Ion–molecule pairs as intermediates in solvolysis. Catalysis by the pyridine leaving group on the elimination reaction

Solvolysis of the 1‐(1‐methyl‐1‐phenylethyl)pyridinium cation (1‐P+) in 25% (v/v) acetonitrile in water at 60°C provides the alcohol 2‐hydroxy‐2‐phenylpropane (1‐OH) as the main product along with the alkene 2‐phenylpropene (3). The formation of the elimination product 3 is promoted by the leaving pyridine. Thus, eight times more 3 is obtained from 1‐P+ than from the protonated ether 1‐OMeH+. Hydron abstraction by the leaving pyridine is only two times less efficient than with AcO− as leaving group. The results indicate that the ion–molecule pair 1+ –P has a significant lifetime. The elimination product is formed mainly from the ion–molecule pair. The free carbocation yields almost exclusively the substitution product.