12.9 : 由羧酸衍生物制备醛和酮

As aldehydes are more reactive than carboxylic acids, the former can get further reduced during its preparation. This makes carboxylic acids ineffective in preparing aldehydes using strong reducing agents.

However, an easily reducible carboxylic acid derivative—acid chloride —successfully gives an aldehyde using a mild reducing agent.

Alternatively, reduction via hydrogenation over poisoned catalyst, known as the Rosenmund reduction, also gives aldehydes. Here, catalytic poisoning prevents the over-reduction of aldehydes.

Acid chlorides through Grignard reactions reduce all the way to tertiary alcohols via ketone intermediates.

This over-reduction can be avoided by using milder reducing agents like the Gilman reagent.

Moreover, aryl ketones can be prepared by Friedel–Crafts acylations.

Another carboxylic acid derivative, ester, can be reduced by DIBAL-H at dry ice temperature to give an aldehyde.

Ketones, however, cannot be prepared directly from esters as they are more reactive than esters and will be over-reduced to tertiary alcohols.