Acidity of some methyl-substituted pyridinium ions in methanol

Abstract

The ionisation constants of the conjugate acids of pyridine, 2-, 3-, and 4- picoline, 2,4-lutidine, and 2,6-lutidine at 25° in methanol have been measured by a spectrophotometric method with 2,6-dinitrophenol [pK_a(25°, MeOH)= 7·750] as indicator. The pK_a values closely parallel the corresponding figures for aqueous solution. There is no evidence for steric effects in the case of 2,6-lutidine.