Electrophilic bromination of gaseous aromatic compounds: Mechanism and linear free energy effects on reaction rates

Abstract

AbstractElectrophilic bromination of monosubstituted aromatic compounds is effected in a pentaquadrupole mass spectrometer using BrCO+ and CH3NH2Br+ as mass‐selected reagent ions. Reaction normally occurs at the ring and the brominated product can be mass selected in turn and caused to dissociate by Br˙ loss upon collision‐induced dissociation. Linear free energy correlations with Brown substituent σ+ constants describe the extent of gas‐phase bromine cation addition under the non‐equilibrium, low‐pressure and solvent‐free conditions which pertain in quadruple collision cells. The electrophilic addition reaction proceeds via a σ‐complex to the ring as suggested by MS3 spectra, except in the case of nitrobenzene, where substituent bromination is suggested by the occurrence of a competitive process in which the nitrosubstituent is displaced by bromine. The reactivity parameters ρ are −0.23 and −0.56 for the gaseous reagents, BrCO+ and CH3NH2Br+, respectively. Both values are much less negative than corresponding values for bromination in solution. The greater reactivity of BrCO+ is evident by the fact that it reacts even with the strongly deactivated substrates and this is consistent with a weak BrCO bond. Competitive protonation occurs in the case of CH3NH2Br+ and, unlike bromination, the rate of this reaction does not correlate with σ+ values. This is suggested to be a consequence of protonation at the ring in some cases and at the substituent in others, including acetophenone and benzonitrile. Evidence for this is that, in contrast to its lack of correlation with substituent constants, the rate of protonation correlates linearly with proton affinity.