Solid‐Gas Reactions, Part III. Interaction of Solid Dibenzoylethylenes with Bromine and Iodine Vapours

The cis and trans isomers of solid 1,2‐dibenzoylethylene, 1,2‐di‐ p ‐chlorobenzoylethylene and 1,2‐di‐ p ‐methyl‐benzoylethylene yield, on exposure to bromine vapour at room temperature, the trans ‐adducts in quantitative yield except in the case of cis ‐1,2‐di‐ p ‐methyl‐benzoylethylene where isomerisation takes place prior to or during the bromination process yielding the same dibromide as the trans ‐isomer. The cis‐trans isomerisation of the three solid cis ‐isomers by iodine vapour has been analysed by determining the rate of the reaction from the change in IR transmittance. The isomerisation follows a first‐order mechanism. A plot of ΔH + + against ΔS + + is linear.