Heats of formation of protonated cyclopropane, methylcyclopropane, and ethane

Abstract

Equilibrium constants have been measured for proton transfer reactions between protonated methanol or protonated formic acid with propylene, trans-2-butene, cyclopropane, methylcyclopropane, and ethane. The single ion source of a mass spectrometer was used as reactor. From the free energies of reaction and assuming negligible entropy change, proton affinities have been determined for the above compounds assuming A//f(7-C3H7+) to be 191.7 kcal/mol. Heats of formation of the various protonated ions calculated from these values show that protonated cyclopropane and methylcyclopropane have heats of formation different from that of the propyl or butyl ions showing that the ring structure for these ions is maintained. The heat of formation of C2H7+ is about 0.2 kcal/mol less than that of C2H5+. This is in accord with the postulated instability of C2H7+. This research has been directed toward establishing thermodynamic properties of certain interesting hydrocarbon positive ions for which no suitable values have yet been determined. Several investigators have postulated protonated cyclopropane or methylcyclopropane as intermediate in the solvolysis of norbornyl derivatives,1 in the addition of acids,2-4 acetyl chloride,3

The kinetics of the solvolysis of propylene and cyclopropane in concentrated aqueous sulfuric acid relative energies of the 2‐propyl cation and protonated cyclopropane

“…Although our figures for propylene and cyclopropane do differ from those of Chong and Franklin [6], the difference is not large and, as Table I11 shows, the relative heats of formation calculated by Bischoff and Dewar [4] are in fairly good agreement with the experimental values while those of Hariharan and co-workers [5] are in less good agreement.…”

“…The activation energies for the solvolysis reaction allow calculation of the relative heats of formation of the 2-propyl cation and protonated cyclopropane. It is of interest to compare these values with the corresponding experimental values in the gas phase as measured by chemionization mass spectrometry [6] and with the calculated values which also pertain to the gas phase. The pertinent figures are assembled in Table 111.…”

“…Both sets of calculations predict that face-protonated (see [3]) cyclopropane is of such high energy as to be excluded as a possibility, but otherwise the figures are in serious disagreement as to the energy of the protonated cyclopropane relative to the propyl cations. The only experimental measurements of the relative energies of these species are those of Chong and Franklin [6] who measured equilibrium constants for proton transfers using chemionization mass spectrometry. We felt that kinetic studies of the solvolysis of propylene and cyclopropane and the determination of the heats and entropies of activation would contribute to the understanding of the energy of protonated cyclopropane relative to the 2-propyl cation and we report the results of such studies here.…”

Pyrolysis of C2H7+ and other Ion–Molecule Reactions in Methane containing Traces of Ethane

“…The present work, which is based on measurements of ion-molecule reactions of methane containing known traces of ethane, was initiated following the report of Chong and Franklin (1) on the proton transfer equilibrium 1 for which these authors found AGIO = -1 kcal/mol (-340 K). Such a very small difference between lExperimenta1 work represents part of Ph.D. dissertation of M.F.…”

Radiation-Sensitized Thermal Cracking of n-Butane. II. Ionic Reactions

“…Further, as the addition of 11.4 molz ammonia caused only a slight change of the yields of higher paraffins, higher olefins, and the straight chain butenes (Table 2), the participation of the accumulated products in the chain mechanism of the isobutane formation may be excluded. If we assume that isobutane is formed by a usual hydride ion transfer reaction of C4H9 + with n-butane molecule, C4H9 + must be a protonated methylcyclopropane or isobutyl cation to secure an exothermicity of the reaction (19,20). However, Lias et al have reported that the isobutyl cation formed as a fragment ion from the radiolysis of isopentane at 27 "C isomerizes promptly to t-butyl cation before it undergoes a hydride ion transfer reaction (21).…”