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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 cs and 0 -in spite of the considerable distance of the site of deprotonation from the phenyl ring. The p 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 Ethan the Z-isomers of the 5-arylmethylene-I -methylhydantoins.

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