Select the reagents needed to effect the transformation from the carboxylic acid to the aldehyde. draw the structure of the intermediate.

Carboxylic Acids can be converted into the Aldehydes in following two steps,

1) Converting Carboxylic Acid into Acyl Halide;

Carboxylic is first reacted with Thionyl chloride in order to replace a weak leaving group (-OH) with a good leaving group i. e. - Cl.

2) Converting Acyl Halide into Aldehyde;

The Acyl Halide formed in first step is treated with Lithium tri tert - butoxy aluminum hydride, which acts as a source of Hydride and prevent further resuction of Aldehyde into Alcohols.

Reactions are as follow,