Electrophilic aromatic bromonium ions

The electrophilic character of chlorine and bromine in their attack on olefins is confirmed by recent studies of substituent effects in styrenes and cinnamic acids [ 102]. The slope of the Hammett plot for chlorination = — 4.01) is consistent with a classical carbonium ion intermediate of type (i) (p. 91), but the smaller value [q — — 2.23) for bromination indicates less delocalisation of charge into the aromatic ring, and is consistent with the bromonium ion intermediate. 

Arenes can be induced to react only with quite voracious electrophiles. For example, Br2 reacts instantly with alkenes but is inert toward benzene. However, Br2 reacts with benzene in the presence of the Lewis acid FeBra, which coordinates to Br2 and makes Br a better leaving group. (A yS-halocarbocation, not a bromonium ion, is an intermediate in the reaction of arenes with Br2.) Aromatic rings are sulfonated (substituted with —SO3H) by treatment with SO3/H2SO4, and nitrated (substituted with —NO2) with HNO3/H2SO4. 

Halogen atoms, usually chlorine or bromine, are introduced by the action of the elemental halogen on the aromatic hydrocarbon in the presence of a Lewis acid catalyst. The chlorinium or bromonium ion is the attacking electrophile. 

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