Photochemistry of halogenated benzene derivatives. Part 2. Photoreactions of α-substituted p-chlorotoluenes

Abstract

Solution-phase photoreactions of α-substituted p-chlorotoluenes of the general structure p-ClC₆H₄CH₂X (X = H, Cl, CN, CO₂H and OH) have been studied at wavelengths around 300 nm. The acetone-sensitized photolyses of the substrates with X = H, CN, CO₂H and OH in deaerated CH₃OH provided reductively dechlorinated photoproducts C₆H₅CH₂X with yields of 10–52% based on photodecomposed starting material. The photoreactions of p-chlorobenzyl chloride (2) proceeded through homolytical and heterolytical cleavage of its side-chain C–Cl bond possibly via a triplet excited state. The main product of the photochemical reactions of (2) in deaerated CH₃OH both with and without acetone present was p-chlorobenzyl methyl ether.

In order to achieve a better understanding of some photochemical aspects of the photolyses of substrate (2), the photoreactions of three halogenobenzaldehydes have been studied. The irradiation of p- and m-chloro- and m-bromo-benzaldehyde in deaerated CH₃OH gave rise to the formation of the corresponding acetals as the sole principal products. The photoreactivity of m-BrC₆H₄CHO was 93 times greater than that of m-ClC₆H₄CHO.