Kinetics and Mechanism of Reactions between Methyl Aromatic Compounds and the Dibromide Radical

The dibromide radical (believed to be HBr 2 • in acetic acid) reacts with methylarenes, aldehydes, and carboxylic acids in acetic acid. The kinetics of these reactions were studied with the use of laser flash photolysis to generate the dibromide radical. The loss of the radical was monitored as a function of time at 360 nm. The data analysis requires an allowance for the concurrent bimolecular disproportionation reaction, 2HBr 2 • → Br 2 + 2HBr, with k d = (2.2 ± 0.5) × 10 9 L mol -1 s -1 at 23 °C. The reactions between HBr 2 • and substrate were first-order with respect to each concentration, with k between 4 × 10 3 (methyl-3-methylbenzoate) and 2 × 10 6 (hexamethylbenzene) L mol -1 s -1 at 23 °C. The variation of rate constant can be correlated with the reported rates for the overall catalytic conversions. Several compounds with deuterium labels in the methyl side chains were investigated. The presence of a significant kinetic isotope effect suggests that the reactions proceed by abstraction of a hydrogen atom from the α-carbon of the R-group on the aromatic residue. Ring substituents such as CHO and CO 2 H have a strong deactivating effect.