Linear free energy relationships. Part VI. Acidity of 4-(5-X-2-hydroxystyryl)-N-methylpyridinium iodides

Abstract

The pK_a values of the title compounds have been determined spectrophotometrically as a function of temperature in a number of buffer systems. In all cases pK_a is a linear function of substituent constant. σ_x, and ranges from 5.8 to 8.7 at room temperature. From the pK_a values of 4-(2-hydroxystyryl)-N-methylpyridinium iodide and the corresponding N-benzyl compound apparent ortho substituent constants, σ^–_ortho, for the 4-vinyl-N-alkylpyridinium groups were assigned (+0.61 ± 0.04 and +0.70 ± 0.01, respectively) and compared to the para-substituent constant (+0.61 ± 0.02) for the 4-vinyl-N-methylpyridinium group evaluated from published data. Differences between thermodynamic parameters (ΔH° and ΔS°) in borate-containing buffers and those in other buffers are attributed to specific interaction with boric acid. All the results are consistent with a significant contribution from quinonoid forms involving the pyridyl ring to the ground states for these compounds and their solvatochromic conjugate bases in aqueous solution: however, this contribution increases upon excitation.