Ionization constants of 5-arylmethylenehydantoins in 80%(w/w) dimethyl sulphoxide–water at 25 °C

Abstract

The pK values of (Z)-5-arylmethylenehydrantoins, (Z)-5-arylmethylene-3-methylhydantoins, and (Z)- and (E)-5-arylmethylene-1-methylhydantoins with various para-substituents in the phenyl ring have been measured. The effects of structure and stereochemistry on acidity are discussed. For each series, a good linear correlation is obtained between the pK values and Hammett constants σ and σ^– in spite of the considerable distance of the site of deprotonation from the phenyl ring. The ρ values show that substituent effects are significantly greater on the acidity of N(1)–H than on that of N(3)–H, and are also greater for the E- than the Z-isomers of the 5-arylmethylene-1-methylhydantoins.