STABILIZATION OF S-2 MOLECULES IN DIPOLAR APROTIC MEDIUM - INVESTIGATIONS ON THE DISSOCIATION OF CYCLOOCTASULFUR IN DIMETHYLACETAMIDE

The partial dissociation of cyclooctasulfur S-8 into S-2 molecules has been investigated in dimethylacetamide by UV-vis spectrophotometry in two sets of experiments. Further studies on the disproportionation of S-8(2-) ions were first carried out in the course of the electrooxida tion of five initial solutions of electrogenerated S-6(2-) ions at 24 degrees C. The evolution of spectra could not be explained by the usua lly admitted equilibria between S-8, S-8(2-) S-6(2-) and S-6(2-)/S-3(. -) stable species in dipolar aprotic media. A good fit between experim ental and calculated absorbances agrees with additional S-8(2-)/S-4(.- ) and S-8/S-2 equilibria, with constants [S-4(.-)](2) [S-8(2-)](-1) = 2 x 10(-3) M and [S-2](4) [S-8](-1) = 10(-7) M(3), corresponding to 50 % of S-8 dissociation at [S-8](0)(T) = 1.5 x 10(-3) M. Secondly, kinet ic studies of the reactions between triarylphosphines Ar3P (Ar = pheny l (1a), 4-chlorophenyl (2a), 3-chlorophenyl (3a)) and sulfur affording triarylphosphine Sulfides showed that the reactions were first order in reactants Ar3P and S-2, whatever the initial values in [S-8](0)(T) (1.0 x 10(-4) M <[S-8](0)(T) < 4.5 x 10(-3) M) and [Ar3P](0) (4.0 x 10 (-4) M and 8.0 x 10(-4) M). The rate constants at 9.2 degrees C, k (1a ) = 29.6 M(-1) s(-1), k (2a) = 3.6 M(-1) s(-1), and k (3a) = 2.2 M(-1) s(-1), were deduced by taking into account the constant [S-2](4) [S-8 ](-1) = 1.6 x 10(-8) M(3).