O || || Dry H / H2O R C OH RMgX + O = C = O R C OMgX ether
Mg (OH) X
"manishkumarphysics.in"
6
CHEMISTRY
CH3 CHCH2CH3 | Cl
1. Mg / diethyl ether
2. CO 2 3. H3O+
2 Chlorobu tan e
CH3CHCH2CH3 | CO 2H
2 Methylbu tanoic acid (76 86%)
1. Mg / diethyl ether
2. CO 2 3. H3O+
CH3 CH COOH | CH3 Isobutyric acid (2 methyl propanoic acid)
Ex.
( i ) CO 2 CH3 CH MgBr ( ii ) H2 O / H | CH3
2.
Synthesis of Carboxylic acids by the hydrolysis of nitriles Mechanism :
+
heat
+ H+
+
Hydrolysis of cyanides (Acid catalysed) :
Ex.
NaCN
DMSO
CH2CN Benzyl cyanide (92%)
Ex.
O || CH2COH Phenylacetic acid (77%)
OH | CH3CCH2CH2CH3 | CN
"manishkumarphysics.in"
7
CHEMISTRY Note:
(1) Alkyl cyanides needed for the purpose can easily be prepared from the corresponding alkyl halides with alcoholic KCN or NaCN. R – Cl + KCN R – C N + KCl (2) This reaction is used to ascend the series having one carbon atom more than the corresponding alkyl halides which are prepared from alcohol on treating with phosphorus halide. ROH + PX5 R – X + POX3 + HX (3) This hydrolysis of alkyl cyanide provides a useful method to get carboxylic acid having one carbon atom more than the original alkyl halide and alcohols.
3.
By oxidation of alkylbenzenes - aromatic acids are produced.
KMnO / OH¯
(i) KMnO / OH ¯
4
4
(ii) H / H2O
(ii) H / H2O
K Cr O
7 2 2
Ex.
H2SO 4
Chemical Reactions 1. Acidic Strength : Acidity of carboxylic acids:-
R – C – O + H
R – C – OH
O (I)
O
(i) R – C – O (I) exists as two equivalent canonical structures I(A) and I(B). This ion is resonance stablised O and resonance hybrid structure is I(C). O O O R–C R–C R–C O O O I(A) I(B) I(C) (ii) R – C – O ion is more stable due to resonance, hence carboxylic acids are acidic in nature. O (iii) Electron withdrawing group (–I effect) stablises the anion and hence, increases acidic nature.
O X Ex.
O F – CH2 – COOH > Cl – CH2COOH > Br – CH2COOH > I – CH2COOH
Cl Ex.
C
Cl
Cl – C – COOH > Cl – CH – COOH > Cl – CH2COOH > CH3COOH Cl
"manishkumarphysics.in"
8
CHEMISTRY (iv) Electron releasing group (+ I effect) destablises the anion and hence decreases acidic nature.
O X
C O
Ex.
HCOOH > CH3COOH > CH3 – CH2 – COOH
COOH Ex.
COOH
COOH
> CH2
CH2 – COOH
> COOH
CH2 – COOH
Ex.
Relative acid strength is:RCOOH > HOH > ROH > HC CH > NH3 > RH Note Acidity of acids is compared by compairing stability of conjugate base. 2. (i) (ii)
Reaction involving removal of proton from –OH group. Action with blue litmus : All carboxylic acids turn blue litmus red. Reaction with metals : 2 CH3COOH + 2Na + H2 2CH3COOH + Zn
(iii) (iv)
(v)
+ H2
Reaction with alkalies : CH3COOH + NaOH CH3COONa + H2O Reaction with carbonates and bicarbonates : 2CH3COOH + Na2CO3 2CH3COONa + CO2 + H2O CH3COOH + NaHCO3 CH3COONa + CO2 + H2O Reaction of carboxylic acid with aqueous sodium carbonate solution produces brisk efferuescence. However most phenols do not produce effervescence. Therefore, the reaction may be used to distinguish between carboxylic acids and phenols. Reaction with grignard reagent : R–CH2MgBr + RCOOH
R–CH3 + RCOOMgBr
Note:
A stronger acid displaces a weaker acid from the salt of the weaker acid. Ex. CH3COOH (Stronger acid) + CH3ONa CH3COONa + CH3–OH (Weaker Acid) Ex. CH3COOH (stronger acid) + NaHCO3 CH3COONa + H2CO3 (Weaker acid) H2O + CO2 (lab. test of carboxylic acid)
3.
