Gas‐phase thermolysis of aryl tert ‐butyl disulphides

The pyrolysis kinetics of RC 6 H 4 SS t C 4 H 9 (RH, p ‐NO 2 , p ‐Cl, p ‐F) were determined at 390–450°C and 7–15 Torr in a stirred‐flow reactor using toluene as carrier gas. The reaction products were 95% isobutene, 5% isobutane and the corresponding RC 6 H 4 SSH disulphanes. The first‐order rate constants, k (s −1 ), based on isobutene production, followed the Arrhenius equations: phenyl tert ‐butyl disulphide, k = 10 13.49±0.31 exp (– 182 ± 4 kJ mol −1 )( RT ) −1 ; p nitrophenyl tert ‐butyl disulphide, k = 10 13.46 ± 0.32 [exp(– 185 ± 5 kJ mol −1 )( RT ) −1 ]; p ‐chlorophenyl tert ‐butyl disulphide, k = 10 14.44 ± 0.66 [exp(– 196 ± 9 kJ mol −1 )( RT ) −1 ; p ‐fluorophenyl tert ‐butyl disulphide, k = 10 10.80 ± 0.16 exp(– 144.5 ± 2 kJ mol −1 )( RT ) −1 ]; The observed reactivities, within the above temperature range, follow the order p F > H > p Cl > p NO 2 . A four‐centre, cuadrupolar cyclic transition state mechanism is proposed for the formation of the isobutene and aryldisulphane products. The optimized ground‐state molecular geometries of the reactants were calculated by using the MINDO/3 procedure.