Protonation and sulphonation reactions of anisole in sulphuric and fluorosulphuric acid

The reactions of anisole in sulphuric and fluorosulphuric acid have been examined. At room temperature sulphonation is so rapid that it is not possible to observe protonated species. 1H N.m.r. spectra in D2SO4 reveal that the ortho- and para-protons are rapidly exchanged. The initial product of sulphonation is p-methoxybenzenesulphonic acid which reacts further in concentrated acids to form 4-methoxybenzene-1,3-disulphonic acid. Anisole reacts cleanly with equivalent amounts of either sulphuric or fluorosulphuric acid in trifluoroacetic acid to form 4-methoxy-benzenesulphonic acid. Our results suggest that Kresge and Hakka's suggestions may be in error.