Carbonyl Reduction Lialh4

Carbonyl Reductions: LiAlH4 then H3O+ (Aldehydes/Ketones -> Alcohols)

Carbonyl Reductions: LiAlH4/H3O+ -> Alcohols


Lithium aluminum hydride (LiAlH4) is a powerful nucleophilic hydride donor. In dry ether solvents (Et2O, THF) it adds H- to carbonyl carbons, converting aldehydes to primary alcohols and ketones to secondary alcohols. The reaction stops at the aluminum-alkoxide stage until a separate acidic workup (H3O+) protonates the oxygen and precipitates aluminum hydroxide salts. Because LiAlH4 reacts violently with water or protic solvents, the reduction and quench steps must be kept strictly separate.

Safety: Handle LiAlH4 in a fume hood with dry glassware. Quench slowly (e.g., wet EtOAc -> H2O -> aqueous acid) to control heat and hydrogen evolution.

Quick Summary


  • Reagents/conditions: LiAlH4 (1-1.5 equiv) in dry Et2O or THF, 0 °C -> rt; then carefully add H3O+ to protonate the aluminum alkoxide.
  • Outcome: Aldehydes -> primary alcohols; ketones -> secondary alcohols. Each carbonyl consumes one hydride equivalent.
  • Selectivity vs NaBH₄: LiAlH4 is much stronger and will reduce esters, acids, amides, and nitriles (not covered here). Choose NaBH₄ when chemoselectivity in protic solvents is preferred.
  • alpha,beta-Unsaturated carbonyls: LiAlH4 predominantly gives 1,2-reduction (allylic alcohol). 1,4-addition is minor under standard conditions.
  • Stereochemistry: New stereocenters form racemic mixtures with achiral LiAlH4. Diastereoselection follows Felkin-Anh vs chelation control in hindered systems.

Mechanism — Hydride Transfer then Acidic Workup (3 Frames)


LiAlH4 transfers hydride to the carbonyl carbon while the C=O π bond moves onto oxygen.
**Step 1 — Hydride delivery:** AlH4- donates H- to the carbonyl carbon while O coordinates to Al, generating an aluminum-bound alkoxide.
The aluminum alkoxide remains bound in dry ether; no protonation yet.
**Step 2 — Aluminum-alkoxide complex:** In anhydrous solvent the alkoxide stays bound to Al; do not draw an -OH until after workup.
Acidic workup protonates the alkoxide to release the alcohol and generates Al(OH)3.
**Step 3 — Acidic workup:** H3O+ protonates the alkoxide to release the alcohol; Al species become insoluble Al(OH)3/salts.

Mechanistic checklist

  • Hydride transfer is polar, not radical; draw AlH4- delivering H- to C=O (no carbocation).
  • Keep the intermediate as an Al-O bond; show the alcohol only after H3O+ workup.
  • One hydride per carbonyl. Excess LiAlH4 is typically employed to guarantee complete reduction.
  • alpha,beta-Unsaturated substrates give allylic alcohols (1,2-addition). Note this in captions/explanations.
  • New stereocenters appear racemic with achiral LiAlH4.

Worked Examples


Benzaldehyde -> Benzyl alcohol

Benzaldehyde LiAlH4/H3O+ reagent Benzyl alcohol

Aldehydes reduce rapidly—benzyl alcohol forms cleanly after a standard aqueous workup.

Cyclohexanone -> Cyclohexanol

Cyclohexanone LiAlH4/H3O+ reagent Cyclohexanol

Ketones give secondary alcohols; no new stereocenter here, but asymmetric ketones produce racemic mixtures with achiral LiAlH4.

Cinnamaldehyde -> Cinnamyl alcohol (1,2-reduction)

Cinnamaldehyde LiAlH4/H3O+ reagent Cinnamyl alcohol

Enals/enones undergo 1,2-reduction to allylic alcohols under LiAlH4 conditions—note this common exam target.

Pinacolone (tert-butyl methyl ketone) -> tert-butyl ethanol analogue

Pinacolone LiAlH4/H3O+ reagent Pinacolol (racemic)

The new stereocenter forms racemically with achiral LAH; mention Felkin-Anh vs chelation control if chiral auxiliaries or Lewis acids are present.

Scope & Limitations


  • Ideal substrates: Aldehydes (aliphatic, benzylic) and ketones (acyclic, cyclic, conjugated) give high conversions.
  • Over-reduction risk: Esters, carboxylic acids, acid chlorides, amides, nitriles are also reduced by LiAlH4—avoid these functional groups if the goal is a selective aldehyde/ketone reduction.
  • Functional groups to watch: Benzylic halides or nitro groups can undergo side reduction; sulfur- and phosphorus-containing substrates may poison the reagent.
  • Solvent: Absolutely anhydrous Et2O/THF; protic solvents destroy LiAlH4 and can ignite.
  • Comparison to NaBH₄: NaBH₄ is milder, compatible with protic solvents, and often selective for aldehydes/ketones only.

Edge Cases & Exam Traps


  • Do not draw the alcohol before workup—the intermediate is an aluminum alkoxide.
  • Conjugate (1,4) additions are minor; highlight 1,2-reduction for enones/enals.
  • Over-reduction of esters/acids/amides is expected under LiAlH4 (unlike NaBH₄).
  • Workup order matters: add quench mediator (wet EtOAc), then water, then acid. Dumping water directly onto LiAlH4 is hazardous.
  • Oxidation-sensitive products (allylic alcohols) may require cool workup to suppress elimination.

Practical Tips


  • Dry glassware and inert atmosphere (N₂/Ar) prevent premature decomposition.
  • Add LiAlH4 slowly at 0 °C, then allow to warm to rt for completion.
  • For equilibrium-prone substrates, monitor by TLC/GC and quench only after consumption.
  • To label the hydroxyl proton, quench with D₂O instead of H2O.
  • Filter the Al(OH)3 slurry, then concentrate the organic layer; residual aluminum salts can be removed with filtration through celite.

Exam-Style Summary


LiAlH4 delivers H- to C=O, forming an aluminum alkoxide; H3O+ work furnishes the alcohol. Compare the milder NaBH4 route: Aldehyde/Ketone → Alcohol with NaBH4/MeOH. Aldehydes -> primary alcohols, ketones -> secondary alcohols, alpha,beta-unsaturated systems give 1,2-reduction. Keep the medium anhydrous until the final quench, and remember that LiAlH4 over-reduces many other carbonyl derivatives.

Interactive Toolbox


  • Mechanism Solver — Animate hydride transfer, the aluminum alkoxide, and the H3O+ workup.
  • Reaction Solver — Flag functional groups LiAlH4 will over-reduce and compare against NaBH₄.
  • IUPAC Namer — Confirm the primary/secondary alcohol names produced in this reduction.