Abstract
The acid-catalysed benzidine rearrangements are proposed to proceed via cation radical intermediates which arise by rapid electron transfer from a neutral nitrogen of the hydrazyl moiety to a proton. Consequently, both the concerted rearrangements, Table 1, benzidines, 4-methoxy-p-semidine formation, Eqns (2), (3) and o,o‘-biaryl-linked products, and non-concerted rearrangements, Table 2, diphenyline, o-semidine, Eqns (5), (6) and perhaps 4-chloro-p-semidine, may proceed via cation radical structures involving radical C – C bond formations and homolytic N – N bond cleavages. This is contrary to the current view of the Polar Transition State theory which postulates heterolytic bond formations and cleavages, Schemes 1, 3 and 4. Mono-substituted and 4,4'-disubstituted hydrazobenzenes are proposed to undergo oxidation of the most basic nitrogen as inferred from the estimated pKas as determined by the SPARC program. This can explain why of the two possible o-semidine rearrangement products, 2,N’ and N,2'-linked, the observed major o-semidine has the substituent para to the amino group, e.g. Eqn (5). However, if one of the para substituents is a halogen, it is the nitrogen para to the halogen which undergoes SET and oxidation. In these second-order acid reactions, this can be explained by protonation of the most basic nitrogen followed by SET from the hydrazyl nitrogen para to the halogen. In the case of 4,4'-disubstitution, the hydrazobenzene may not lose its substituents readily and undergoes a rapid one-electron reduction and consequent disproportionation reaction as shown in Eqn (8). Thus the kinetics, first-order in hydrazoarene and the same kinetic order in acid for both disproportionation and rearrangement, are explained.