Rate (k) and equilibrium constants ( K ) have been measured for 17 non-degenerate hydridetransfer reactions, the donors and acceptors generally being variously substituted and elaborated pyridinium ions. Rate constants have also been measured for 6 degenerate reactions of the same type. These results have been used to test the Marcus theory of atom transfer. The Brsnsted plot shows a good deal of scatter, but around K = 1.0 it has about the predicted slope (a) of 0.5. Using the measured values of / c for degenerate reactions and assuming intrinsic reactivity to be constant within structural families, values of k for 12 non-degenerate reactions were calculated without introducing adjustable parameters. They reproduce 99 % of the variation in the measured values, the measured values generally being larger than the calculated values. The data do not permit a test of the predicted variation of a with K .A proposed mechanism involving electron transfer followed by proton transfer followed by another electron transfer, with metastable intermediates between the steps, can be excluded. Such a mechanism would lead to values of k below those predicted by the theory.* We thank Mr M. A, Mahmud for this measurement.

No abstract

The rates of reaction of methyl trifluoromethanesulphonate with a number of cyclic and open-chain disulphides have been studied at various temperatures. The main products of the reactions have been identified as the corresponding methylsulphonium salts. The difference in reactivity between the fivemembered and other disulphides arises from a combination of both enthalpy and entropy effects.

No abstract

The mercuriation of a series of aromatic derivatives by mercury(l1) trifluoroacetate in trifluoroacetic acid has been studied using titrirnetric and spectrophotornetric methods. The latter method has revealed the presence of a rapidly formed x-complex intermediate and values of the thermodynamic parameters of its formation have been computed. Only small kinetic isotope effects were found for the complex-forming step in contrast to the large values obtained for the slow step. The effect of added salts was investigated and the mechanism of the reaction discussed in detail in the light of these results. Mercuriation by a readily synthesised mixed acetate, Hg(OCOCF,)-(OCOCH,), is also reported as well as some preliminary studies of triptycene derivatives.