STOICHIOMETRY AND KINETIC-STUDIES OF OXIDATION OF STRUCTURALLY RELATED ORGANIC SUBSTRATES, VIZ THIOUREA, N-PHENYLTIOUREA AND N,N'-DIPHENYL-THIOUREA WITH AMMONIUM HEXANITRATO-CERATE(IV) IN PERCHLORIC-ACID MEDIUM

In planning optimal analytical conditions through stoichiometry as wel l as in the study of structure-reactivity correlations of oxidation of thiourea (TU), N-phenylthiourea (N-PTU), N,N'-diphenylthiourea (N,N'- DPTU) with cerium(IV) in perchloric acid medium, systematic spectropho tometric kinetic studies at 295 nm are of paramount importance. Second order rate constants (k(2)) obtained in these investigations are util ised to correlate the effects of variations of [thiourea or substitute d thioureas], [cerium(IV)], [H+], mu = [ClO4-], [cerium(III)] and vari ation of temperature. The oxidation reactions of thiourea and N-phenyl thiourea (sym.) with cerium(IV) are essentially by the substitution a t the oxidant centre in an overall inner-sphere process. The reactive species, (CeOH)(3+) oxidises the protonated thioureas considerably mor e rapidly than Ce4+. The mechanism of the oxidation reaction Is that i t takes place through the formation of a free radical intermediate due to the cleavage of a S-H bond (present in a monoprotonated species) f ollowed by the conversion into the corresponding disulphide due to dim erisation. These disulphides undergo subsequent oxidation to the corre sponding sulphones and finally to hydroxy compounds with the formation of H2SO4 and the corresponding oxygen analogues, e.g. urea, di(p-hydr oxylphenyl)urea. Oxidation reaction rates, as well as thermodynamic pa rameters are correlated to the structure-reactivity of the aforementio ned organic substrates. A suitable rate law in consonance with the exp erimental observations is also suggested.