Reduction

Carboxylic acids and their derivatives bear a formal +3 oxidation state at the C=O carbon. This is the highest possible oxidation state of a carbon in an organic functional group.

The oxidation of a carboxylic function can be achieved, but always at the cost of degrading the molecule to H₂CO₃, i.e. CO₂ + H₂O.

Scale of carbon oxidation states, from C⁴⁻ to C⁴⁺: alkanes, alkenes, alcohols, alkynes, aldehydes and ketones, acids and derivatives and, finally, CO, CO₂ and H₂CO₃

However, a carboxylic acid can be reduced to another functional group with the key carbon in a lower oxidation state, provided there exists the appropriate reagent.

Reduction to aldehydes

Reduction of an acid to an aldehyde with lithium in methylamine: the imine R–CH=NMe forms and is hydrolysed to R–CH=O

Alkali metals are excellent reducing agents because of their great tendency to release their valence electron.

The electron-deficient carboxyl carbon can accept an electron and thus be reduced. The amine can supply the hydrogens needed. Can you propose a reasonable mechanism?

Reduction of an acid to an aldehyde with diisobutylaluminium hydride, (i-Bu)₂AlH

Diisobutylaluminium hydride (DIBAL-H), much less reactive than lithium aluminium hydride (LAH), partially reduces the carboxyl group ending up in the corresponding aldehyde.

Reduction to alcohols

Reduction of a carboxylic acid to a primary alcohol, R–COOH → R–CH₂OH

Reducing a carboxyl group to an alcohol requires stronger reducing agents. Be careful: other functional groups can be reduced as well!

LiAlH₄ / ether. reduces too:

Groups also reduced by LiAlH₄: ketones and aldehydes, acid chlorides, esters, amides, nitriles, nitro compounds, halides and epoxides

BH₃ / THF. reduces too:

Groups also reduced by BH₃: alkenes, alkynes, ketones and aldehydes, nitriles and epoxides

Caution! LAH reacts very vigorously with carboxylic acids, and explosions or fires can easily occur.

Click here to review other uses of LAH.