Reduction to Alcohols

Carboxylic acids can be reduced to primary alcohols, but only with LiAlH₄ (lithium aluminum hydride). NaBH₄ is not strong enough, and catalytic hydrogenation (H₂/Pd) does not touch the COOH group. The net transformation:

R-COOH → R-CH₂OH (primary alcohol).

This reduction takes a carbon from oxidation state +3 (carboxylic acid) to oxidation state −1 (primary alcohol), a 4-electron reduction.

Why LiAlH₄ Works and NaBH₄ Does Not

NaBH₄ is mild enough to reduce only highly electrophilic carbonyls (aldehydes, ketones). Carboxylic acids have a problem: their acidic -OH proton (pKa 4-5) would deprotonate the borohydride or react with any basic reducing agent, effectively neutralizing it. More importantly, the neutral COOH is less electrophilic than an aldehyde or ketone because one of the oxygens has a lone pair that can donate into the carbonyl.

LiAlH₄ is strong enough to overcome both issues:

1. LiAlH₄ first acts as a strong base to deprotonate the -COOH, converting it to the lithium carboxylate R-COO⁻ Li⁺ (with H₂ gas released).
2. Then LiAlH₄ attacks the carboxylate’s carbonyl carbon, reducing it via multiple hydride transfers, through aldehyde-like intermediates, down to the primary alcohol alkoxide.
3. Aqueous workup protonates the alkoxide to give the alcohol.

Net stoichiometry: one COOH needs two to four equivalents of hydride (depending on mechanism specifics) plus a proton from workup.

Mechanism Sketch

Greatly simplified:

1. LiAlH₄ + R-COOH → R-COO⁻ Li⁺ + AlH₃ + H₂↑ (deprotonation).
2. AlH₃ + R-COO⁻ → R-CHO (aldehyde) + AlH₂(OR’… coordination shifts).
3. AlH₂ + R-CHO → R-CH₂OAlH (hydride addition to the aldehyde intermediate).
4. Aqueous workup → R-CH₂OH.

The aldehyde intermediate is not isolated because LiAlH₄ is aggressive enough to reduce it faster than it can be isolated.

LiAlH₄ Reduces Many Functional Groups

The strong reducing power of LiAlH₄ is not selective:

• Carboxylic acids → primary alcohols.
• Esters → primary alcohols (+ alcohol byproduct from the OR group).
• Amides → amines.
• Nitriles → primary amines.
• Ketones → secondary alcohols.
• Aldehydes → primary alcohols.
• Epoxides → alcohols.

If a substrate has multiple reducible groups, LiAlH₄ reduces all of them. To reduce only a ketone selectively (not a carboxylic acid or ester), use NaBH₄ instead.

DIBAL-H: Stopping at the Aldehyde

DIBAL-H (diisobutylaluminum hydride, iBu₂AlH) is a “single-hydride” reducing agent. At low temperature (-78°C) and with carefully controlled stoichiometry (one equivalent), DIBAL-H can reduce an ester to an aldehyde (stopping at the aldehyde stage) because only one hydride is delivered before the intermediate escapes the Al coordination sphere.

The analogous reaction on carboxylic acids (1 equiv DIBAL-H) gives the aldehyde as well, but this requires precise conditions and is rarely a clean MCAT question.

Borane (BH₃) or B₂H₆: An Alternative

Borane (BH₃) in THF reduces carboxylic acids (but not esters, amides, or nitriles, selectively) to primary alcohols. Unlike LiAlH₄, borane is SELECTIVE for carboxylic acids over esters, making it useful when both are present.

So if you need to reduce ONLY a carboxylic acid in a molecule that also has an ester, use BH₃/THF (or BH₃·Me₂S). NaBH₄ would not touch either. LiAlH₄ would reduce both.

Biological Reduction

Cells do not reduce carboxylic acids back to alcohols directly. The typical metabolic sequence reverses the oxidation pathway: acetate is activated as acetyl-CoA (a thioester) before any reduction. The thioester is a much better electrophile than the free carboxylic acid, and enzyme systems (like fatty acid synthase) use NADPH to reduce it in steps.

The lab LiAlH₄ reduction skips this activation by using an aggressive reagent. The biological equivalent takes more steps but uses less energy per step.