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The protonation equilibria of alkyl methyl sulphides have been studied in sulphuric acid solutions. Their -Ho values a t half protonation lie in the range -5-4 to -5-7 and their pKBa+ values, as determined with the aid of the Bunnett and Olsen equation, in the range -6-7 to -6.9. Steric effects are not important even for the t-butyl derivative and the polar effects are small (p* = 0.86 f 0.09). The solvation requirements of the protonated sulphides are small as evidenced by the negative r ) values obtained (-0.26 to -0.29).THE behaviour in acids of sulphides and related bivalent sulphur compounds is of interest on theoretical l y 2 as well as on practical3 grounds, acid treatment of hydrocarbons being a possible means for removing sulphur compounds from petroleum product^.^The available information is scanty. In an extended tabulation of pKBH+ of weak organic bases, Arnett reported values of -5.10 and -4.52 for thiacyclopentane, obtained via spectroscopic and partition methods re~pectively,~ as well as values of -5.3, -6-15, and -6-7 for Me&, Et,S,, and MeSH, respectively, obtained probably with the solvent extraction technique developed in Arnett's laboratory. Moreover, no information is available as to the kind of acidity function necessary to describe the protonation equilibria behaviour of sulphides in concentrated acids.The reliability of the results obtained with the solvent extraction technique seems to depend on the degree of specific interactions between the substrate and the extracting solvent. At least in the case of alcoholsJ5 the basicity values determined via this technique were found widely different from those obtained via Raman and n.m.r.' spectroscopy.

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