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

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 a s direct substrates in sulfonation reactions is discussed together with det ails of the sulfonation mechanism for 2,3-dimethoxynaphthalene (2,3-DMON), which is presented as a characteristic example. Steady sulfonation rate con stants of 2,6- and 2,7-DMON in 80-88% H2SO4 are a result of the DMON proton ation equilibria. Binding energies and heats of formation were calculated f or several mono- and disulfonic acids of DMONs. Substituent effects are dis cussed.