What reaction must carboxylic acid go through to create an aldehyde?

Carboxylic acids cannot produce aldehydes in a single-step reaction because a reduction of carboxylic acid continues until a primary alcohol is produced. In order to produce aldehydes from carboxylic acid, the carboxylic acid must pass through an intermediate step.

Some possible methods to synthesize aldehydes from carboxylic acid include reducing the carboxylic acid to form an alcohol, and then oxidizing the alcohol to form an aldehyde. Another method to produce aldehydes from carboxylic acid is to convert the carboxylic acid to an ester via esterification. The ester product can then be reduced with a mild reducing agent to produce an aldehyde.