Ionization constants of 5‐pyridylmethylenehydantoins in 80% (w/w) dimethyl sulphoxide–water at 25 °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 p K   a Iand p K   a IIvalues 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)CH 3 …N attraction is also postulated for the Z ‐isomer of 1‐methyl‐5‐(2‐pyridylmethylene)hydantoin.