Free energy relationship of the equilibrium ionization constants of disulfonyl carbon acids in 80% (w/w) dimethyl sulfoxide – water solvent at 25°C

A series of eight carbon acids, α,α-bis(benzylsulfonyl)toluene and 7 substituted α,α-bis-(benzylsulfonyl)toluenes, was synthesized and their equilibrium ionization constants were determined by spectrophotometric measurement in 80% (w/w) dimethyl sulfoxide – water solvent at 25 °C. From a plot of −log K versus σ for the 'well-behaved' substituents and σ− for para-cyano and para-nitro substituents, a ρ value of 2.53 is obtained. The correlation co-efficient is 0.99. Comparing this ρ value with the reported ρ value of 12 for the substituted toluenes in dimethyl sulfoxide, it shows that the acidities of the disulfonyl toluenes are at least 8 to 9 orders of magnitude less sensitive to substituent effects than are the acidities of substituted toluenes implying that, for disulfonyl toluenes, the negative charge on the benzylic carbon is extensively delocalised into the two adjacent sulfonyl groups.