Ester Hydrolysis Acid

Ester Reactions: Acid Hydrolysis (Ester → Carboxylic Acid)

Ester → Carboxylic Acid (H₃O⁺) | OrgoSolver

Carbonyl Reactions: Ester → Carboxylic Acid Using H₃O⁺

Acid-catalyzed hydrolysis is the reverse of Fischer esterification. Protonating the carbonyl oxygen increases electrophilicity; water adds to give a tetrahedral oxonium; rapid proton shuttles convert that intermediate into the form where ROH is protonated and primed to depart; collapse re-forms the carbonyl and expels ROH; deprotonation reveals the carboxylic acid and regenerates H₃O⁺. Because the process is reversible, excess water and/or removal of ROH (distillation, extraction) drive the equilibrium towards hydrolysis.


Key Emphasis (Teaching Pivots)

  • Reversibility / equilibrium. This is Fischer hydrolysis. Product ratios are equilibrium-controlled; use excess H₂O or remove ROH to favor RCO₂H formation.
  • Mechanistic spine. Protonate C=O → H₂O addition → proton-shuffled tetrahedral intermediates → collapse expelling ROH → deprotonate to regenerate H₃O⁺.
  • Rate logic. Acid makes the acyl carbon more electrophilic and turns OR′ into ROH (a better leaving group) via protonation.
  • Exception worth memorizing. tert-Alkyl esters can hydrolyze via alkyl–oxygen cleavage (AAL1/SN1) with R₃C⁺ formation, especially t-Bu esters used as acid-labile protecting groups.

Quick Summary

  • Reagents/conditions: H₃O⁺ (aq) catalytic; 0 °C → reflux depending on substrate; run with excess water or remove ROH to drive hydrolysis.
  • Outcome: R–C(=O)OR′ + H₂O ⇌ R–C(=O)OH + R′OH (acid catalyst regenerated).
  • Special case: t-Bu esters (and other tertiary/benzylic esters) may hydrolyze by AAL1/SN1 alkyl–oxygen cleavage, often very fast under strong acid.

Mechanism — Six Steps (Acid Addition–Elimination)

Step 1: Protonate the carbonyl to activate the acyl carbon.
Step 1 — Carbonyl oxygen removes H⁺ from H₃O⁺ (activated C=O).
Step 2: Water attacks the protonated carbonyl, generating a tetrahedral intermediate.
Step 2 — H₂O adds to the protonated carbonyl (tetrahedral oxonium).
Step 3: Proton transfers convert the addition OH to neutral and OR′ to ROH.
Step 3 — Proton relays: deprotonate the new OH, protonate OR′ → ROH (good leaving group).
Step 4: Lone pair on the acyl oxygen prepares to re-form C=O while ROH departs.
Step 4 — Collapse begins: the acyl oxygen attacks back into C=O while ROH is protonated and poised to leave.
Step 5: Collapse of the tetrahedral intermediate expels ROH.
Step 5 — Collapse completes: ROH leaves and the protonated carboxylic acid appears.
Step 6: Water removes the extra proton to regenerate H₃O⁺.
Step 6 — H₂O removes the extra proton to give the neutral carboxylic acid and regenerate H₃O⁺.

Step commentary:

  • Step 1. Oxonium formation (protonate C=O) is the “activation” step for both directions of Fischer chemistry.
  • Step 2. Water (not HO⁻) attacks the protonated carbonyl, producing a tetrahedral oxonium.
  • Step 3. Proton transfers interconvert the TIs so that ROH is the weaker base and therefore the leaving group.
  • Step 4. Lone pair on the acyl oxygen begins the collapse while ROH remains protonated.
  • Step 5. Collapse completes: ROH leaves and the protonated acid remains.
  • Step 6. Deprotonation of the acyl group gives RCO₂H and regenerates the acid catalyst.

Mechanistic Checklist (Exam Focus)

  • Reversible with Fischer esterification; cite Le Châtelier control (excess H₂O or removal of ROH).
  • Addition–elimination at the acyl carbon: always pass through the tetrahedral intermediate.
  • Leaving group logic: protonated ROH leaves more easily than H₂O in acid.
  • Acid’s role: increase electrophilicity and protonate OR′; no HO⁻ in the mechanism.
  • Exception: t-Bu (and other tertiary/benzylic) esters often hydrolyze via AAL1/SN1 at the alkyl–O bond.

Worked Examples

Ethyl acetate → acetic acid + ethanol

Classic Fischer hydrolysis: reflux in aqueous H₃O⁺, use water as solvent or distill ethanol to pull the equilibrium.

Example A reactant: ethyl acetate Reagent: H3O+ (aq), heat Example A products: acetic acid + ethanol

Methyl benzoate → benzoic acid + methanol

Aryl esters hydrolyze readily; warm aqueous acid and excess water are sufficient.

Example B reactant: methyl benzoate Reagent: H3O+ (aq) Example B products: benzoic acid + methanol

t-Butyl p-methoxybenzoate → p-anisic acid + t-BuOH (AAL1)

Strong acid, minimal water: protonate the alkoxy oxygen, form t-Bu⁺ (AAL1/SN1), capture with H₂O to give t-BuOH / isobutene while the acyl fragment becomes the free acid.

Example C reactant: t-Bu p-methoxybenzoate Reagent: strong acid Example C products: p-anisic acid + t-BuOH

Scope & Limitations

  • Works well: Aliphatic and aryl esters; lactones give hydroxy acids (ring opening). Transesterifications follow the same manifold but use ROH as the nucleophile.
  • Fast substrates: Aryl esters (better acyl activation) and tert-alkyl esters (AAL1) hydrolyze rapidly.
  • Sensitive groups: Acid-labile protecting groups (acetals, t-Boc) may fall off—plan the order of operations.
  • Equilibrium caveat: In ROH solvent the reaction can run backwards or transesterify.

Edge Cases & Exam Traps

  • Drawing RO⁻ as a leaving group in acid (should be ROH or ROH₂⁺).
  • Forgetting that hydrolysis is reversible; do not promise “quantitative” conversion without Le Châtelier reasoning.
  • Missing the tert-alkyl/benzylic (AAL1/SN1) cleavage path.
  • Confusing acidic hydrolysis with basic saponification (irreversible, yields carboxylate).

Practical Tips

  • Use excess water and heat; co-distill or extract ROH to drive hydrolysis.
  • Keep acid catalytic (typically 5–50 mol % strong mineral acid). Too much acid promotes dehydration or deprotection elsewhere.
  • For t-Bu esters, p-TsOH or TFA under anhydrous conditions cleanly removes the protecting group via AAL1.
  • After hydrolysis, neutralize and extract; adjust pH to isolate the free acid.

Exam-Style Summary

H₃O⁺ protonates the ester carbonyl → H₂O adds → proton transfers set up ROH as the leaving group → collapse re-forms C=O and expels ROH → deprotonation yields RCO₂H. The reaction is reversible with Fischer esterification; push toward acids with excess water or ROH removal. Remember the tert-alkyl AAL1 exception.


Interactive Toolbox

  • Mechanism Solver — Animate Steps 1–5 for acid hydrolysis and toggle the “tert-alkyl (AAL1)” branch to see alkyl–oxygen cleavage.
  • Reaction Solver — Compare acid hydrolysis with base saponification or esterification by adjusting H₂O/ROH equivalents and temperature.
  • IUPAC Namer — Confirm the systematic names of both the carboxylic acid and the liberated alcohol for reports or flashcards.