Reaction involving replacement of –OH group
(i)
Formation of acid chlorides :
"manishkumarphysics.in"
9
CHEMISTRY
(ii)
Ex.
+ SOCl2
Ex.
+ PCl5
+ SO2 + HCl
+ POCl3 + HCl
Fisher Esterification Carboxylic acid react with alcohol to form esters through a condensation reaction known as esterification.
General Reaction :
+ R – OH
+ H2O
Specific Examples: + CH3CH2–OH + CH3–OH Mechanism : (Acid catalysed esterification)
If we follow the forward reactions in this mechanism, we have the mechanism for the acid catalysed esterification of an acid. If however, we follow the reverse reactions, we have the mechanism for the acid catalysed hydrolysis of an ester. Acid catalysed ester hydrolysis.
which resut we obtain will depend on the condition we choose. If we want to esterify an acid, we use an excess of the alcohol and, if possible remove the water as it is formed. If we want to hydrolyse an ester, we use a large excess of water that is we reflux the ester with dilute aqueous HCl or dilute aqueous H2SO4.
"manishkumarphysics.in"
10
CHEMISTRY (iii)
Formation of amides :
In fact amides can not be prepared from carboxylic acids and amines unless the ammonium salt is heated strongly to dehydrate it. This is not usually a good method of preparing amides. (iv)
Formation of acid anhydride :
4. (i)
Decarboxylation reactions : Soda-lime decarboxylation : General reaction :
In this reaction carbanion intermediate is formed. Rate of reaction depends upon the stability of carbanion intermediate. Electron with drawing group at R–COOH will increases the rate of decarboxylation.
Ex.
Rate of decarboxylation. I > II > III > IV (ii)
(a) Decarboxylation of -keto carboxylic acids: Acids whose molecules have a carbonyl group one carbon removed from the carboxylic acid group, called -keto acids, decarboxylate readily when they are heated to 100–150ºC.
There are two reasons for ease of decarboxylation. When the acid estelf decarboxylates, it can do so through a six membered cyclic trensition state :
This reaction produces an enol directly and avoids an anionic intermediate. The enol then tautomerises to a methyl ketone.
"manishkumarphysics.in"
11
CHEMISTRY When the carboxylate anion decarboxylates, it forms a resonance stabilized enolate anion.
Alphatic acids that do undergo successful decarboxylation have certain functional groups or double or triple bonds in the or positions.
(iii)
Kolbe’s electrolysis Electrolysis R – R + 2CO2 + H2 + 2KOH 2RCOOK + 2HOH R CO2– + K+
Mechanism :
R CO2K
At Anode :
R CO2– R CO2 + e–
(oxidation)
(I) R
CO2
R + CO2
(II)
R + R R – R If n is the number of carbon atoms in the salt of carboxylic acid, the alkane formed has 2(n–1) carbon atoms. Ex.
Electrolysis
CH3CH3 + 2CO2 + H2 + 2KOH. 2CH3 – COOK + 2H2O
"manishkumarphysics.in"
12
CHEMISTRY (iv)
Hunsdiecker Reaction (Bromo-decarboxylation) : R–COOAg + Br2 R–Br + CO2 + AgBr Mechanism :
Step 1 :
R.COOAg + X2
+ AgX
Step 2 : Step 3 : Step 4 : Although bromine is the most often used halogen, chlorine and iodine have also been used. When iodine is the reagent, the ratio between the reactant is very important and determines the product A 1 : 1 ratio of salt to iodine gives alkyl halide, as above. A 2 : 1 ratio, however gives the ester RCOOR. This is called simonini reaction and sometimes used to prepare carboxylic ester.
5.
