Consider the following compounds :
(A) 
(B) 
(C) 
(D) 
Arrange these compounds in the increasing order of rate of hydrolysis :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) < (D) < (A) < (C)
Hydrolysis of an acyl chloride is a nucleophilic attack of water on the carbonyl carbon:
\( \displaystyle \ce{Ar-COCl + H2O -> Ar-COOH + HCl} \)
The more positive (electrophilic) the carbonyl carbon, the faster the attack. A substituent at the para position changes this: an electron withdrawing group increases the positive charge and the rate; an electron releasing group decreases them.
(B) 4-hydroxybenzoyl chloride: –OH releases electrons by resonance (+R), so it is the slowest: first.
(D) benzoyl chloride: no substituent: second.
(A) 4-formylbenzoyl chloride: –CHO withdraws electrons: third.
(C) 4-nitrobenzoyl chloride: –\( \ce{NO2} \) withdraws electrons most strongly, so it is the fastest: last.
Shortcut: The –OH compound (B) is the slowest and the –\( \ce{NO2} \) compound (C) the fastest. Options (1) and (2) start with (B); only Option (2) ends with (C).