Aromatic sulphonation. Part 75. Intramolecular kinetic isotope effects of hydrogen in the aprotic sulphonation of 1,2,4,5-tetramethylbenzene, naphthalene, and 1,6-methanoannulene. Implications for the mechanism of the sulphur trioxide sulphonation
The intramolecular substrate kinetic isotope effects of hydrogen for the sulphonation of 1,2,4,5-tetramethyl[3-2H]benzene and [1,4-2H2]naphthalene with sulphur trioxide in nitromethane as solvent at 0 °C have been determined as kH/kD= 5.6 ± 0.6 and 1.8 ± 0.1, respectively. With trichlorofluoromethane as solvent at –25 °C the intramolecular isotope effects for these two substrates are 3.1 ± 0.3 and 2.0 ± 0.1 respectively. For the sulphonation of 1,6-methano[2,7-2H2]annulene in dioxan at 12 °C kH/kD= 3.8 ± 0.3. The observed primary kinetic isotope effects are discussed in terms of the previously established σ,σ-mechanism for aromatic sulphonation with sulphur trioxide in aprotic solvents. They are ascribed to steric hindrance accompanying the intramolecular proton shift of the 1-arenium-1-pyrosulphonate σ-complex, yielding the arenepyrosulphonic acid. For trichlorofluoromethane as solvent the occurrence of an ‘encounter complex’ is discussed.