HVZ Reaction (Halogenation of aliphatic acids and Substituted acids) In the presence of a small amount of phosphorus, aliphatic carboxylic acids react smoothly with chlorine or bromine to yield a compound in which -hydrogen has been replaced by halogen. This is the Hell-VolhardZelinsky reaction. Because of its regioselectivity-only alpha halogenation-and the readiness with which it takes place, it is of considerable importance in synthesis. Cl , P
Cl , P
Cl , P
2 2 ClCH2COOH 2 Cl3CCOOH Cl2CHCOOH CH3COOH
The halogen of these halogenated acids ungergoes nucleophilic displacement and elimination much as it does in the simpler alkyl halides. Halogenation is therefore the first step in the conversion of a carboxylic acid into many important substituted carboxylic acids. + large excess of NH3 RCHCOOH | NH2 An ha log enated An a min o acid
RCHCOOH | Br acid
H RCHCOOH NaOH RCHCOONa RCHCOOH | | | Br OH OH An hydroxy acid H
RCH CHCOOH RCH2CHCOOH KOH (alc ) RCH = CHCOO¯ An , unsaturate d acid | Br
"manishkumarphysics.in"
13
CHEMISTRY Summary of reactions of carboxylic acids :
Carboxylic Acid Derivatives Closely related to the carboxylic acids and to each other are a number of chemical families known as functional derivatives of carboxylic acids : acid chlorides, anhydrides, amides, and esters, These derivatives are compounds in which the —OH of a carboxyl group has been replaced by —CI,—OOCR, — NH2, or —OR`.
"manishkumarphysics.in"
14
CHEMISTRY
Acid chloride
Anhydride
Amide
Ester
They all contain the acyl group, Like the acid to which it is related, an acid derivative may be aliphatic or aromatic, substituted or unsubstituted; whatever the structure of the rest of the molecule, the properties of the functional group remain essentilly the same. Characteristic reaction of acid deerivatives (Nucleophilic acyl substitution) : Nucleophilic acyl substitution usually takes place by an addition-elimination mechanism.The incoming nucleophile adds to the carbonyl to form a tetrasubstituted intermediate with a tetrahedral carbon.
The tetrahedral intermediate formed when a nucleophile attacks the carbonyl carbon of a carboxylic acid derivative is not stable and cannot be isolated. A pair of nonbonding electrons on the oxygen reforms the p bond, and either or is eliminated with its bonding electrons. Whether or is eliminated depends on their relative basicities. The weaker base is preferentially eliminated because the weaker the base, the better it is a leaving group.
Thus carboxylic acid derivative will undergo a nucleophilic acyl substitution reaction provided that the incoming nucleophile is a stronger base than the group that is to be replaced. If the incoming nucleophile and the group attached to acyl group in the starting material have similar basicities, the tetrahedral intermediate can expect either group with similar ease. A mixture of starting material and substitution product will result.
(i)
(ii)
"manishkumarphysics.in"
15
CHEMISTRY
(iii)
(iv)
(v)
Condition for acyl nucleophilic substitution reaction :
(i) L must be better leaving group than (ii)
, i.e., basicity of Nu should be more than that of
must be a strong enough nucleophilic to attack RCOL.
(iii) Carbonyl carbon must be enough electrophilic to react with
.
(A) Acid halides Methods of preparation of Acyl halides (i) RCOOH + PCl5 RCOCl + POCl3 + HCl (ii)
3RCOOH + PCl3 3RCOCl + H3PO3
(iii)
RCOOH + SOCl2 RCOCl + SO2 + HCl
Ex.
Distil 3CH3COONa + PCl3 3CH3 COCl Na 3PO 3
Pyridine
Acetyl chloride
Ex.
Distil 2C 6H5 COONa POCl3 2C 6H5 COCl NaCl NaPO 3 Sod. benzoate Benzoyl chloride
Chemical Reactions (1) Reaction with carboxylic acids Acyl chlorides react with carboxylic acids to yield acid anhydrides. When this reaction is used for preparative purposes, a weak organic base such as pyridine is normally added. Pyridine is a catalyst for the reaction and also acts as a base to neutralize the hydrogen chloride that is formed.
