Kinetic and mechanism of reactions between methyl aromatic compounds and the dibromide radical

The dibromide radical (believed to be HBr2. in acetic acid) reacts with met hylarenes, aldehydes, and carboxylic acids in acetic acid. The kinetics of these reactions were studied with the use of laser flash photolysis to gene rate the dibromide radical. The loss of the radical was monitored as a func tion of time at 360 nm. The data analysis requires an allowance for the con current bimolecular disproportionation reaction, 2HBr(2)(.) --> Br-2 + 2HBr , with k(d) = (2.2 +/- 0.5) x 10(9) L mol(-1) s(-1) at 23 degreesC. The rea ctions between HBr2. and substrate were first-order with respect to each co ncentration, with k between 4 x 10(3) (methyl-3-methylbenzoate) and 2 x 10( 6) (hexamethylbenzene) L mol(-1) s(-1) at 23 OC. The variation of rate cons tant can be correlated with the reported rates for the overall catalytic co nversions. Several compounds with deuterium labels in the methyl side chain s were investigated. The presence of a significant kinetic isotope effect s uggests that the reactions proceed by abstraction of a hydrogen atom from t he a-carbon of the R-group on the aromatic residue. Ring substituents such as CHO and CO2H have a strong deactivating effect.