Protonation of Methyl Substituted Phenols in Super Acids

The protonation of a series of methyl substituted phenols and anisoles in FSO 3 H has been examined. When the para position of the phenol is unsubstituted, protonation was found to take place exclusively at this site. A methyl group on C-4 reduces the basicity of this carbon such that protonation on C-2, C-6 or oxygen can now compete with para protonation. Despite claims to the contrary, in FSO 3 H no significant amount of meta protonation was observed for any of the phenols. In FSO 3 H–SbF 5 , 2,4,6-trimethylphenol is protonated on C-3 or C-5; however, it would appear that this meta protonation is accompanied by a modification of the hydroxy group by either further protonation or reaction with the Lewis acid SbF 5 .