+
+
O CH3(CH2)5CCl + Heptanoyl chloride
"manishkumarphysics.in"
16
CHEMISTRY (2) Reaction with alcohols Acyl chlorides react with alcohols to form esters. The reaction is typically carried out in the presence of pyridine.
+
+
+
(3) Reaction with ammonia and amines
+
+
+
+
(4) Hydrolysis Acyl chlorides react with water to yield carboxylic acids. In base, the acid is converted to its carboxylate salt. The reaction has little preparative value because the acyl chloride is nearly always prepared from the carboxylic acid rather than vice versa.
+ H2O
+
water
+
H2O
+
water (5) Reaction of acid halide with organometallic (a) with Grignard reagent
(b) Reaction with Gilmann reagent
"manishkumarphysics.in"
17
CHEMISTRY (6) Reduetion of acid halides (a) Reduction by LiAIH4
(b) Reduction with H2 /Pd / BaSO4 (Rosenmund reduction)
Summary of reactions of acid halide
(B) Acid amides Methods of preparation of acids amides 1. By reaction of esters with ammonia and amines
Ex.
+
+
Ammonia is more nucleophilic than water, making it possible to carry out this reaction using aqueous ammonia.
Ex.
O || H2C = C – COCH3 | CH3 Methyl 2-methylpropenoate
+
O || H2C = C – CNH2 + | CH3 2-Methylpropenamide (75%)
Amines, which are substituted derivatives of ammonia, react similarly :
Ex.
+
"manishkumarphysics.in"
+
18
CHEMISTRY
Ex.
+
Ex.
+
Ex.
+
2.
3.
+
+
+
From acid halides RCOCl + 2NH3
From anhydride
(RCO)2O + 2NH3 4.
6.
RCONH2 + RCOONH4
From esters
RCOOR + NH3 5.
RCONH2 + NH4Cl
RCONH2 + ROH
From ammonium salt of carboxylic acid
RCOONH4
CH3 COONH 4 Amm.cyanides acetate From
R – C N + H2O
CH3C N H2O
7.
RCONH2 + H2O
CH3CONH2 Acetamide
Conc . HCl
or H2O 2 NaOH
Conc. H SO
4 2
R – CONH2
CH3 CONH2
+
Chemical Reactions 1.
Hoffmann rearrangement In the Hofmann rearrangement an unsubstituted amide is treated with sodium hydroxide and bromine to give a primary amine that has one carbon fewer than starting amide General reaction.
+ NaOH + Br2
R–N=C=O isocyanate
R – NH2
"manishkumarphysics.in"
19
CHEMISTRY Mech : OH
CO2R– NH
R – NH2 (2) Hydrolysis of amides
+
+
In acid, however, the amine is protonated, giving an ammonium ion, R2
+
:
+
In base the carboxylic acid is deprotonated, giving a carboxylate ion :
+
+
The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible. In both cases the amine product is protonated in acid ; the carboxylic acid is deprotonated in base.
Ex.
Ex.
H2O / H2SO 4
+
+
"manishkumarphysics.in"
20
CHEMISTRY Summary of reaction of amide:
Ex.
Ans.
CH3 CH2 C NH2 || O
(C) Esters Methods of Preparation (i)
H CH3COOC2H5 H2O CH3 COOH C 2H5 OH Acetic acid
H C6H5 COOCH3 H2O C6H5 COOH CH3OH Methyl benzoate
Pyridine
(ii) CH3COCl + C2H5OH CH3COOC2H5 + HCl Alcohols react with acyl chlorides by nucleophilic acyl substitution to yield esters. These reactions are typically performed in the presence of a weak base such as pyridine.
+
Ex.
Ex.
+
+
+
NaOH C6H5COCl + CH3CH2OH C6H5COOCH2CH3 + HCl
"manishkumarphysics.in"
21
CHEMISTRY Chemical Reactions 1.
