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A method for the location of the positions of carbonxarbon double bonds using high pressure mass spectrometry is proposed. A known olefinic molecular ion reacts with a second, unknown olefin to form a four-centre complex, which fragments with retention of the structural identity of methylene and substituted methylene groups to eliminate a new olefin molecule and to form an unsaturated ion from which the position of the double bond in the unknown olefin can be inferred. Vinyl methyl ether proved to be a convenient reagent gas and its molecular ion undergoes the required reaction with several classes of olefinic compound. Conjugated dienes and unsaturated compounds containing electronegative groups do not undergo this reaction.

The kinetics of a number of gas-phase chloride transfer reactions R,+ + RCI = R,CI + R+ were measured with a pulsed electron high pressure mass spectrometer. Most of the reactions were found to occur near the collision limit, i.e., with rate constants k = m~lecules-~ cm3 s-'. However, several reactions were much slower and were exhibiting negative temperature dependence, Le., decreasing rate with increasing temperature. Hydride and chloride transfer equilibria were determined for various carbocations Rt (R = isopropyl, cyclopentyl, tert-butyl, 1 -methylcyclopentyl, substituted benzyls, norbornyl, 2-methyl-2-norbornyl, and adamantyl). Excellent agreement with earlier hydride transfer measurements of Solomon, Meot-Ner, and Field were observed. The chloride affinities generally support conclusions based on the hydride transfer data. Discussion of the data in connection with benzyl cation substituent effects, the norbornyl cation stability, and solvent effects on carbocation stability is presented. The data show that the 2-norbornyl cation is unusually stable. Significant differences between carbocation R+ stabilities in gas phase and solution are found which suggest that both differential nucleophilic solvent stabilization and differential nonspecific solvation occur in solution.(1) Kebarle, P. Annu. Rev. Chem. 1977, 28, 445. (2) Yamdagni, R.; Kebarle, P. J. Am. Chem. SOC. 1976, 98, 1320. Wolfe, J. F.; Staley, R. H.; Koppel, I.; Taagepera, M.; McIver, R. T.; Beauchamp,

Can. J. Chem. 60,2325(1982.The kinetics and equilibria of the gas phase reaction [I] tert-C4H9+ + C6H6 = tert-C4HgC6H6+ were studied with a high ion source pressure pulsed electron beam mass spectrometer. Equilibria [I] could be observed in the temperature range 285-325 K. van't Hoff plots of the equilibrium constants led to AH? = -22f2 kcal mol-I and ASP = -49f 5 cal K-I mol-I. The rate constants at 305 K were k I f = 1.5 x molecules-2 cm6 s-I and k,, = 2.9 x 10-l9 molecules-' cm3 s-I. tert-C4H9C6H6+ dissociates easily via [Ir] not only because of the low dissociation energy (-AH?) but also because of the unusually favorable entropy (-ASP). The occurrence of transalkylation reactions: tert-C,HgC6H6+ + alkylbenzene = tert-C4H9 alkylbenzene+ + benzene, was discovered in the present work. D. K. SEN SHARMA, S. IKUTA et P. KEBARLE. Can. J. Chem. 60,2325 (1982). On a etudik la cinktique et I'kquilibre de la rkaction en phase gazeuse [I] tert-C4Hg+ + C6H6 = tert-C4HgC6H6+ en faisant appel6 un spectromttre de masse a faisceau d'klectrons pulsks et ayant une source d'ions haute pression. Les kquilibres [I] peuvent Etre observes dans I'intervalle de tempkrature de 285-326 K. Les courbes de van't Hoff des constantes d'kquilibre conduisent a = 22 + 2 kcal mol-' e t a ASP = -49 + 5 cal K-I mol-I. Les constantes de vitesse a 305 K sont: k I f = 1,5 x molkcules-2 cm3 s-I et k , , = 2,9 x lot9 molecules-' cm3 s-I. L'ion tert-C4HgC6H6+ se dissocie facilement via [Ir] non seulement parce que I'knergie de dissociation est faible (-AH?] mais kgalement par suite d'une entropie exceptionnellement favorable (-ASP). Dans cette ktude, on a decouvert la manifestation des rkactions de transalkylation: tert-C4HgC6H6+ + alkylbenztne = tert-C4Hg alkylbenztne+ + benzene.[Traduit par le journal]

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