Bromination reactions can occur in a variety of aqueous solutions and can contribute to the synthesis of bromine containing specialty chemicals (i.e., pharmaceuticals), formation of disinfection byproducts (DBPs) in drinking water, and damage to biomolecules associated with inflammatory diseases. Free bromine refers to the collection of electrophilic species containing at least one bromine atom with an oxidation state of 0 or +I and includes species such as HOBr, BrO–, BrCl, Br2, BrOCl, and Br2O. Most previous bromination studies assume HOBr is the only active brominating agent in solutions of aqueous free bromine. Nevertheless, previous research has shown that free bromine species other than HOBr are highly electrophilic and are capable of influencing overall bromination rates under a range of solution conditions. The purpose of this research is to quantify the reaction rates of individual free bromine species with alkylbenzenes (arenes), specifically ethyl-, isopropyl-, and tert-butylbenzene. In order to examine the rates, regioselectivity, and steric effects of arene bromination, time course reactions were performed in which chloride, bromide, and free available chlorine concentrations, and pH were treated as independent variables. Reactors were analyzed using gas chromatography with mass spectrometry (GC-MS). Formation rates of brominated products were determined and were used to calculate regiospecific second-order rate constants. For many solution conditions, the contribution of HOBr toward overall bromination rates was too small to quantify, whereas BrCl was typically the most reactive bromine species. These results suggest that mono-substituted arenes are more selective in reactions with free bromine species relative to more nucleophilic aromatic compounds (e.g., aromatic ethers). The ortho and para positions of ethylbenzene exhibited approximately equal reactivity in solutions of free bromine; however, the para positions of isopropyl- and tert-butylbenzene were more reactive relative to their ortho positions. These findings suggest that steric effects can influence the regioselectivity of these bromination reactions.

Factors controlling the regioselectivity and rate of arene bromination by aqueous BrCl and related brominating agents: Influence of steric effects

253rd National Meeting (2017)

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