ELECTROPHILIC BROMINATION OF GASEOUS AROMATIC-COMPOUNDS - MECHANISM AND LINEAR FREE-ENERGY EFFECTS ON REACTION-RATES

Electrophilic bromination of monosubstituted aromatic compounds is eff ected in a pentaquadrupole mass spectrometer using BrCO+ and CH3NH2Br as mass-selected reagent ions. Reaction normally occurs at the ring a nd the brominated product can be mass selected in turn and caused to d issociate by Br. loss upon collision-induced dissociation. Linear free energy correlations with Brown substituent sigma+ constants describe the extent of gas-phase bromine cation addition under the non-equilibr ium, low-pressure and solvent-free conditions which pertain in quadrup ole collision cells. The electrophilic addition reaction proceeds via a sigma-complex to the ring as suggested by MS3 spectra, except in the case of nitrobenzene, where substituent bromination is suggested by t he occurrence of a competitive process in which the nitrosubstituent i s displaced by bromine. The reactivity parameters rho are -0.23 and -0 .56 for the gaseous reagents, BrCO+ and CH3NH2Br+, respectively. Both values are much less negative than corresponding values for brominatio n 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 i n the case of CH3NH2Br+ and, unlike bromination, the rate of this reac tion does not correlate with sigma+ values. This is suggested to be a consequence of protonation at the ring in some cases and at the substi tuent 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 af finity.