Acid catalysed hydrolysis of ester (AAc2): Because H2O and R—OH have approximately the same basicity, it will be eqully easy for tetrahedral imtermediate I to collapse to reform the ester as it will be for tetrahedral intermediate II to collapse to form the carboxylic acid. Consequently, when the reaction has reached equilibrium, both ester and carboxylic acid will be obtained. CH3COOH + ROH Excess water will force the equilibrium to the right.
CH3COOH + ROH Mechanism:
2.
Base-Promoted Hydrolysis of Esters : Saponification (BAc2): Esters not only undergo acid hydrolysis, they also undergo base-promoted hydrolysis. Base-promoted hydrolysis is sometimes called saponification, from the Latin word sapo, soap. Refluxing an ester with aqueous sodium hydroxide, for example, produces an alcohol and the sodium salt of the acid :
The carboxylate ion is very unreactive toward nucleophilic substitution because it is negatively charged. Base-promoted hydrolysis of an ester, as a result, is an essentially irreversible reaction. The mechanism for base-promoted hydrolysis of an ester also involves a nucleophilic addition-elimination at the acyl carbon. Mechanism :
"manishkumarphysics.in"
22
CHEMISTRY Evidence for this mechanism comes from studies done with isotopically labeled esters. When ethyl propanoate labled with 18O in the ether-type oxygen of the ester(below) is subjected to hydrolysis with aqueous NaOH all of the 18O shows up in the ethanol that is produced. None of the 18O appears in the propanoate ion.
This labeling result is completely consistent with the mechanism given above . If the hydroxyide ion had attacked the alkyl carbon instead of the acyl carbon, the alcohol obtained would not have been labled. Attack at the alkyl carbon is almost never observed.
Although nucleophilic attack at the alkyl carbon seldom occurs with esters of carboxylic acids, it is the preferred mode of attack with esters of sulfonic acids (e.g. tosylates and mesylates)
Summary of reaction of esters :
(D) Acid anhydrides Methods of Preparation of acid anhydrides 1. From carboxylic acids P2O5 , CH3 COOH HOOCCH3 CH3 CO . O . CO . CH3 H2O Acetic acid Acetic anhydride
"manishkumarphysics.in"
23
CHEMISTRY
PO ,
2 5
Ex.
PO ,
2 5
Ex.
PO ,
2 5
Ex.
Ex.
+
PO ,
2 5
five or six membered cyclic anhydride are stable 2. From acid and acid halide Pyridine
CH3COOH + CH3COCl CH3CO.O.COCH3 + HCl Ex.
CH3COCl + CH3COONa CH3CO.O.COCH3 + NaCl
Chemical Reactions (1) Reaction with aromatic compounds (Friedel crafts acylation)
+ ArH
Ex.
+
+
+
(2) Reaction with alcohols Acid anhydrides react with alcohols to form esters. The reaction may be carried out in the presence of pyridine or it may be catalysed by acids. In the example shown, only one acyl group of acetic anhydride becomes incorporated into the ester ; the other becomes the acyl group of an acetic acid molecule.
"manishkumarphysics.in"
24
CHEMISTRY
+
+
Ex.
+
+ CH3COOH
(3) Reaction with ammonia and amines Acid anhydrides react with ammonia and amines to form amides. Two molar equivalents of amine are required. In the example shown, only one acyl group of acetic anhydride becomes incorporated into the amide and the other becomes the acyl group of the amine salt of acetic acid.
+
+
+
+ CH3COOH
(4) Hydrolysis Acid anhydrides react with water to yield two carboxylic acid functions. Cyclic anhydrides yield dicarboxylic acids.
+
+
+
7.
Heating Effects : (a) Heating effect on monocarboxylic acid 2R – COOH
"manishkumarphysics.in"
25
CHEMISTRY (b) Heating effect on dicarboxylic acid
CH3 – COOH (c) Heating effects on Hydroxy acids
(1) – Hydroxy acid
(2) – Hydroxy acid
(3) – Hydroxy acid Since 4 or 8 membered rings are less stable the refore -Hydroxy acids on heating produce unsaturated carboxylic acid.
(4) – Hydroxy acid
(d) Heating effects on esters
R` – COOH to R` – CH = CH2
Mech :
R` – CH = CH2 + R – COOH
This reaction follows syn elimination & hoffman product is formed.
