The Wallach rearrangement. Part XIV. Rearrangements of azoxynaphthalenes in sulphuric acid. Kinetics and mechanisms

Abstract

The rates of rearrangement of the 1-naphthyl-, 2-naphthyl-, and phenyl-substituted azoxy-compounds (1)–(6) in moderately concentrated sulphuric acid solutions have been investigated. A mechanism involving quinonoid type intermediates is used to explain the acidity dependence of the observed rate constants for compounds (1), (2), and (4)–(6) at H₂SO₄ concentrations below ca. 83% w/w. Equilibrium protonation of the substrates (K_SH⁺) is followed by nucleophilic attack (HSO_4^–) on aromatic carbon yielding the uncharged quinonoid intermediates (S′). Another equilibrium protonation of S′(K_SH⁺) on the N–OH function is followed by rate-determining abstraction of an aryl-bound hydrogen with simultaneous loss of H₂O. In contrast, compound (3) exhibits a linear log (rate) correlation with log a_H2SO4 at all acidities, indicative of general acid catalysed formation of a dicationic intermediate; the other compounds also appear to react by this mechanism above 83% H₂SO₄. Thus in the low acid region the isomeric reactants (2) and (3) follow different reaction pathways while yielding a common product.