A change from stepwise to concerted mechanism in the acid-catalysed benzidine rearrangement: a theoretical study.

The monoprotonated mechanism of the benzidine acid-catalysed rearrangement of hydrazobenzene (corresponding to a second order kinetics) is studied and compared with the diprotonated mechanism ( corresponding to a third order reaction and previously studied). The nature of the two mechanisms is found to be completely different: a concerted closed-shell sigmatropic shift in the monoprotonated, a stepwise radical cation recoupling in the diprotonated. The activation energies, combined with the energetics of the protonation equilibria, are also very different: 35 kcal mol-1 for the former and 26 kcal mol-1 for the latter (in good agreement with the experimental data). These values make the third order diprotonated mechanism favoured with respect to the second order monoprotonated mechanism for the rearrangement of hydrazobenzene, as found at typical experimental acid concentrations.