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A Kinetic Study of S ‐Nitrosothiol Decomposition
Author(s) -
Grossi Loris,
Montevecchi Pier Carlo
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020118)8:2<380::aid-chem380>3.0.co;2-p
Subject(s) - chemistry , autocatalysis , homolysis , radical , medicinal chemistry , steric effects , decomposition , alkyl , thermal decomposition , reaction rate constant , nitric oxide , arrhenius equation , hydroxylation , photochemistry , kinetics , stereochemistry , organic chemistry , catalysis , activation energy , physics , quantum mechanics , enzyme
Under anaerobic conditions S ‐nitrosothiols 1 a – e undergo thermal decomposition by homolytic cleavage of the S−N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k 1 , have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1 c ( k 1(69 °C) =13×10 −3 min −1 ) and 1 d ( k 1(69 °C) =91×10 −3 min −1 ) decomposed faster than the primary nitrosothiols 1 a ( k 1(69 °C) =3.0×10 −3 min −1 ) and 1 b ( k 1(69 °C) =6.5×10 −3 min −1 ). The activation energies ( E # =20.5–22.8 Kcal mol −1 ) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S ‐nitrosothiols 1 a – e takes place by an autocatalytic chain‐decomposition process catalyzed by N 2 O 3 . The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p ‐cresol, β ‐styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.