Controlled Reduction of Aldehydes and Ketones

Aldehydes and ketones can be made from carboxylic acid derivatives by careful reduction. The key idea is to use reagents that reduce the starting material without pushing the reaction too far. Strong reducing agents can over-reduce aldehydes to alcohols, so the choice of reagent matters.

Carboxyl...

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.