12.8 : 由腈和羧酸制备醛和酮

Treating a carboxylic acid with lithium aluminum hydride yields an aldehyde, which instantly reduces to a primary alcohol.

This over-reduction occurs because LAH is a strong reductant and aldehydes are more easily reduced than acids.

For successfully preparing aldehydes, easily reducible carboxylic acid derivatives are treated with milder aluminum hydride reducing agents.

Aluminum hydride derivatives are less reactive than lithium aluminum hydride due to their increased steric bulk.

Unlike aldehydes, ketones are formed directly from carboxylic acids by reduction using organolithium reagents.

The acid reacts rapidly with two equivalents of the organolithium reagent to form a dianion. Subsequent protonation yields a hydrate, which on loss of water, gives a ketone.

Both aldehydes and ketones can be prepared from nitriles using suitable reducing agents.

Reducing a nitrile using DIBAL-H forms an aluminum complex, which, following hydrolysis, yields an aldehyde.

Similarly, reducing nitriles with Grignard or organolithium reagents forms an imine intermediate that undergoes acid hydrolysis to give the corresponding ketones.