Question

The correct increasing order of the acid strength of benzoic acid $(I), 4$ -nitrobenzoic acid $(II), 3,4$ -dinitrobenzoic acid(III) and $4$-methoxybenzoic acid $(IV)$ is

• A. I < II < III < IV
• B. II < I < IV < III
• C. IV < I < II < III
• D. IV < II < I < III

Answer 1

Answer: (C)

IV < I < II < III

Answer 2

Acidity of carboxylic acid (or benzoic acid) depends upon the stability of benzoate ion formed. More the number of electron withdrawing groups attached to benzene nucleus, more is the dispersal of negative charge and hence, more is stability of carboxylate ion which in turn increases the acidity of the respective carboxylic (or benzoic) acid.

Presence of electron releasing substituent like $-OR$ group however decreases the acidity by decreasing the stability of generated benzoate ion.

Thus, the correct increasing order of acidity of given compounds is $IV < I < II < III$.