Crafts of catalytic reaction to realizing the separation of complex components have received constant attention in recent years. β-bromoethylbenzene, an essential organic intermediate, is mainly obtained by the anti-Markovnikov addition of HBr to styrene, however, the by-product α-bromoethylbenzene isomer will be inevitably produced via the Markovnikov’s rule during the synthesis process, making the separation of β-bromoethylbenzene from α-β-bromoethylbenzene mixtures become one of the most important, yet very challenging process. In this work, an efficient ZnO-induced reactive separation method for the selective decomposition of α-bromoethylbenzene to purify β-bromoethylbenzene from α-β-bromoethylbenzene mixtures is discovered. Zn2+ is verified to be the actual active species in the homogeneous process. The experimental results identify that the selective decomposition of α-bromoethylbenzene follows a radical reaction mechanism, that is, Zn2+ acts as the catalyst to facilitate the homolysis of the C-Br bond of α-bromoethylbenzene to produce α-methylbenzyl radical and Br radical, which then trigger complex radical polymerization reactions. Thence the main product is macromolecular polymer rather than styrene as previously reported. In addition to Zn2+, Fe3+ and Al3+ can also perform the radical polymerization likewise though cause varying extents of loss of β-bromoethylbenzene. This Zn2+-mediated radical polymerization process can be successfully extended to apply in other reaction systems with allylic iodo, brominated hydrocarbons, offering an efficient reactive separation method to enable the separation of molecular isomers with almost identical physical properties and lower the operational costs in large-scale industrial processes.

ZnO-induced reactive separation of β-bromoethylbenzene from α-β-bromoethylbenzene mixtures: A Zn2+-mediated radical polymerization mechanism

ACS Spring 2022

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