Oxonium and quinonoid intermediates in the sulfonation of dimethoxynaphthalenes (DMONs)

Abstract

Monoprotonated oxonium cations (such as 2) and quinonoid dications (such as 5) of 2,3-, 2,6- and 2,7-dimethoxynaphthalenes are described. Their role as direct substrates in sulfonation reactions is discussed together with details of the sulfonation mechanism for 2,3-dimethoxynaphthalene (2,3-DMON), which is presented as a characteristic example. Steady sulfonation rate constants of 2,6- and 2,7-DMON in 80–88% H₂SO₄ are a result of the DMON protonation equilibria. Binding energies and heats of formation were calculated for several mono- and disulfonic acids of DMONs. Substituent effects are discussed.