pKa VALUES OF 2- AND 2,6-DISUBSTITUTED PYRIDINE DERIVATIVES CONTAINING SULFENYL AND SULFINYL GROUPS AND σ* AND EsVALUES OF SEVERAL SULFENYL AND SULFINYL GROUPS

pKa Values of pyridine derivatives having sulfenyl and sulfinyl groups attached to either 2- or 2,6-positions in the pyridine nuclei were measured affording the following values; 2-SCH3 (3.64), 2-S(O)CH3 (0.17), 2-CH2SCH3 (5.40), 2-CH2S(O)CH3 (3.10), 2-CH2OCH3 (4.35), 2,6-(SCH3)2 (2.37), 2,6-(CH2SCH3)2 (4.10), 2,6-(CH2S(O)CH3)2 (1.53), 2,6-(CH2OCH3)2 (3.50). These pKa values together with several other 2-substituted pyridines: 2-X-C5H4N in which X is Cl, Br, I, CN, CO2C2H5, CH3, H, OCH3, were plotted against Taft [sgrave]* values to afford a good straight line giving ρ*=-4.5. From fitting the above pKa values on this line, the [sgrave]* values of these groups were determined. Furthermore, Es values of CH3S(O)−, CH3SCH2− , CH3S(O)CH2− , CH3OCH2− groups were determined and the application of the [sgrave]* values was tested by measuring both acid and alkaline hydrolyses rates of the corresponding p-nitrophenyl acetates, the modified Taft method. The additivity of these σ* values in the 2,6-positions in the pyridine nuclei was examined. The cation-transfer experiments were undertaken by using 2,6-bis(methylsulfinylmethyl)pyridine as mediator.