Denitrosation and nitrosation: a kinetic and mechanistic investigation

Abstract

Section one of this thesis is concerned with the kinetics and mechanism of the process of denitrosation. At high concentrations of sufficiently reactive nucleophiles the rate of reaction of several N-alkyl-N-nitrosoanilines becomes independent of both the nature and the concentration of such species. This has been intrepreted in terms of a shift in the rate-determining to an earlier one in the reaction pathway, namely the protonation. Support for such a proposal comes from the solvent isotope effects K(_H(_2)O) : K(_D(_2)O) of 1.4 and 0.7, respectively. The denitrosation of D,L-N-acetyl-N-nitrosotryptophan (NANT) has been studied in water in the acid range 4 x 10(^-2) - 1M H(_2)SO(_4) and also at the lower acidities in buffer solution pH 2-6. The reaction was irreversible, giving D,L=N-acetyltryptophan (NAT) and nitrous acid quantitatively. At higher acidities the rate of reaction was independent of added parent amine NAT and also of the addition of various nucleophiles. The kinetic solvent isotope effect k(_H(_2)O) : k(_D(_2)O) was 1.3 and 1.1 at 0.7M H(_2)SO(_4) and 0.1M H(_2)SO(_4), respectively. At pH 6, however, the addition of various nucleophiles did catalyse the reaction, with increasing efficiency along the series C1(^-)<Br(^-)<SCN(^-)< 1(^-)~(^N)3(^-). As the concentration of nucleophile increased the reaction rate constant tended to become independent of the [nucleophile]. The results arc discussed in terms of two acid-catalysed reaction pathways, one predominate in the region pH 4-7 and the other at acidities greater than pH 1, as clearly shown by the pH - rate profile. At the higher acidities, N.ANT was used to nitrosate 4 - nitroaniline but only in the absence of a nitrous acid trap, thereby implicating the intermediacy of free nitrous acid. Section two is concerned with the reverse process, nitrosation. Thioureais shown to be a very efficient catalyst in the nitrosation of morpholine in acid solution. Compared with other known catalysts the order of efficiency