Question

Describe a test for carboxylic acids.

Answer 1

Answer:

Any of the following test can be carried out to detect the carboxylic functional group.

Litmus test

Sodium bicarbonate test (or) Sodium hydrogencarbonate test

Ester test

Fluorescein test

(a) Litmus Test:

The carboxylic acid turns blue litmus red. Because the hydroxyl group present in -COOH is far acidic than in alcohol.

The chemical reaction is given below.

R-COOH + H2O → R-COO– + H3O+

Note: If the colour of the blue litmus paper changes to red then carboxylic group is present. Phenol also gives this test.

(b) Sodium Bicarbonate Test:

When carboxylic acid reacts with sodium bicarbonate solution carbon dioxide is evolved with a brisk effervescence along with sodium acetate is formed.

The chemical reaction is given below.

RCOOH + NaHCO3 → RCOONa + H2O + CO2↑ (brisk effervescence)

Note: This test is used to distinguish between carboxylic acid from phenol. Phenol does not answer to this test.

(c) Ester Test:

Carboxylic acid reacts with alcohol in the presence of concentrated sulfuric acid forms a pleasant smelling ester. This reaction is known as esterification.

The chemical reaction is given below.

RCOOH + R-OH + H2SO4 → RCOO-R (Ester) + H2O

Note: Formation of a sweet smelling compound indicates the presence of carboxylic group in the given organic compound.

(d) Fluorescein Test:

This test is given by dicarboxylic acid. Dicarboxylic acid on heating gives acid anhydride. When this anhydride is treated with resorcinol in the presence of concentrated sulfuric acid a fluorescent

dye is formed and so this reaction is called fluorescein test.

The chemical reaction is given below.

Fluorescein Test

Answer 2

Explanation:

• You can identify carboxylic acids through ester test.
• Take a small amount of ethyl alcohol i.e., ethanol in a test tube, add 2-3 drops of concentrated sulphuric acid to it and warm it up.
• Then you will observe a vapour which has a sweet smell that confirms the presence of carboxylic acid.
• We get this sweet smelling vapour because some ester is formed due to the reaction between the conc. sulphuric acid and ethanol.
• You can also identify carboxylic acids by litmus test and sodium bicarbonate.