Question

Consider the acidity of the carboxylic acids
(i) $ PhCOOH $
(ii) $ o $ - $ NO_2C_6H_4COOH $
(iii) $ p $ - $ NO_2C_6H_4COOH $
(iv) $ m $ - $ NO_2C_6H_4COOH $
Which of the following order is correct ?

• A. (i) $ > $ (ii) $ > $ (iii) $ > $ (iv)
• B. (ii) $ > $ (iv) $ > $ (iii) $ > $ (i)
• C. (ii) $ > $ (iv) $ > $ (iii) $ > $ (v)
• D. (ii) $ > $ (iii) $ > $ (iv) $ > $ (i)

Answer 1

Answer: (D)

$—NO_2$ group at any position shows electron withdrawing effect, thus acid strength is increased. But $o$-nitro benzoate ion is stabilised by intramolecular $H$-bonding like forces, hence its acid strength is maximum.
Thus, the order of acid strength is $(II) > (III) > (IV) > (I)$.