Question

Substituents on carboxylic acids have much effect on their acidity. Substantiate the statement with the following examples.

(a) HCOOH, CH₃COOH, CH₂CICOOH

(b) 

Answer 1

CH₂CICOOH > HCOOH > CH₃COOH

The methyl group will intensify the negative charge on the carboxylate ion and destabilise it as compared to formate ion. So HCOOH is stronger than CH₃COOH. The electron withdrawing effect of a Cl makes chloroacetic acid stronger than HCOOH and CH₃COOH.

In the case of aromatic carboxylic acids, presence of electron withdrawing groups at ortho and para position increases their acidity while presence of electron donating groups decreases their acidity.

In 4-nitro benzonic acid acid strength is greater than that of benzoic acid due to the electron withdrawing nature of -NO₂ group while in 4-methoxy benzoic acid acid strength decreases due to the electron donating nature of the methoxy group.