Photo-induced, liquid phase transformation of aromatic aldehydes into aroyl chlorides promoted by polyhalomethanes and dioxygen

Benzoyl chloride was prepared in 50% selectivity when a CCl₄ solution of benzaldehyde was treated with molecular oxygen under atmospheric pressure and irradiated for 3 h at 72° C (74% benzaldehyde conversion). The benzoyl chloride selectivity could be tuned by changing parameters such as oxygen flow rate, temperature or polyhalomethane nature. PhCCl₃ and CCl₄ proved to be the most active and selective chlorinating agents. Aromatic aldehydes with electron donating groups gave maximum aroyl chloride selectivity. The reaction seems to be of a free-radical nature, accelerated by irradiation or chemical radical initiators. The possibility of using this system as an efficient method for symmetric aromatic anhydrides synthesis in the presence of a solid base is briefly discussed.