Carbon-bromine bond formation

Two pathways are observed for nucleophile addition to 48 in water (Scheme 49) (i) uncatalyzed nucleophile addition to form the oxygen anion 48 that undergoes rapid protonation (ii) specific acid-catalyzed nucleophile addition. The SDIE on the specific acid-catalyzed addition of solvent and bromide anion to 48 are kH/kD = 0.68 and 1.0, respectively, for reactions in 50/50 (v/v) water trifluoroethanol,67 but a smaller SDIE of kH/kD = 0.41 is observed for the specific acid-catalyzed addition of an aqueous solvent to l.52 The larger SDIE for acid-catalyzed addition of Br to 48 is consistent with a concerted reaction mechanism, in which protonation of oxygen and carbon-bromine bond formation occur in a single step with a rate constant kHBr (Scheme 49). 

The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated in the reaction of bromine with various aromatic compounds. With benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxidative addition occurs (6) however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C8Br (II, 12). 

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