ACID-BASE BEHAVIOR OF SULFOXIDES . MEASUREMENT OF PKA VALUES BY ULTRAVIOLET AND NUCLEAR MAGNETIC RESONANCE TECHNIQUES

The protonation of several aliphatic and aromatic sulfoxides has been measured by nmr and uv techniques in aqueous sulfuric and perchloric acids. The sulfoxides do not follow the Ha acidity function and the thermodynamic pKa's have been calculated by the linear free energy relationship (l.f.e.r.) proposed by Bunnett and Olsen. The pKa's range from –1.8 for DMSO to –2.9 for p-nitrophenylmethyl sulfoxide, and the φ values lie within the range 0.4-0.6. Such φ values are very similar to the ones reported for amides and indeed it seems that the HA function satisfactorily represents the protonation behavior of sulfoxides. Structural effects on the basicity of the SO group are not very large; the Hammett ρ value for substituted phenyl methyl sulfoxides is +0.85. The evaluated thermodynamic pKa's give good correlations with half-neutralization potentials (HNP) in acetic anhydride and with the shift of OH stretching frequencies in solutions of sulfoxides with phenol in CCI4. © 1969, American Chemical Society. All rights reserved.