ELECTRICAL EFFECTS IN MONOSUBSTITUTED THIAZOLES - PKA VALUES OF SOME S-SUBSTITUTED AND 2-SUBSTITUTED THIAZOLES

pKa Values of a number of representative 2- and 5-substituted thiazoles were determined spectrophotometrically and/or potentiometrically in aqueous solution at 25°C. The pKa is very sensitive to changes of substituent, the full range of values observed covering ca. 10 logarithmic units. The substituent in position 2 appears to interact with the reaction centre through mesomeric as well as inductive effects; the latter is the dominant effect. Steric effects appear to be absent. The relative sensitivities to variation of the substituent in positions 2 and 5 of these thiazoles and of other ' aza ' substituted heterocyclic systems are briefly compared. The usefulness of an ' ortho ' polar effect derived from the pKa of 2-' aza ' substituted heterocyclic bases is critically examined.