IONIZATION-CONSTANTS OF 5-PYRIDYLMETHYLENEHYDANTOINS IN 80-PERCENT (W/W) DIMETHYL-SULFOXIDE WATER AT 25-DEGREES-C

The ionization constants of nine amphoteric 5‐pyridylmethylenehydantoins were measured in 80% (w/w) dimethyl sulphoxide–water at 25 °C. The effects of structure and Z/E configuration on both pK aI and pK aII values are discussed. The especially low basicity and acidity of the Z‐isomers of the N‐unsubstituted and 3‐methyl‐substituted 5‐(2‐pyridylmethylene)hydantoin are attributed to the formation of intramolecular N(1)H…N hydrogen bond in the s‐cis conformation of these compounds. The existence of N(1)CH3…N attraction is also postulated for the Z‐isomer of 1‐methyl‐5‐(2‐pyridylmethylene)hydantoin. Copyright © 1990 John Wiley & Sons Ltd.