Chlorine used to disinfect drinking water can react with bromide and natural organic matter present in water sources, forming toxic disinfection byproducts (DBPs). Bromide can be oxidized by chlorine into free bromine which can subsequently react with organic compounds, forming brominated DBPs. Free bromine can be in the form of BrCl, Br2O, Br2, BrOCl, and HOBr (among others), but most previous studies assume HOBr to be the only relevant brominating agent in solutions of free bromine. The relative contributions of these brominating agents to overall bromination rates can be calculated after performing reactions with organic compounds under varying solution conditions, such as [Cl–], [Br–], and pH. The purpose of this research is to evaluate the role of steric and electronic effects in bromination of benzene, alkylbenzenes (ethyl, isopropyl, tert-butyl), and alkoxybenzenes (ethoxy, isopropoxy, tert-butoxy), and to elucidate the inherent reactivities of brominating agents towards these aromatic compounds. For mono-substituted alkyl- or alkoxybenzenes, bromine substitution occurs at the para- or ortho- positions. BrCl was the most inherently reactive brominating agent towards the examined alkyl- and alkoxybenzene species. Rate constants for reactions of aromatic nucleophiles with BrCl ranged from 5.7 ± 1.3 M–1 s–1 (for benzene) to (6 ± 2) × 105 M–1 s–1 (for iso-propoxybenzene). For para-substituted alkylbenzenes, overall reactivity increased from tert-butyl &lt; ethyl ≈ isopropyl. For para-substituted alkoxybenzenes, reactivity increased from tert-butoxy &lt; ethoxy &lt; isopropoxy. In going from ethyl to tert-butyl and ethoxy to isopropoxy, unfavorable steric effects can attenuate the favorable electronic effects imparted by the substituents. When comparing unsubstituted benzene, alkyl-, and alkoxybenzenes, the structure of the substituent has a significant effect on rates and regioselectivity of bromination. Collectively, these findings indicate that steric effects, electronic effects, and brominating agents other than HOBr can influence aromatic compound bromination in solutions of free bromine.

Steric and electronic effects on reaction rates of substituted benzenes with often-overlooked brominating agents

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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