Selective Aldehyde Formation from Acid Chlorides with Bulky Aluminum Hydrides (LiAl(OR)₃H; H₂O workup)

Bulky aluminum hydrides such as lithium tri‑alkoxy aluminum hydride, LiAl(OR)₃H (classically LiAlH(O‑t‑Bu)₃H), selectively reduce acid chlorides (R′COCl) to aldehydes when the reagent is kept cold and strictly anhydrous. The reagent delivers one hydride by nucleophilic acyl substitution, ejecting chloride to form the aldehyde, and the sterically encumbered, electronically attenuated hydride does not add again. Aqueous workup (H₂O or mild acid) simply removes aluminum salts and releases the neutral aldehyde.

Exam contrast: LiAlH₄ reduces acid chlorides all the way to primary alcohols, whereas LiAl(OR)₃H stops at aldehydes. DIBAL‑H can also give aldehydes (acid chlorides or esters) at ≤ −78 °C, but it belongs to a different reagent family. RMgX delivers two additions to yield tertiary alcohols.

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Quick Summary

• Reagents/conditions: Acid chloride (or mixed anhydride), 1.0–1.2 eq LiAl(OR)₃H (e.g., LiAlH(O‑t‑Bu)₃H), dry Et₂O/THF, −78 → −20 °C, inert atmosphere; then cold H₂O (or dilute NH₄Cl) workup.
• Outcome: R′COCl → R′CHO, with chloride captured by aluminum and no over‑reduction under properly cold, controlled conditions.
• Mechanism: Hydride transfer → tetrahedral intermediate → collapse with chloride (or carboxylate) departure → aldehyde coordinated to aluminum; H₂O hydrolyzes the Al–O bonds.
• Stereochemistry: The new C–H bond forms at the carbonyl carbon; no new stereogenic center is created in this closed-shell addition–elimination sequence.
• Contrasts: LiAlH₄ reduces acid chlorides to primary alcohols; DIBAL‑H can reduce esters or acid chlorides to aldehydes at ≤ −78 °C; RMgX delivers two additions and gives tertiary alcohols.
• Common pitfalls: Warming above −20 °C, using excess reagent, or substituting LiAlH₄ all risk over-reduction; protic impurities quench the hydride instantly.

Mechanism — Three Frames (Hydride Add, Collapse, Product Release)

All frames come directly from the RDKit builder that powers Mechanism Solver, so the arrows, overlays, and seafoam highlights (Color 1) match the interactive experience.

Mechanistic Checklist (Exam Focus)

• Draw hydride attack → tetrahedral intermediate → collapse that ejects Cl⁻ (or a carboxylate if a mixed anhydride is used) → aldehyde.
• Emphasize that no second hydride is transferred under cold, hindered conditions—one hydride per aluminum center.
• Show the aqueous workup that removes Li/Al salts; the product is already an aldehyde, not an alkoxide.
• Contrast clearly with LiAlH₄ (over‑reduces to primary alcohols) and DIBAL‑H (can reduce esters to aldehydes at ≤ −78 °C).

Worked Examples

Reactant

Reagent

Variant shown

LiAlH(O‑t‑Bu)₃ (bulky, classic choice)

Product

Benzaldehyde forms cleanly once chloride leaves; no new carbon fragment is added.

Reactant

Reagent

Variant shown

LiAlH(O‑i‑Pr)₃ (iso-propoxy, slightly less bulky)

Product

Hexanal appears after workup; the carbon skeleton is unchanged.

Reactant

Reagent

Variant shown

LiAlH(OEt)₃ (electronically softer; still stops at aldehyde)

Products

Benzaldehyde plus acetate—only one acyl fragment accepts the hydride, leaving the partner as acetate.

The carbon backbone stays identical from reactant to product; LiAl(OR)₃H simply replaces the chloride with hydrogen to reveal the aldehyde.

Scope & Limitations

• Electrophiles: Acid chlorides respond best; mixed anhydrides also reduce cleanly to aldehydes plus carboxylate. Esters and amides generally do not stop at the aldehyde with LiAl(OR)₃H (use DIBAL‑H for esters).
• Functional groups: Protect or avoid protic sites (–OH, –NH, –CO₂H); they instantly quench the hydride.
• Temperature: Keep ≤ −20 °C (ideally −78 → −50 °C) during addition so the aldehyde is not further reduced.
• Stoichiometry: 1.0–1.2 equiv is typical; excess hydride or prolonged times increase the chance of undesired reduction.
• Solvent: Dry ether or THF under inert gas; water or alcohol contamination destroys the reagent.

Practical Tips & Pitfalls

• Add the acid chloride solution slowly to the cold LiAl(OR)₃H suspension to control the exotherm and keep hydride delivery selective.
• Flame-dry glassware, keep solvent rigorously dry, and maintain an inert atmosphere; even trace moisture or oxygen quenches the reagent.
• Quench cautiously at ≤0 °C with H₂O or cold NH₄Cl(aq); vigorous quenching causes runaway heat and can hydrate sensitive aldehydes.
• Do not substitute LiAlH₄ in this setup—its more reactive hydrides push straight to the primary alcohol.
• Need aldehydes from esters? Switch to DIBAL‑H at −78 °C instead of LiAl(OR)₃H.

Exam-Style Summary

R′COCl —(LiAl(OR)₃H, cold)→ tetrahedral aluminate —collapse→ R′CHO —(H₂O)→ R′CHO (isolated). One hydride per aluminum, weak donor ability, and steric hindrance are the reasons the reaction stops at the aldehyde.

Pitfalls to remember:

• Predicting a primary alcohol is wrong here—LiAl(OR)₃H stops at the aldehyde; LiAlH₄ is the alcohol reagent.
• Allowing the mixture to warm or dumping in excess hydride erodes selectivity and can over-reduce the aldehyde.
• Forgetting to remove protic groups (or using wet solvent) just destroys the reagent and yields no reduction.

FAQ

Why doesn’t LiAl(OR)₃H reduce the aldehyde again like LiAlH₄?
The bulky OR ligands and single hydride on aluminum make the reagent both sterically hindered and electronically weak. At −78 → −20 °C, the aldehyde lacks a leaving group and is not electrophilic enough for another addition, so the reaction stops after chloride expulsion.

How do I stop at a ketone instead of an aldehyde?
Use a cuprate (Gilman reagent) to add an R₂CuLi fragment once, or use the Rosenmund hydrogenation. LiAl(OR)₃H always delivers a hydride, so it cannot give ketones from acid chlorides.

What happens if moisture sneaks in?
Water (or protic additives) instantly quenches the reagent to give Al–OH species before hydride transfer occurs. Keep glassware dry and add the acid chloride into the cold hydride suspension to avoid wasting the reagent.

Interactive Toolbox

Use these tools to double-check your setup:

• Mechanism Solver — replay the four LiAl(OR)₃H frames (hydride attack, collapse, cold stop, workup) with the same seafoam overlays shown above.
• Reaction Solver — compare LiAl(OR)₃H vs LiAlH₄ vs DIBAL‑H to see how the product changes (aldehyde vs primary alcohol).
• IUPAC Namer — confirm aldehyde product names (benzaldehyde, hexanal, etc.) without exposing SMILES to learners.

Related Reading

• Acid Chloride → Primary Alcohol with LiAlH₄ — see why the classic hydride reagent over-reduces.
• Acid Chloride → Ketone with Gilman Reagents — another way to stop after a single carbon–carbon bond formation.
• Ester → Aldehyde with DIBAL‑H — partial reduction of esters under even colder conditions.