Intermediates in aromatic substitution reactions

Abstract

A systematic study has been made of the protonation of methyl- and ethylphenols and methylnaphthols in fluorosulphuric acid. As well as the expected steric and electronic directional influences of the substituents, solvent effects are also found to have an important bearing on the site of electrophilic attack. Protonation may take place either on the oxygen substituent or on the aromatic ring; comparison with other studies shows the former to be favoured in good hydrogen-bonding solvents. The preferred site for ring-attack is para to the hydroxyl group, but alkyl substitution of this site deactivates it towards proton attack by restricting solvation of the cation formed. The major site of attack will then be either ortho to the hydroxyl group or on oxygen unless protonation of a substituted site is favoured by the release of steric strain. The alkyl groups are much more weakly ortho-para directing than the oxygen substituent, so their electronic influences are not as important as their steric clashes, both with solvating species and with neighbouring substituents, which can be major factors controlling the site of protonation.