Kinetic study of the reaction of α-bromoxylene and 2,4,6-tribromophenol via phase transfer catalysis at low alkalinity

The reaction of 2,4,6-tribromophenol and alpha-bromoxylenes (o-, p- and m-) catalyzed by tetrabutylammonium bromide was carried out in an alkaline solution of KOH of low concentration/ organic solvent two-phase medium. The concentration of tetrabutylammonium 2,4,6-tribromophenoxide (ArOQ) is constant throughout the path of reaction while more than 98% of the catalyst exists as tetrabutylammonium 2,4,6-tribromophenoxide (ArOQ) in the organic phase. The mass transfer resistance of ArOQ between the two phases is negligible compared to the rate of organic-phase reaction. The reaction in the organic phase is a rate-determining step. A pseudo-first-order rate law is proposed to express the kinetic data. The effects of potassium hydroxide, organic solvents, agitation speed, amount of aqueous-phase reactant (2,4,6-tribromophenol), amount of catalyst, organic-phase reactant (alpha-bromoxylene), inorganic salts and temperature on the conversion are investigated in detail. It is found that the reaction rate is not well-correlated to the polarity of the organic solvent. The solvolysis effect, which concerns with the polarity and Lewis-base of the organic solvents, is used to explain the characteristics of the reaction satisfactorily at low alkaline concentration. The stability of carbonium ions of different alpha-bromoxylenes is used to explain their reaction rates satisfactorily.