The lipophilicity of Sudan I and its tautomeric forms

Abstract

Partition coefficients from water to wet octanol and to 24 dry solvents have been obtained for Sudan I from the ratio of solubilities in water and the solvents. From these (global) partition coefficients for Sudan I and the equilibrium constants for the azo (Z) ⇌ quinone (Q) tautomeric equilibrium, partition coefficients for the Z and Q tautomers have been calculated. The three sets of partition coefficients have been analyzed using the solvation equation of Abraham to yield descriptors for Sudan I, the Z and the Q forms. It is shown that the water–wet octanol partition coefficient, as log P_oct, for Sudan I is 5.60, that is almost 5 log units more than the recorded literature value. The Z and Q tautomers have very small hydrogen bond acidities, suggesting that they form strong intramolecular hydrogen bonds. The Q tautomer is a stronger hydrogen bond base than the Z tautomer, and consequently partition coefficients are smaller for the Q tautomer; values of log P_oct are 5.30 and 5.94 respectively. It is suggested that the literature value of 0.64, obtained by the shake flask method, is wrong because the very low aqueous solubility of Sudan I, that we have determined to be 5.1 × 10⁻⁸ mol dm⁻³, precludes any accurate shake flask determination.