The Electrophilic Aromatic Bromination of Benzenes: Mechanistic and Regioselective Insights from Density Functional Theory

X. Deraet, E. Desmedt, R. Van Lommel, V. Van Speybroeck, F. De Proft
Physical Chemistry Chemical Physics (PCCP)
25, 28581 - 28594


The electrophilic aromatic substitution of benzenes is part of any undergraduate organic chemistry textbook, yet the mechanism, and more precisely the Wheland intermediate, remains a matter of debate. In this paper, we have computed different reaction paths for the bromination of benzene, anisole and nitrobenzene at the B97X-D/cc-pVTZ level of theory. This revealed, independently of the considered benzenes, a clear kinetic preference for an addition-elimination mechanism, rather than a substitution. Moreover, both mechanisms do not involve a charged Wheland-like intermediate, not in the gas phase nor in the investigated solvents (CCl4 and acetonitrile). Insight into the regioselectivity of the bromination was provided using a combination of conceptual DFT reactivity indices, aromaticity indices, Wiberg bond indices and the non-covalent interaction index. The ortho/para directing effect of the electron-donating methoxy-group in anisole was retrieved and ascribed to a synergy between strong electron delocalisation and attractive interactions. In contrast, the preferred meta-addition on nitrobenzene could not be traced back to any of these effects, nor to the intrinsic reactivity property of the reactant. In this case, an electrostatic clash between the ipso-carbon of the ring and the nitrogen atom resulting from the later nature of the rate-determining step, with respect to anisole, appeared to play a crucial role.