Butanoic Acid is converted to Butanal in two steps;
Step 1: Conversion of Butanoic Acid into Butyryl Chloride:
Butanoic Acid when treated with Thionyl Chloride gives Butyryl Chloride, SO₂ and HCl gases. This step is selected due to conversion of a bad leaving group (-OH) into a good leaving group (-Cl).
Step 2: Conversion of Butyryl Chloride to Butanal:
In this reaction the Butyryl Chloride is reduced to Butanal by treating Butyryl Chloride with a mild reducing agent i.e. Lithium tri tert-butoxy aluminum hydride.