Pyrylium-mediated transformations of natural products. Part 9. The effect of temperature and ionic strength on the mechanism and rate of reactions of pyridinium ions in aqueous solution

Increasing ionic strength decreases nucleophilic substitution rates both of the formally neutral (1) and of the formally negatively charged pyridinium substrate (2) with azide, indicating the dominating effect of interactions with the positively charged nitrogen monopole. For (1) and (2), both solvolysis and bimolecular reaction components show small entropies of activation, whereas solvolysis of (3) displays a large negative entropy of activation.