Nucleophilic acyl substitution is the major reaction pattern of carboxylic acid derivatives, including acid chlorides, anhydrides, esters, amides, and thioesters. This cheat sheet helps students compare mechanisms, predict products, and rank reactivity across related functional groups. It is especially useful in organic chemistry because many synthesis and biochemistry reactions depend on acyl transfer.
Key Facts
- The general mechanism is nucleophilic addition to followed by elimination of to restore the carbonyl.
- The tetrahedral intermediate has the general form before the leaving group is expelled.
- Carboxylic acid derivative reactivity usually follows .
- A better leaving group has a weaker basicity, so leaves more readily than or $NR_2^-.
- Acyl substitution is favored when the incoming nucleophile is stronger than the leaving group or when the leaving group is removed by acid-base reaction.
- Acid-catalyzed acyl substitution uses carbonyl protonation to make more electrophilic, then proton transfers convert a poor leaving group into a better one.
- Base-promoted ester hydrolysis is driven forward because the carboxylic acid product is deprotonated to .
- Amides are least reactive because resonance donation from nitrogen gives the bond partial double-bond character.
Vocabulary
- Nucleophilic acyl substitution
- A reaction in which a nucleophile replaces the leaving group on a carboxylic acid derivative through addition and elimination.
- Acyl group
- The carbonyl-containing group that is transferred or modified in acyl substitution reactions.
- Tetrahedral intermediate
- A temporary intermediate formed when a nucleophile adds to a planar carbonyl carbon, giving a tetrahedral carbon bearing .
- Leaving group
- The atom or group that departs from during the elimination step of acyl substitution.
- Acyl chloride
- A highly reactive carboxylic acid derivative with the structure .
- Transesterification
- An acyl substitution reaction in which one ester is converted into another ester by alcohol exchange.
Common Mistakes to Avoid
- Treating acyl substitution as simple carbonyl addition is wrong because carboxylic acid derivatives usually reform the by expelling a leaving group.
- Ignoring leaving group basicity is wrong because poor leaving groups such as usually cannot depart without activation or strong reaction conditions.
- Predicting an amide from an ester with a weak amine under mild conditions can be wrong because ester aminolysis may be slow and equilibrium-dependent.
- Forgetting proton transfers in acid-catalyzed mechanisms is wrong because neutral leaving groups such as or often require protonation before departure.
- Ranking carboxylates as highly reactive acylating agents is wrong because is resonance-stabilized and strongly deactivated toward nucleophilic attack.
Practice Questions
- 1 Rank , , , and from most reactive to least reactive toward nucleophilic acyl substitution.
- 2 Predict the major organic product when reacts with excess .
- 3 Write the tetrahedral intermediate formed when reacts with before chloride leaves.
- 4 Explain why amides are much less reactive than acid chlorides in nucleophilic acyl substitution.