Borderline between E1 and E2 Mechanisms. Bimolecular Base-Promoted Elimination via Ion Pairs Competing with Concerted E2 Elimination

Article abstract of DOI:10.1021/ja00196a054

The solvolysis of 2-chloro-2-methyl-1-phenylpropane (1) in methanol or in 25 vol percent acetonitrile in water at 25 deg C yields substitution product 2 (ether or alcohol) and the two elimination products 3 and 4.The olefin fraction in the aqueous solution does not increase significantly in the presence of 0.75 M sodium hydroxide.In methanol, on the other hand, the fraction of 3 and 4, the ratio <3>/<4>, and the rate of disappearance of 1 increase with methoxide ion concentration.The kinetic isotope effects (k₁₂^H + k₁₃H _{+ k}14_H)/(k₁₂^d₂ + k₁₃^d₂ +k₁₄^d₂) = 1.41 and 1.42, measured for the solvolysis of the dideuterated analogue d₂-1 in water/acetonitrile and in methanol, respectively, are too large to be secondary effects on a rate-limiting ionization step, but they suggest reaction via a reversibly formed carbocationic intermediate.The solvolysis of the hexadeuterated analogue d₆-1 exhibits "normal" secondary kinetic isotope effects, (k₁₂^H + k₁₃^H + k₁₄^H)/(k₁₂^d₆ + k₁₃^d₆ + k₁₄^d₆) = 1.79 and 1.81 in water/acetonitrile and methanol, respectively.The presence of 2 M NaOMe increases the isotope effect for d₂-1 to 2.4 but decreases the effect for d₆-1 to 1.35.These results indicate that 3 is formed by a parallel methoxide-promoted one-step concerted rout (E2) and that such a route is not significant for the formation of 4.The isotope effect for the E2 reaction is estimated as k^H/k^d₂ = 4.9 (on the basis of the assumption that the elimination product ratio <3>/<4> for the carbocationic route is not affected by MeO(1-)).