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Title: Kinetic studies on sulphur-nitroso compounds
Author: Garley, M. S.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1982
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Previous kinetic studies of the formation of thionitrites, (NR2)2ChO (where R = H or Me), by reaction of nitrous acid with thiourea and tetramethylthiourea have been extended to investigate the effect of chloride, bromide and thiocyanate catalysis. The results, obtained by stopped-flow spectrophotometry, indicated that the reaction between the nitrosyl halide and the thiourea substrate occurs close to the encounter limit. The thionitrite subsequently decomposes to a disulphide, (NR 2)2tSS&NR2)2, and nitric oxide. Initial rate measurements in an excess of thiourea indicated that decay occurs via two mechanisms. One mechanism involves the radical cation [(NR2)2CSSC(NR2)2] , which is formed by the nucleophilic attack of the thiourea on the thionitrite, with the displacement of nitric oxide. This cation is further oxidised by the thionitrite to yield the disulphide. The other mechanism involves a bimolecular reaction between two thionitrite cations. A study of the rate inhibiting effect of nitric oxide led to the establishment of a decomposition scheme for the complete decay reaction. A corresponding integrated rate equation was derived, which accurately predicted the experimental results over a wide range of conditions. The interpretation of the results obtained in an excess of nitrous acid was less certain. Under these conditions, two further mechanisms were evident. As a preliminary interpretation, it was proposed that: (i) Nitric oxide is removed from the thionitrite as the species NO.NO+ by complexation with nitrous acid. As a result, the radical cation [(NR2)2CS]+ is formed. This yields the disulphide either by undergoing further oxidation by the thionitrite, or by dimerisation. (ii) The thionitrite undergoes homolytic dissociation, producing the radical cation [(NR 2)2CS]+ and nitric oxide. The disulphide is formed from this radical by a process similar to that described in (i) above. In general, the tetramethylthiourea system was found to be more reactive than the thiourea system. This was attributed to inductive effects produced by electron release from the methyl groups. The development of a stopped-flow spectrophotometer system incorporating computerised data processing is also discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available