question_answer

Consider the acidity of the carboxylic adds (i) \[PhCOOH\] (ii) \[o-N{{O}_{2}}{{C}_{6}}{{H}_{4}}COOH\] (iii) \[p-N{{O}_{2}}{{C}_{6}}{{H}_{4}}COOH\] (iv) \[mN{{O}_{2}}{{C}_{6}}{{H}_{4}}COOH\] Which of the following orders is correct?

A)  (i) > (ii) > (iii) > (iv)

B)  (ii) > (iv) > (iii) > (i)

C)  (ii) > (iv) > (i) > (iii)

D)  (ii) > (iii) > (iv) > (i)

Correct Answer: D

Solution :

  (i) (ii) (iii) (iv) \[-\text{N}{{\text{O}}_{\text{2}}}\]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 (i) > (ii) > (iv) > (i).