Electrical effects in monosubstituted thiazoles. pKa Values of some 5- and 2-substituted thiazoles

Abstract

pK_a Values of a number of representative 2- and 5-substituted thiazoles were determined spectrophotometrically and/or potentiometrically in aqueous solution at 25 °C. The pK_a 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 pK_a of 2-‘aza’ substituted heterocyclic bases is critically examined.