Question

List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed.

Answer 1

Carboxylic acid can be distinguished from an alcohol by performing the following tests:-

i. Test with NaHCO₃ solution in water.

On adding carboxylic acid to baking soda, carbon dioxide is liberated with brisk effervescence.

On adding a solution of baking soda to alcohol, no brisk effervescence occurs.

ii. Test with blue litmus solution.

Carboxylic acid turns blue litmus red.

There is no change in colour when a blue litmus solution is added to alcohol.

Answer 2

Two tests for experimentally distinguishing between an alcohol and a carboxylic acid are as follows:-

a) Baeyer's reagent test: When few drops of 5% alkaline potassium permanganate solution are added to few ​millilitres of an alcohol contained in a test tube kept in a hot water bath, it is observed that the pink colour of potassium permanganate disappears. This happens due to oxidation of alcohol to carboxylic acid. On the other hand, potassium permanganate solution does not get decolourised by the effect of a carboxylic acid.

Carboxylic acid + Alk. Potassium permanganate solution → No colour change
             Alcohol + Alk. Potassium permanganate solution → Pink colour disappears

b) Sodium hydrogen carbonate test: When a solution of baking soda is treated with a few drops of a carboxylic acid, brisk effervescence is observed due to the evolution of carbon dioxide gas. When this gas is passed in limewater, the latter turns milky, whereas no effervescence occurs upon treating an alcohol with baking soda solution.

Carboxylic acid + Baking soda → Brisk effervescence (Due to formation of CO₂ gas)
             Alcohol + Baking soda → No reaction