"manishkumarphysics.in"
26
CHEMISTRY
MISCELLANEOUS SOLVED PROBLEMS (MSPS) 1.
Select the correct statement about the following compounds I, II, III.
Ans.
(A) (I) decarboxylates faster than (II) on heating. (B) Only *CO2 is eliminated on heating of compound (I). (C) Compound (I) eliminates a mixture of CO2 and *CO2 on heating. (D) The rate of decarboxylation of (II) is faster than (III). (A)
Sol.
No decarboxylation CO2 rate of decarboxylation : III > I > II
2.
final product is :
(A)
Ans.
CO2
(B)
(C)
(D)
(B)
Sol.
3.
final product is
(A)
Ans.
(B)
(C)
(D)
(B)
Sol.
"manishkumarphysics.in"
27
CHEMISTRY 4.
Identify (A), (B), (C) and (D). ( i ) CO [O ] Mg / dryether C3H5Cl (A) (C) C8H12(D) (B) 2 ( ii) H2 O / H
Saturated Ans.
(A) =
; (B) =
; (C) =
; (D) =
5.
Preparation of propanoic acid from ethyl alcohol follows :
Sol.
PCl 5 KCN H2 O / H CH3 – CH2OH CH3 – CH2 – Cl CH3 – CH2 – CN CH3 – CH2 – COOH
6.
Identify (A), (B) and (C).
KCN H2 O / OH C3H6Cl2 (A) (B) (C) 2-Methylpropanoic acid
CO2
Sol.
Cl CN COOH | | | KCN H2 O / OH CH3 – C – CH3 CH3 – C – CH3 CH3 – C – CH3 2-Methylpropanoic acid CO2 | | | Cl CN COOH
7.
Find the rate of soda-lime decarboxylation.
Sol.
Rate of soda-lime decarboxylation. I > II > III > IV > V
8.
Identify (A), (B) and (C).
Br2 (1 eqV ) / P KCN H2 O / H / CH3 – CH2 – COOH (A) (B) (C)
Sol.
(A) CH3 — CH — COOH ; | Br
9.
Write the structures of (A) C3H7NO which on acid hydrolysis gives acid (B) and amine (C). Acid (B) gives (+)ve silver–mirror test.
Ans.
A=
10.
Predict A , B , C , D and E.
(B) CH3 — CH — COOH ; | CN
(C) CH3—CH2—COOH
or
Mesitylene / AlCl3 Acid (A) B
Sol.
(A) = CH3COOH;
(B) = CH3 — C — O — C — CH3 ; || || O O
(C) =
(D) =
"manishkumarphysics.in"
28
CHEMISTRY 11.
Which of these represents correct reaction ? (A) (B)
conc .NaOD DCOO– + DCH2 OD NaOH C(CH2OH)4 + HCOO–
+ (excess)
P Br2
(C)
(D) Ans.
(A,B,C,D)
Sol.
(A) (B)
conc . H2SO 4
+
H2O
– conc .NaOD DCOO + DCH2 OD
(Cannizzaro reaction)
– NaOH C(CH2OH)4 + HCOO
+
(Aldol + Cannizzaro reaction)
(excess) P Br2
(C)
(D) 12.
(HVZ reaction)
conc . H2SO 4
+
(Esterification reaction)
H2O
Which are correct against property metioned ? (A) CH3COCl > (CH3CO)2O > CH3COOEt > CH3CONH2
(Rate of hydrolysis)
(B) CH3–CH2–COOH >
(Rate of esterification)
(C)
(D)
>
>
>
>
(Rate of esterification)
> Ph–CH2–COOH
(Rate of decarboxylation)
Ans.
(A,B)
13.
Match the product of column II with the reaction of column I. Column I Column II
Ans.
(A)
(p) ester with O18
(B)
(q) A -diketone with –18OH group
(C)
(D)
OH /
(r) A cyclic anhydride with –18OH group
(s) A cyclic ester without O18
(A) – r ; (B) – s ; (C) – p ; (D) – q.
"manishkumarphysics.in"
29
