Abstract
Solvolysis of 1-(1-methyl-1-phenylethyl)pyridinium cation (1-P+) in 25 vol % acetonitrile in water at 60 °C provides the substitution product 2-hydroxy-2-phenylpropane (1-OH) and the elimination product 2-phenylpropene (3). The fraction of elimination is about half of that obtained with an acetate leaving group of similar basicity. The total rate of reaction is dependent on the basicity of the leaving group, 1-(1-methyl-1-(4-cyanophenyl)ethyl)pyridinium cation (1-P-CN+) reacts 1100 times faster than 1-P+ which corresponds to a Brønsted parameter of βlg = −0.93. Also the fraction of elimination is leaving-group dependent as expressed by a Brønsted parameter of β = 0.12 for the dehydronation of the ion−molecule pair by the leaving group. Addition of substituted pyridines has only a minor effect on the product ratio. The reactions are concluded to occur stepwise through a common carbocation intermediate of ion−molecule pair type. The overall kinetic deuterium isotope effects using the hexadeuteriated analogs were measured as kobsH/kobsD6 = 1.85 ± 0.10 (60 °C) with the pyridinium ion 1-P+and as kobsH/kobsD6 = 1.53 ± 0.06 (40 °C) with the 4-cyanopyridinium ion 1-P-CN+. The kinetic deuterium isotope effects on the elimination step (assuming the reaction from intermediate to alcohol is insensitive to isotopic substitution) were measured as keH/keD6 = 2.7 ± 0.2 for 1-P+ (60 °C) and 3.4 ± 0.2 for 1-P-CN+ (40 °C).
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