Journal of the American Chemical Society
2540
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102:8
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April 9, 1980
Proton-Transfer Reactions Involving Alkyl Ions and Alkenes. Rate Constants, Isomerization Processes, and the Derivation of Thermochemical Data S. G . Lias,* D. M. Shold, and P.Ausloos Contribution from the National Bureau of S t a n d a r d s , N a t i o n a l Measurement Laboratory, Washington, D.C. 20234. Receiced October I , I979
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Abstract: Rate constants and mechanisms have been determined for proton-transfer reactions of the type A H + M G M H + A, where A is propylene, isobutene, trans-2-butene, cyclopentene, and cyclohexene. In order to avoid competing side effects the A H + reactant ions are generated i n alkanes and alkyl halides. It is observed that the rate constants for exothermic direct proton transfer reactions, from A H + to M or from M H + to A, are equal to the collision rate only when the total rotational, vibrational, and electronic entropy change associated with the reaction is positive, and when the exothermicity of the reaction exceeds 2-4 kcal/mol. On the basis of rate constants in the forward and reverse direction for multiple reaction pairs, internally consistent values are obtained for the proton affinities of propylene, isobutene, and cyclopentene. Protonation of trans-2-C4Hs by H 3 0 + (AHR, = -10 kcal/mol, 41.84 kJ/mol) yields sec-C4H9+ exclusively. However, carbon skeletal rearrangement to the t-CdHg+ configuration becomes important when the AH of the proton transfer is
