Acidity constants of benzimidazolium ketone and pyridinium aldehyde hydrates

Acidity constants (pKa) of the hydroxyl groups of the quaternary heterocyclonium carbinols 2‐hydroxy‐methyl‐1‐methylpyridinium and 2‐(1‐hydroxyethyl)‐1,3‐dimethylbenzimidazolium ions are 11.5 ± 0.5. Acidity constants of the corresponding gem diols, the hydrates of 2‐formyl‐1‐methyl‐pyridinium and 2‐acetyl‐1,3‐dimethylbenzimidazolium ions hence are estimated to be 9 ± 0.5. Accordingly, acidity constants of the hydroxyl groups of pyridine aldehyde salt hydrates are 12–13, not 4–5 as previously reported; and conversely, acidity constants of the pyridinium groups are 4–5, not 12–13.