Nuclear magnetic resonance study of the ionization kinetics of tri-phenylmethanols in concentrated acid solutions

Abstract

The n.m.r. line shape method is used to determine the rate constants for the appearance and disappearance of substituted triphenylmethyl ions in concentrated H₂SO₄ and HClO₄ solutions. The ionization reaction of tris-(4-methylphenyl)methanol is studied in 60%(w/w) dioxan–water solutions and that of tris-(4-methyl-3-sulphophenyl)methanol in water. From the closer correlation of log k_f(for ion formation) with H₀ than with H_R, it is concluded that the transition state approximates much more closely to a protonated alcohol than to the carbonium ion structure. Further evidence for a special interaction between perchlorate and triphenylmethyl ions is also provided by the n.m.r. chemical shifts observed in solutions of tris-(4-methylphenyl) and triphenymethyl carbonium ions in mixtures of 98% H₂SO₄ with 70% HClO₄. The shifts observed are concordant with those calculated from Buckingham's equation when the perchlorate ion is assumed to be ca. 6 Å from the charged carbon atom.