KINETICS AND MECHANISM OF THE NITROSATION OF 2-MERCAPTOPYRIDINE [PYRIDINE-2(1H)-THIONE]

2-Mercaptopyridine (MP) reacts rapidly with nitrous acid in mildly aci d aqueous solution (via the thione tautomer) to give an unstable S-nit roso ion (SNO+) in a reversible process with an equilibrium constant ( K-N) of ca. 1 x 10(5) dm(6) mol(-2). SNO+ is readily detected by two p eaks in the UV spectrum at 295 and 240 nm with extinction coefficients 9600 and 9300 dm(3) mol(-1) cm(-1) respectively. MP is regenerated wh en the solution is made alkaline. Kinetic measurements made on the nit rosation reaction give a value of 8200 dm(6) mol(-2) s(-1) for the thi rd order rate constant k(3) (defined by rate = k(3) [MP] [HNO2] [H+]), which is close to that believed to be the diffusion-controlled limit for attack by NO+ (or H2NO2+). As expected there is marked catalysis b y Cl- and Br-, and analysis of the kinetic results obtained from varia tion of measured rate constants with [halide ion] gave values of 3.5 x 10(9) and 3.7 x 10(9) dm(3) mol(-1) s(-1) respectively for the bimole cular rate constants for NOCl and NOBr reactions with MP, again values close to the diffusion limit The same experimental results also yield ed values of 30 and 2400 dm(3) mol(-1) s(-1) for the second order rate constants, for the reverse process of Cl- and Br- reaction with SNO+. Values for K-N of 1.3 x 10(5) and 7.9 x 10(4) dm(6) mol(-2) were obta ined from the halide catalysed reactions. In acid solution SNO+ decomp osed to the disulfide (2,2'-dipyridyl disulfide) and NO (measured with a NO-electrode). Quenching of SNO+ at pH 7.4 gave UV spectroscopic ev idence for the neutral deprotonated form (SNO) of SNO+ and there was a transformation to give mainly MP together with some disulfide. There was clear evidence that SNO+ (and maybe SNO) can act as an efficient n itrosating species: addition of the thiol N-acetylcysteine (at pH 6.15 ) resulted in the almost instantaneous decomposition of SNO. Addition of N-methylaniline (NMA) to an acidified solution of SNO+ resulted in quantitative N-methyl-N-nitrosoaniline formation and kinetic measureme nts of the nitrosation of NMA in the presence of MP showed marked cata lysis at low [MP], which disappeared at higher [MP]. These results are quantitatively consistent with nitrosation via SNO+: catalysis disapp ears at higher [MP] when the nitrous acid is virtually completely conv erted to SNO+. A value of 1.7 x 10(5) dm(3) mol(-1) s(-1) was obtained for the bimolecular rate constant for reaction of SNO+ with the free base form of NMA. MP is thus an excellent catalyst for electrophilic n itrosation. Under somewhat different conditions SNO+ can then act as a source of HNO2/NO2-, NO or NO+. The chemistry reported in this paper bears many similarities to that involved in the nitrosation of thioure as, and subsequent reactions of the S-nitrosothiouronium ions.