Proton-Transfer Reactions Involving Alkyl Ions and Alkenes. Rate Constants, Isomerization Processes, and the Derivation of Thermochemical Data

Article abstract of DOI:10.1021/ja00528a005

Rate constants and mechanisms have been determined for proton-transfer reactions of type AH⁺ + M ->/<- MH⁺ + A, where A is propylene, isobutene, cyclopentene, and cyclohexene.In order to avoid competing side effects the AH⁺ reactant ions are generated in alkanes and alkyl halides.It is observed that the rate constants for exothermic direct proton transfer reactions, from AH⁺ to M or from MH⁺ 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 revers direction for multiple reaction pairs, internally consistent values are obtained for the proton affinities of propylene, isobutene, and cyclopentene.Protonation of trans-2-C4H8 by H3O⁺ (Δ_Rn = -10 kcal/mol, 41.84 kJ/mol) yields sec-C4H9⁺ exclusively.However, carbon skeletal rearrangement to the t-C4H9⁺ cofiguration becomes important when the ΔH of the proton transfer is <10 kcal/mol.The experimental evidence indicates that the isomerization occurs before departure of C4H9⁺ from the reaction complex.The isomerization process which also occurs upon collision of sec-C4H9⁺ with polar molecules is dependent on the dipole moment of M.For sec-C4H9⁺/trans-2-C4H8 and c-C6H11⁺/c-C6H10, where these isomerization processes are especially important, thermochemical information is derived from qualitative considerations of trends in reaction rates.Scales of relative proton affinities of the olefins and relative heats of formation of the alkyl ions are derived and compared with data of absolute heats of formation.Taking a value of ΔH_f(t-C4H9⁺) = 162.1 +/- 0.8 kcal/mol (678.2 kJ/mol), the values derived for the heats of formation in kcal/mol (1 kcal/mol = 4.184 kJ/mol) follows: sec-C3H7⁺, 184.5 +/- 2.1; c-C5H9⁺, 183.8 +/- 1.9; sec-C4H9⁺, 176.7 +/- 2.9; c-C6H11⁺, 171 +/- 3; c-C5H8CH3⁺, 164.2 +/- 1.7; t-C5H11⁺, 154.6 +/- 2.0; c-C6H10CH3⁺, 152.7 +/- 7.1; t-C6H13⁺, 148.5 +/- 1.8 (the last four from an analysis of data from the literature).Heats of formation of some of the corresponding radicals, in kcal/mol, calculated from ionic heats of formation and ionization potential data, follow: C3H7, 14.8 +/- 2.6; c-C5H9, 17.5 +/- 2.4; c-C6H11, ca.6; t-C4H9 7.6 +/- 2.2.