Reduction To Alcohol

Acid Chloride Reactions: Reduction of Acid Chlorides using LiAlH4 to form Alcohols

Acid Chlorides → Primary Alcohols with LiAlH₄, then H₃O⁺ | OrgoSolver

Acid Chlorides → Primary Alcohols with LiAlH₄, then H₃O⁺

Lithium aluminum hydride (LiAlH₄) reduces acid chlorides (RCOCl) rapidly and completely to primary alcohols. Two hydride equivalents add across the acyl unit: the first creates a tetrahedral intermediate that collapses to an aldehyde, the second immediately reduces that aldehyde to an alkoxide. Aqueous workup (H₃O⁺) protonates the alkoxide to give the neutral alcohol.

Teaching pivots: Emphasize two hydride additions (acid chloride → aldehyde → alkoxide → alcohol), chloride expulsion, and careful staged quench. LiAlH₄ will not stop at the aldehyde; use selective hydrides if that’s the goal.

Quick Summary

Reagents/Conditions	Outcome	Notes
LiAlH₄ (Et₂O/THF, strictly anhydrous, 0 °C → rt), then H₃O⁺ quench	RCOCl → RCH₂OH (primary alcohol)	Two hydride transfers; aldehyde not isolable; violent with water—quench cold and slowly.

Mechanism — 5 Steps (Closed-Shell Nucleophilic Acyl Substitution)

Hydride addition to the acyl carbon: AlH₄⁻ delivers H⁻ (explicit H–Al bond) to the carbonyl carbon; π electrons shift to oxygen → tetrahedral alkoxide with chloride still attached.
Collapse; chloride leaves to give an aldehyde: The O⁻ re-forms C=O, expelling Cl⁻ (captured by aluminum); hydride remains on the carbon as the aldehyde forms.
Second hydride addition to the aldehyde: Another AlH₄⁻ adds H⁻ to the aldehyde carbonyl, forming a primary alkoxide coordinated to aluminum.
Protonation of the alkoxide: Hydronium transfers H⁺ to the alkoxide oxygen; the H–O bond in H₃O⁺ collapses back to water.
Product frame: Primary alcohol (RCH₂OH) after workup; aluminum salts precipitate as hydroxides/oxides.

Mechanistic Checklist (Exam Focus)

Draw two hydride additions: acid chloride → aldehyde (in situ) → alkoxide → alcohol (workup).
Show a tetrahedral intermediate and chloride expulsion; no radicals or rearrangements.
LiAlH₄ does not stop at the aldehyde; use LiAlH(O‑tBu)₃ or Rosenmund if you need one.
Acid or water before completion quenches LiAlH₄ and stalls the reduction.
No stereocenter survives at the acyl carbon—product carbon becomes CH₂.

Worked Examples

Scope & Limitations

Works: Aliphatic and aromatic acid chlorides; rapid and high-yielding under dry ether/THF.
Chemoselectivity: LiAlH₄ also reduces esters and anhydrides to alcohols and amides/nitriles to amines—avoid sensitive groups or protect them.
Aldehyde access: Choose LiAlH(O‑tBu)₃ at low temperature or Rosenmund hydrogenation; LiAlH₄ is too strong to stop early.
Stoichiometry: ≥2 hydride equivalents per acyl group; practical excess is common.
Safety: Pyrophoric; violent with water. Quench cold, slowly, and in stages.

Practical Tips (Lab)

Dry glassware and inert atmosphere; charge LiAlH₄ first, then add the acid chloride solution slowly at 0 °C.
Control exotherm and gas evolution; never add LiAlH₄ to water.
Stage the quench: cautious water addition → mild base (e.g., NaOH) to break Al complexes → full acidic workup (H₃O⁺) to release the alcohol.
Use ≥2 eq hydride per acyl unit; more if other reducible groups are present.

Exam-Style Summary

RCOCl —(LiAlH₄, dry ether/THF)→ [tetrahedral] → aldehyde (in situ) —(LiAlH₄)→ RCH₂O⁻ —(H₃O⁺)→ RCH₂OH. Two hydride transfers; chloride leaves in the collapse; aldehyde is not isolated.

FAQ

Does LiAlH₄ stop at an aldehyde?
No. It reduces acid chlorides past the aldehyde to the primary alcohol. Use LiAlH(O‑tBu)₃ or Rosenmund to isolate an aldehyde.

How many hydride equivalents are needed?
At least two per acyl group; use excess to account for side consumption.

Which solvents are typical?
Dry diethyl ether or THF under N₂/Ar.

How should I quench LiAlH₄ safely?
Cold, dropwise water, then mild base to dismantle Al complexes, then acidic workup (H₃O⁺).

Interactive Toolbox

Mechanism Solver — animate both hydride additions, chloride expulsion, and the acidic workup.
Reaction Solver — predict primary alcohols from acid chlorides; flag co-present esters/amides as over-reduction risks.
IUPAC Namer — caption products such as benzyl alcohol or 1-octanol without exposing SMILES.

Study Tools