KINETICS AND MECHANISM OF IODIDE ION REDUCTION OF S,S-DIMETHYLSULFIMINIUM PERCHLORATE

The kinetics of the reaction (CH3)2SNH2+ + 31¯ + 2H +→ (CH3)2S + I3¯ + NH4+have been measured spectrophotometrically in water and in DMSO-water solvents over the range XDMSO = 0-1.00. The observed rate law is d[I3¯]/dt = k1[(CH3)2SNH2+][H+][I¯]. In aqueous solution at 25¯ and 0.120 M ionic strength, kI = 0.56 ± 0.02 M−1 sec−1, ΔH‡ = 12.2 ± 0.4 kcal/mole, and ΔS‡ = -19 ± 1 eu. The deuterium solvent isotope effect observed is kH/kD = 0.27. The reaction is catalyzed by added thiourea, but not by bromide or chloride ions. The mechanism proposed involves nucleophilic displacement at the sulfur center by a soft base nucleophile, iodide ion or thiourea. © 1969, American Chemical Society. All rights reserved.