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For the substitution reaction R–C(=O)–X + Nu− → R–C(=O)–Nu + X−, the rate is highest when the leaving group X is:

1. –OCOR
2. –OC2H5
3. –NH2
4. –Cl

Correct answer: –Cl

Solution

Reactivity of acyl derivatives toward nucleophilic acyl substitution follows leaving-group ability: acid chloride > anhydride > ester > amide. -Cl is the best leaving group, so X = -Cl gives the highest rate.

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