Proton-transfer reactions involving alkyl ions and alkenes. Rate constants, isomerization processes, and the derivation of thermochemical data

Abstract

Rate constants and mechanisms have been determined for proton-transfer reactions of the type AH+ + M <=; MH + + A, where A is propylene, isobutene, írani-2-butene, 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 w hen 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-C4Hx by H30+ (A/Vr" = -10 kcal/mol, 41.84 kJ/mol) yields sec-C4Hg+ exclusively. However, carbon skeletal rearrangement to the Z-C4H9+ configuration becomes important when the AH of the proton transfer is <10 kcal/mol. The experimental evidence indicates that the isomerization occurs before departure of C4Hg+ from the reaction complex. The isomerization process which also occurs upon collision of iec-C4Hg+ with polar molecules is dependent on the dipole moment of M. For 5coC4H9+/ trans-2-C4H8 and c-C6H n+/c-CaH ¡o, 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 on absolute heats of formation. Taking a value of A//f(r-C4H9+) = 162.1 ± 0.8 kcal/mol (678.2 kJ/mol), the values derived for the heats of formation in kcal/mol

Ethyl cation transfer from diethylchloronium to alkyl aromatics studied by high pressure mass spectrometry

“…This is consistent with extensive studies on the isomerization of sec-C4H; to t-C4H; (25). Even if the former ion were the immediate product of reaction [lo], the high temperature and the presence of the dipolar ethyl chloride (kD = 2.1 D) would cause very rapid isomerization (26).…”

Gas‐phase basicities of polyfunctional molecules. Part 2: Saturated basic sites

“… a kJ/mol; b J K −1 mol −1 ; c Meot‐Ner (1983); d Hunter & Lias (1998) (entropy terms into parentheses are estimated values indicated as such in the original reference); e MP2/6‐31+G(d) calculations (Ganguly & Fuchs, 1997); f Lias, Shold, & Ausloos (1980); g Bromilow et al (1981); h Kabli et al (2006); i Sharma, Blades, & Kebarle (1984); j Adötoledo et al (1998). …”

MNDO calculations of proton and methyl‐and ethyl‐cation affinities of neutral carbon, nitrogen, and oxygen bases

“…This agrees well with other values ( 192.3,9 193.312) obtainedlo by combining the relative heats of formation of t-C4H$ and i-C3H: (ref. 11) with the absolute heat of formation of the latter.9J' However, a direct determination of AHf ( t -C4H;) by Traeger In the MNDO formalism AHf(H+) is defined by the parametric value of the one-center core-electron attraction integral Us, chosen to accommodate best the heats of formation of hydrocarbons.3(h) This procedure, however, leads to an empirical value of Us, , which differs significantly from that appropriate to an isolated hydrogen atom. This results in a calculated heat of formation of the proton which compares very poorly with the 365.7 kcal mol-l determined e~perimenta1ly.l~ We therefore decided to base the "calculated" PAS not on the MNIIo value but on the experimental value of AH,(H+).…”