The first example of bromination of aromatic compounds with unprecedented atom economy using molecular bromine

Abstract

Bromination of organic compounds is usually effected using equimolar ratios of bromine with the substrate. The generated HBr is used either in the preparation of value added brominated products or is disposed aswaste, causing serious environmental problems.Acatalyst for an inbuilt recycle of HBr by oxidation and subsequent bromination of the organic compounds has been designed and developed. The efficacy is demonstrated in the production of tetrabromobisphenol-A (TBBA) in an integrated approach of bromination coupled with oxybromination of bisphenol-A (BA), aimed at utilisation of both bromine atoms of the bromine molecule. Here 2.05 mol of bromine and 2.1 mol of H₂O₂ as an oxidant were used per mol of BA catalysed by tungstate-exchanged Mg-Al layered double hydroxide (LDH-WO₄) with unprecedented atom economy in a biphase system comprising dichloroethane and water under auto-reflux for the first time. This is quite impressive when compared with the currently used process, in which > 4 mol of bromine per mol of BA is being used. The TBBA thus obtained allows in-process control of pollutants in the integrated process and conforms to all the 14 specifications that include American Public Health Administration (APHA) colour values and hydrolysable bromide, very important specifications for marketability. Effects of mole ratios of H₂O₂/BA and Br₂/BA, concentration of the catalyst, solvent and temperature on yields and quality of the product were studied in an effort to optimise the parameters. Environmental and economical factors of the process evolved are highlighted. The Mg-Al and Ni-Al-LDH-WO₄ catalysts used in the above process and their precursors were characterised bymeans of powder-XRD, TGA-DTA, FT-IR spectroscopy, scanning electron microscopy (SEM), and particle size distribution.The heterogeneity and reusability of the Mg-Al-LDH-WO₄ were well established. The catalyst displayed consistent activity and selectivity for four recycles.