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Consider the following amide compounds: (i) C6H5CH2CONH2 (phenylacetamide), (ii) 2-methylbenzamide, (iii) 3-methylbenzamide, (iv) 4-methylbenzamide. Which of the following statements correctly compares their rates of acid hydrolysis? The compound with the fastest rate is

1. Compound (i), because the CH2 group insulates the ring from electron withdrawal
2. Compound (iv), because the para-methyl group donates electrons most effectively to the carbonyl
3. Compound (iii), because meta-substitution has minimal steric and electronic effect
4. Compound (ii), because ortho-methyl provides steric acceleration

Correct answer: Compound (i), because the CH2 group insulates the ring from electron withdrawal

Solution

Acid hydrolysis of amides proceeds via nucleophilic addition of water to the carbonyl carbon. A more electrophilic carbonyl reacts faster. In C6H5CH2CONH2, the phenyl ring is separated from the carbonyl by a CH2 group, so there is no direct conjugation and the carbonyl remains more electrophilic. In benzamides (ii, iii, iv), direct conjugation of the ring with the carbonyl reduces its electrophilicity. Among benzamides, the para-methyl (iv) donates electrons most effectively (through conjugation), making the carbonyl least electrophilic. The meta-methyl (iii) has a smaller effect, and the ortho-methyl (ii) has steric and inductive effects. Overall, (i) hydrolyses fastest.

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