Tests for the Alcoholic Group [R-OH]

Alcohols

Compounds in which the hydroxyl group (—OH) is linked to aliphatic carbon chain or in the side chain of an organic compound are called alcohols. The alcohols containing one, two or three hydroxyl groups per molecule are called mono, di or trihydric alcohols respectively.
Alcohols are further classified as primary (1°), secondary (2°) and tertiary (3°) according as the —OH group is attached to primary, secondary and tertiary carbon atoms respectively.

Tests for the Alcoholic [R-OH] Group

The alcoholic group can be detected by any of the following tests:
1. Sodium metal test.
2. Ester test.
3. Ceric ammonium nitrate test.
4. Acetyl chloride test.
5. Iodoform test for alcohols containing the

Materials Required

1. Sodium metal
2. Acetic acid
3. Ceric ammonium nitrate
4. Acetyl chloride
5. Ammonium hydroxide
6. Iodine
7. Sodium hydroxide
8. Calcium sulfate
9. Lucas reagent (zinc chloride and Con.HCl)
10. Test tubes
11. Test tube holder
12. Filler

Preparation of Reagents:

1. Ceric Ammonium Nitrate: Take 20gm of orange crystals of ceric ammonium nitrate in 200ml of warm dilute nitric acid.
2. Lucas Reagent: Take 135gm of anhydrous zinc chloride and dissolved in 100ml of concentrated hydrochloric acid.
3. Iodine Solution: Dissolve 5gm of potassium iodide in 40ml of water. To that solution add 1gm of solid iodine and dissolve it completely.
4. 5% sodium hydroxide solution. Dissolve 5 g of solid sodium hydroxide in about 50 ml water and then make up the volume to 100 ml.

Sodium metal test

This test is based on the appearance of effervescence due to liberation of hydrogen gas when the alcohol is reacted with active metals like sodium.

The alcohol to be tested should be dry because water also reacts with sodium. Sodium should be handled carefully, unreacted sodium should be destroyed by adding excess alcohol. This test is favourable if phenyl or carboxyl groups are absent.

Procedure

1. Take the organic compound to be tested in a dry test tube.
2. Add 1gm of anhydrous calcium sulfate and shake well to remove excess water.
3. Decant the solution to another clean test tube.
4. Add a small piece of sodium metal.

Results: If brisk effervescence appears due to the evolution of hydrogen gas indicate the presence of alcoholic group.

Ester test

Alcohols react with carboxylic acids to form sweet smelling esters. The reaction between an alcoholic group and a carboxylic acid group is called esterification. This is a slow and reversible reaction and is catalysed by an acid such as cone, sulphuric acid.

The chemical reaction is given below:

Procedure

1. Take 1ml of the organic liquid to be tested in a clean dry test tube.
2. Add 1ml of glacial acetic acid and 2-3 drops of conc. sulfuric acid.
3. Heat the mixture in a water bath for 10 minutes.
4. The hot mixture is poured into a beaker containing cold water.
5. Smell the water in the beaker.
6. A fruity smell confirms the presence of alcoholic group.

Ceric ammonium nitrate test

Alcohol or reaction with ceric ammonium nitrate forms a pink or red colour precipitate due to the formation of a complex compound and ammonium nitrate.

Here is the chemical reaction

Procedure

1. Take 1ml of given compound in a dry test tube.
2. Add a few drops of ceric ammonium nitrate reagent and shake the solution well.
3. Observe the solution.
4. If red precipitate appears then the presence of alcoholic group is conformed.

Results: The appearance of wine red colour precipitate shows the presence of alcoholic group.

Acetyl chloride test

Alcohol reacts with acetyl chloride results in the formation of ester and hydrogen chloride. The resulting hydrogen chloride on contact with ammonium hydroxide forms a white fumes of ammonium chloride and water.

Procedure

1. Take 2ml of given organic compound in a clean test tube
2. Add 1gm of anhydrous calcium sulfate and shake well.
3. Filter the solution. To the filtrate add 3 to 4 drops of acetyl chloride and shake well.
4. Take a glass rod dipped in ammonium hydroxide solution.
5. Bring the glass rod near the mouth of the test tube.

Results: The formation of white fumes indicates the presence of alcohol.

 Iodoform test

This test is given by secondary alcohols, ketones and acetaldehyde. First the compound is heated with sodium hydroxide solution and iodine. A formation of yellow precipitate of iodoform shows the presence of alcohol.

Procedure

1. Take 1ml of given organic compound in a clean dry test tube.
2. Add 1ml of 1% iodine solution to it.
3. Add dilute sodium hydroxide solution drop wise until the brown colour of iodine is discharged.
4. Heat the mixture gently in water bath.

Results: The formation of yellow precipitate shows the presence of alcohol, acetaldehyde or methyl ketones.

Distinction Between Primary , Secondary and Tertiary Alcohols

Lucas Test

This test is based upon relative reactivities of various alcohols towards HCl in the presence of ZnCl₂. In this test, the alcohol is treated with Lucas reagent which is equimolar mixture of HCl and ZnCl₂. The reagent It reacts with primary, secondary and tertiary alcohols at different rates. This reagent forms a cloudiness on reacting with alcohols. Tertiary alcohols reacts immediately and give cloudiness, secondary alcohols reacts slowly and gives cloudiness after 5 to 10 minutes and there is no reaction with primary alcohols.

Procedure

• Take about 1 ml of dry alcohol in a clean dry test-tube and add 8-10 drops of Lucas reagent.
• Shake the mixture well.
• Note down the appearance of cloudiness.

Result Interpretation

1. If cloudiness appears immediately, tertiary alcohol is indicated.
2. If cloudiness appears within five minutes, secondary alcohol is indicated.
3. If cloudiness appears only upon heating, primary alcohol is indicated.

Summary Of Tests For Alcoholic Groups