Kinetics of catalytic oxidation 4-bromethylbenzene by ozone in acetic acid

Abstract

The kinetic regularities of catalytic oxidation of 4-bromoethylbenzene by ozone to create an eco-logical, low-temperature technology for the synthesis of 4-bromoacetophenone have been studied. The experiment was performed in a glass reactor with a porous membrane under conditions of kinetic regime at a temperature of 293-333 K. The concentration of ozone in the gas phase was determined by spectrophotometric method. Analysis of 4-bromoacetophenone and its oxidation products was performed by gas-liquid chromatography. Oxidation of 4-bromoethylbenzene by ozone in a solution of acetic acid at a temperature of 293 K in the presence of catalytic impurities of manganese (II) ace-tate proceeds mainly on the ethyl group with the formation of a mixture of 4-bromoacetophenone (95.6 %) and 1-(4-bromophenylethanoacetate 4.2 % ). Prevention of destructive oxidation of the ben-zene ring (ozonolysis) with the involvement of the catalyst is explained by the fact that ozone under ca-talysis mainly reacts with a salt of manganese (II) and not with the substrate to form the active form of manganese Mn(IV) which has high substrate selectivity to alkylarenes, directs the oxidation of 4-bromoethylbenzene to the ethyl group. High selectivity of side chain oxidation is achieved only at ele-vated catalyst concentrations, which is largely due to the higher reaction rate of the substrate with ozone than with Mn(IV). The composition of the products of catalytic oxidation of 4-bromoethylbenzene depends on the temperature: at 293 K the reaction stops at the stage of formation of the corresponding ketone and acylated alcohol, increasing the temperature promotes further oxidation of 4-bromoacetophenone to 4-bromobenzoic acid, thus forming a mixture containing 4-bromoacetophenone (82.5 %), 1-(4-bromophenyl)ethanolacetate (4.2 %) and 4-bromo-benzoic acid (11.8 %). The research allowed to formulate general regularities of the reaction of catalytic oxidation of 4-bromomethylbenzene by ozone in acetic acid, to explain the role of the catalyst in the system and to propose a chemical scheme of oxidation corresponding to experimental data.