Hydrolysis

Hydrolysis is the cleavage of a carboxylic acid derivative by water, breaking the C-LG bond and giving the parent carboxylic acid + the leaving group. The rate depends on the derivative (reactivity ladder) and the conditions (acid vs. base catalysis).

Rate Order for Hydrolysis

Acyl halide >>> anhydride > ester >> amide

At physiological pH and room temperature:

• Acyl halides: hydrolyze in seconds to minutes (without catalysis).
• Anhydrides: hydrolyze in minutes to hours.
• Esters: hydrolyze in days to weeks without catalysis; minutes with acid/base catalysis at reflux.
• Amides: hydrolyze in hundreds of years without catalysis; hours with strong acid/base at reflux.

This rate hierarchy parallels the reactivity ladder exactly.

Base-Catalyzed Hydrolysis (Saponification for Esters)

Mechanism:

1. OH⁻ attacks the carbonyl C.
2. Tetrahedral alkoxide intermediate forms.
3. LG departs, reforming the C=O.
4. The product carboxylic acid is immediately deprotonated by another OH⁻, giving the stable carboxylate.

Step 4 is irreversible for ester hydrolysis (saponification). Once the carboxylate forms, it cannot re-attack the alcohol.

For amide hydrolysis under basic conditions, the rate is slow (amides resist), but eventually the same mechanism gives carboxylate + amine.

Acid-Catalyzed Hydrolysis

Mechanism:

1. Acid protonates carbonyl O, activating the electrophile.
2. Neutral water attacks the activated C.
3. Proton transfers shuffle: the water-derived OH gets deprotonated; the original LG (OR or NR₂) gets protonated to a better LG.
4. LG departs as neutral (ROH or R₂NH for ester/amide).
5. C=O reforms and gets deprotonated, giving carboxylic acid.

For Fischer ester hydrolysis, this mechanism is reversible (same mechanism, run in reverse, is Fischer esterification). Drive forward with excess water.

For amide hydrolysis under acid, water loss is very slow because amide N is a poor LG even after protonation.

Rate Differences Explained

Acyl halides hydrolyze fastest: excellent LG (Cl⁻), minimal resonance from Cl. Even uncatalyzed water attack is fast. This is why acyl halides must be kept dry.

Anhydrides hydrolyze moderately fast: good LG (RCOO⁻), weak resonance donation. Acetic anhydride reacts with moist air slowly but completely.

Esters hydrolyze slowly: poor LG (RO⁻), moderate resonance from OR. Needs acid or base catalysis to proceed at reasonable rates.

Amides hydrolyze extremely slowly: terrible LG (R₂N⁻ is a strong base), strong resonance from N. This is why proteins and peptides are stable.

Enzyme-Catalyzed Hydrolysis

Biological hydrolysis uses enzymes that can accelerate the reaction by factors of $10^{10}$ or more:

• Proteases (pepsin, trypsin, chymotrypsin) hydrolyze amide/peptide bonds. Usually at a serine active site (serine proteases) or a cysteine/aspartate active site (cysteine/aspartyl proteases). Neutral pH, 37°C.
• Esterases and lipases hydrolyze esters. Pancreatic lipase digests dietary fats at pH 7-8.
• Amidases hydrolyze amides. Slower, less common than proteases.

The enzymes’ power comes from: active site positioning that brings water and substrate together; general acid/base catalysis; stabilization of the tetrahedral intermediate via an “oxyanion hole”; and covalent catalysis (serine proteases form a covalent acyl-enzyme intermediate).