Classification Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds Compounds Introduction and Procedure: A chemical test is a qualitative or quantitative or quantitative procedure designed to prove the existence of, or to quantify, a chemical a chemical test is a qualitative compound or chemical group with the aid of a specific reagent. specific reagent. The following tests were conducted to distinguish whether hydroxyl – – or carbonyl – – containing, to identify the type of alcohol it contains, to determine if aldehyde or ketone and to explain the mechanisms involved. Due to the limited availability of chemicals, only the following tests were conducted: 1.
Solubility of Alcohol in Water
Most organic molecules are relatively non-polar and are usually soluble in organic solvents (e.g. ( e.g. diethyl ether, dichloromethane, chloroform, petroleum ether, hexanes etc.) etc.) but not in polar solvents like water. However, some organic molecules are more polar and therefore soluble in water. This denotes a rather high ratio of polar group(s) to the non-polar hydrocarbon chain, i.e., i.e., a low molecular weight compound containing an -OH, -NH2 or -CO2H group, or a larger molecule containing several polar groups. Label 3 test tubes and add 10 1 0 drops of ethanol, secbutyl alcohol and terttert-butyl alcohol in each using a Pasteur pipette.
Add 1-mL of water dropwise to the test tube containing the alcohol, shaking the mixture throughly in each addition.
If cloudiness results, continue adding adding 0.25-mL water at each time with vigorous shaking until homogeneous dispersion results.
Note the total volume of water. If no cloudiness results after the addition of 2.0-mL water, the alcohol is soluble in water. Note down the results.
2.
Chromic Acid Test (Jones Oxidation)
The orange-yellow Jones reagent will immediately turn green in the presence of 1 and 2 alcohols and aldehydes. The
color change is typically observed only for these functional groups and this color change constitutes a “positive” result. Tertiary alcohols do not react with Jones reagent. Sometimes amines, ethers, ketones, alkenes and alkynes will give a positive test after 2-5 seconds due to contamination with alcohols.
Dissolve a drop of liquid sample in 1-mL acetone in a test tube.
3.
Add 5 drops of Jones reagent.
Perform test on n-butyl alcohol, -butyl alcohol and acetone. tert -butyl
2,4-dinitrophenylhydrazone (or 2,4-DNP) Test
2,4-Dinitrophenylhydrazine can be used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. A positive test is signaled by a yellow, orange or red precipitate. If the carbonyl compound is aromatic, then the precipitate will be red; if aliphatic, then the precipitate will have a more yellow color.
Place a drop of sample into a small test tube.
Add 5 drops of 95% ethanol and shake well.
Add 3 drops of 2,4-dinitrophenylhydrazine. If no yellow or orange-red precipitate forms, allow the solution to stand for at least 15 minutes.
Perform the test with acetone and acetaldehyde.
Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds Discussion of Results: The term hydroxyl group is used to describe the functional group –OH when it is a substituent in an organic compound . Organic molecules containing a hydroxyl group are known as alcohols, the simplest of which have the formula CnH2n+1 –OH. Hydroxyl groups are especially important in biological chemistry because of their tendency to form hydrogen
bonds,
both
as
a
donor
and
acceptor,
which
are
related
to
their
ability
to
increase hydrophilicity and water solubility. Alcohol
Structural Formula
mL H2O needed to produce homogeneous dispersion
Solubility in water
1-mL
Miscible
1-mL
Miscible
9-mL
Miscible
ethanol
n-butyl alcohol
tert -butyl alcohol
Two opposing solubility trends in alcohols are: the tendency of the polar -OH to promote solubility in water, and the tendency of the carbon chain to resist it. Thus, ethanol, and n-butyl alcohol are miscible in water because the hydroxyl group wins out over the short carbon chain while the miscibility of tert -butyl is decreased. As seen from the structure, n-butyl alcohol is a four carbon straight chain with the hydroxyl group at the end. The ratio of four carbons exposed to only one -OH group exposed limits the solubility of the straight chain. Carbons chains are more in line with the alkanes and are more miscible with oils than water. On the other hand the tert-butyl alcohol has the OH group right in the middle of the chain and tends to shield the carbons more and makes the -OH group more accessible to mixing with water and therefore more soluble. Alcohols with one to three carbons are miscible in any polar solvent. The miscibility of alcohols with more than four carbons starts to decrease, w hereas those with more than seven c arbons are considered immiscible. Structural Formula n-butyl alcohol
Chromic Acid Test Oxidized; green precipitate forms
tert-butyl alcohol
Not oxidized; precipitate forms
acetone
Not oxidized; precipitate forms
Alcohols are classified into primary (1°), secondary ( sec-, s- or 2°), and tertiary (tert-, t- or 3°), based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl functional group.
Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds While a carbonyl group is a functional group composed of a carbon atom doublebonded to an oxygen atom, C=O. The Jones reagent is chromium trioxide (CrO3) in sulfuric acid (H 2SO4). It is a potent oxidizing agent which rapidly oxidizes primary alcohols and aldehydes to carboxylic acids, and secondary alcohols to ketones. Alcohols react with the yellow-orange Jones reagent containing a Cr (VI) atom. Over the course of the organic oxidation, the Cr is reduced to Cr (III). The first two steps of the reaction mechanism help to explain why tert iary alcohols do not undergo oxidation with the Jones reagent. In ste p 2, water reacts with a proton of the chromate ester that is bonded to the carbon atom of the former alcohol functional group. Tertiary alcohols do not have a hydrogen atom bonded to this carbon; therefore the reaction could not proceed beyond this point. a.
Summarization of Jones Oxidation with primary and secondary alcohols
b. Jones Oxidation Mechanism on primary alcohols
Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds c.
Jones Oxidation Mechanism on secondary alcohols
3+
Thus, n-butyl alcohol, being a primary alcohol, is oxidized indicated by the formation of a Cr ion, having a color green appearance, whereas
tert -butyl
alcohol is not oxidized, being a tertiary alcohol. And acetone, as well, is not oxidized,
being typically used as the organic solvent. Structural Formula
2,4-DNP Test
Acetaldehyde
Positive; orange
Acetone
Positive; yellow
Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds
Carbonyl groups are common to several classes of organic compounds, as part of many larger functional groups, aldehydes (characterized by the presence of a carbonyl functional group at the end of a compound's carbon skeleton) and ketones (a type of organic compound where a carbonyl group bonds to two other carbon atoms of the carbon backbone) for example. 2,4-dinitrophenylhydrazine is often abbreviated to 2,4-DNP or 2,4-DNPH. A solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulfuric acid is known as Brady's reagent. R and R' can be any combination of hydrogen or hydrocarbon groups (such as alkyl groups). If at least one of them is hydrogen, then the original compound is an aldehyde. If both are hydrocarbon groups, then it is a ketone.
The reaction is known as a condensation reaction, one in which two molecules join together with the loss of a small molecule in the process. In this case, water. In terms of mechanisms, it is a nucleophilic addition-elimination reaction wherein 2,4dinitrophenylhydrazine is first added across the carbon-oxygen double bond (the addition stage) to give an intermediate compound and then lose a molecule of water (the elimination stage). Hence, acetaldehyde is an example of aldehyde, thus has a positive result. The same goes to acetone, being a ketone. Summary and Conclusion: There are different types of tests to classify unknown compounds. The simplest examples include solubility in water to determine the polarity of a compound; chromic acid test to distinguish primary and secondary alcohols by oxidation to produce aldehydes and ketones through the appearance of green chromium (III) ion; and 2,4dinitrophenylhydrazine test to differentiate aldehydes and ketones from other compounds by the presence of yellow to red precipitate. Thus, organic compounds can be identified and classified through similarities within its structure and properties shown by chemical tests. PRE-LAB ASSIGNMENT 1.
Differentiate the three types of alcohol. Give an example for each.
Alcohols are classified into primary (1°), secondary (sec-, s- or 2°), and tertiary (tert-, t- or 3°), based upon the number of carbon atoms connected to the c arbon atom that bears the hydroxyl functional group.
Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds Type
Primary
Definition
Example 1-propanol
Alcohols in which the carbon carrying the -OH group is only attached to one alkyl gro up
2-butanol Secondary
Alcohols in which the carbon carrying the -OH group is attached to two alkyl group which may be the same or different 2-methyl-2-propanol
Tertiary
2.
Alcohols in which the carbon carrying the -OH group is attached to three alkyl group which may be the combination of same or different
Differentiate aldehydes from ketones based on chemical structure. Definition
Aldehyde
A type of organic compound where a carbonyl functional group is at the end of the carbon skeleton
Ketone
A type of organic compound where a carbonyl functional group bonds two other carbon atoms of the carbon backbone
Structure
POST LAB ASSIGNMENT 1.
How do structure and number of car bons affect the solubility of alcohols in water?
The physical properties of alcohols are influenced by the hydrogen bonding ability of the -OH group. The longer the carbon chain in an alcohol is, the lower the solubility in polar solvents and the higher the solubility in nonpolar solvents. 2.
Based on the samples used, which alcohol is most readily oxidized? Why do you think so? Out of three samples tested, n-butyl alcohol is most readily oxidized because of being an example of a primary alcohol.
