Thermochemistry and kinetics of the reaction of methyl mercaptan with iodine

Abstract

The gas-phase reaction CH3SH + IZ has been studied spectrophotometrically over the temperature range of 476-604 K. It was found that the reaction undergoes H abstraction by I at 5575 K, leading to the formation of MeSI and followed by a secondary reaction which leads to the formation of MeSSMe:Taking into consideration the effect of reaction (2), the equilibrium constant Kl(554 K) has been evaluated to be 0.025 f 0.004. This value was combined with the estimated values Sg8 (CHBSI, g) = 73.7 f 1.0 eu and (AC0,1,554) = 0.87 f 0.3 eu to obtain AH~,zss = 4.03 f 0.73 kcal/mol. This yields AHym (CH3S1, g) = 7.16 f 0.73 kcal/mol when combined with known thermochemical values for CHBH, HI, and 12. A kinetic study was vitiated by the concurrent heterogeneous reaction of MeSH and 12 at lower temperatures and the rather complicated chemistry occurring at elevated temperatures. However, attempts at measuring rate constants at 554 K lead to a lower limit of (CHsS., g) t 29.5 f 2 kcaVmol when an estimated value of A = 1010.8*0.z L/mol.s for the reaction I t CH3SH S? CH3S t HI is used. D H L (CH3S-I) is estimated to be 49.3 f 1.7 kcal/mol. The bond strengths of some divalent sulfurs and the reaction mechanisms are discussed. A crude estimate of DHO(H-CH2SH) = 96 f 1 kcal has been obtained from the kinetic data.

Photooxidation of dimethyl sulfide and dimethyl disulfide. I: Mechanism development

“…The difference in the formation enthalpy between CH,S (31.0 kcal/mole) and CH, (34.8 kcal/ mole) (Shum and Benson, 1983;Baulch et al, 1984) is small, and the bond dissociation energies between S-C and S-S bonds of the adducts are not expect to be very large (considering their corresponding reactants and the similar reasoning discussed in Section 2.2.3). Thus, the unimolecular decomposition of CH,S(OH)CH, to CH,SOH and CH, is probably not negligible, and may contribute partially to go unimolecular decomposition to CH,S and H,O via intramolecular H-bonding.…”

“…Since it is difficult to evaluate the bond dissociation energies of adducts for pathway (2), a rough comparison of bond dissociation energy from the corresponding reactants can be made as follows (Benson, 1978;Shum and Benson, 1983): If the broken bonds are C-S in the CH,SH and CH,SCH, adducts and S-S in the CH,SSCH, adducts, the difference between those bond dissociation energies is small and similar rate constants will be expected. C-S bond scission is indeed expected for the CH,SH adduct.…”

“…The bond dissociation energy of the C-S bond in CH,SSCH, is about 18 kcal/mole less than that in CH,SCH,. This is due to the higher stability of the produced radical, CH,SS, because of the partial double or x bond formed in the radical (Benson, 1978;Shum and Benson, 1983 …”

The gas-phase oxidation of organic sulfur compounds

“…A surprising feature of the studies was that the reaction of O(3p) with mercaptans is slower by a factor of 30 at 300°K than the reaction of O(3p) with sulfides. This occurs in spite of the fact that the S-H bond is weak (88.6 ± l kcal/mole [8]), so that the H-atom on the sulfur is easily abstractable. This provides further evidence that the major reaction pathway is addition.…”

H‐atom acceptor capacity of free radicals used in antioxidant measurements

“…The intrinsic BDE(ROOAH) was experimentally estimated, mainly for HOOAH, but also for other peroxy radicals including CH 3 OOAH and CH 3 CH 2 OOAH [29][30][31][32][33][34]. The values obtained in the gas phase by different methods (photoelectron and photoionization mass spectroscopies, and VLPR (very low pressure reactor)) were relatively homogeneous.…”