Which statement ranks reactivity correctly from most to least reactive for nucleophilic acyl substitution?

• A. Acid chloride > Anhydride > Ester > Amide
• B. Amide > Ester > Anhydride > Acid chloride
• C. Ester > Amide > Anhydride > Acid chloride
• D. Anhydride > Acid chloride > Ester > Amide

Answer: (A)

In nucleophilic acyl substitution, how easily the leaving group departs and how electrophilic the carbonyl is mainly control the rate. The best leaving group and the most activated carbonyl give the fastest reaction.

Acid chlorides are at the top because chloride leaves very readily as Cl−, a very stable conjugate base, so the departure step is highly favorable. The carbonyl in an acid chloride is also strongly electron-poor, thanks to the highly electronegative chlorine, making it very reactive toward nucleophiles.

Acid anhydrides follow because the leaving group is carboxylate (RCO2−), which is resonance-stabilized and thus a good leaving group, and the carbonyls are still quite activated by adjacent acyl groups. However, chloride is a better leaving group than carboxylate, so reactivity is slightly lower than for acid chlorides.

Esters come next. The leaving group is alkoxide (RO−), which is a stronger base and less stable, so it leaves less readily. Also, the alkoxy substituent can donate electron density into the carbonyl by resonance, reducing the electrophilicity of the carbonyl carbon and slowing substitution.

Amides are the least reactive because the nitrogen can donate electron density to the carbonyl via resonance very effectively, greatly stabilizing the carbonyl and making the leaving group (amido) poorly leaving. This resonance delocalization dampens the carbonyl’s susceptibility to nucleophilic attack and lowers the overall reactivity.

Thus, the order from most to least reactive in nucleophilic acyl substitution is: acid chloride > anhydride > ester > amide.