Nucleophilic Acyl Substitution Reactions
High-Yield Summary
- All carboxylic acid derivative interconversions run through nucleophilic acyl substitution: nucleophile attacks carbonyl carbon → tetrahedral intermediate → intermediate collapses, ejecting a leaving group and restoring the carbonyl.
- Anhydride + ammonia → amide + carboxylic acid. Anhydride + alcohol → ester + carboxylic acid. Anhydride + water (hydrolysis) → two equivalents of carboxylic acid. Only the nucleophile changes; mechanism shape is identical across all three.
- Transesterification: an alcohol displaces an ester's original alkoxy group, forming a new ester + releasing the original alcohol — key to triglyceride metabolism and biodiesel production.
- Amide hydrolysis requires strongly acidic conditions (amides are the least reactive derivative) — acid protonates the carbonyl (raising electrophilicity) and helps convert nitrogen into a workable leaving group. Result: carboxylic acid + ammonia.
General Anhydride Nucleophilic Acyl Substitution Pattern
- 1Nucleophile attacks one of the anhydride's carbonyl carbons, pushing carbonyl electrons onto the carbonyl oxygen — forms a tetrahedral intermediate.
- 2Intermediate collapses: carbonyl double bond reforms, remaining carboxylate portion is ejected as a leaving group.
- 3Proton transfer neutralizes charges, giving the final neutral products (identity of nucleophile determines whether product is an amide, ester, or more carboxylic acid).
Amide Hydrolysis Under Acidic Conditions
- 1Protonation of the carbonyl oxygen — increases carbonyl carbon's electrophilicity.
- 2Water attacks the activated carbonyl carbon, pushing electrons onto oxygen — forms a tetrahedral intermediate (one O positive, N neutral).
- 3Proton transfers convert the poor-leaving-group nitrogen into a better one (effectively protonating it so it leaves as ammonia).
- 4Intermediate collapses: carbonyl double bond reforms as protonated nitrogen leaves, expelling ammonia.
- 5Final deprotonation of the carbonyl oxygen yields the neutral carboxylic acid product.
Anhydride Reactions by Nucleophile
| Nucleophile | Overall products |
|---|---|
| Ammonia (NH₃) | Amide + carboxylic acid |
| Alcohol (e.g., methanol) | Ester + carboxylic acid |
| Water (hydrolysis) | Two equivalents of carboxylic acid |
Key Terms
- Nucleophilic acyl substitution
- The core mechanism for all carboxylic acid derivative interconversions: nucleophilic attack on the carbonyl carbon, tetrahedral intermediate, collapse with leaving-group ejection.
- Tetrahedral intermediate
- The sp3 intermediate formed when a nucleophile adds to a carbonyl carbon, before the leaving group is ejected and the carbonyl reforms.
- Transesterification
- Conversion of one ester into another via reaction with an alcohol, which displaces the original alkoxy group.
Common MCAT Trap
- All three anhydride reactions (ammonia, alcohol, water) share the exact same mechanism shape — only the incoming nucleophile changes, which then determines whether the product is an amide, ester, or carboxylic acid. Don't memorize three separate mechanisms.
- Amide hydrolysis needs BOTH acid roles to work: protonating the carbonyl (step 1, raises electrophilicity) AND helping convert nitrogen into a leaving group (step 3) — omitting either role in an explanation misses why strong acid specifically is required, not just any catalyst.
- Transesterification releases the ORIGINAL alcohol as byproduct, not the incoming one — the incoming alcohol's group ends up installed in the new ester.
Quick Recall
What are the overall products when an anhydride reacts with water?
Why does amide hydrolysis require strongly acidic conditions while anhydride reactions don't?
What is transesterification and what byproduct does it release?