5037
Carbonium Ions in Radiation Chemistry. Reactions of t-Butyl Ions with Hydrocarbons’ P. Ausloos and S. G. Lias Contribution from the National Bureau of Standards, Washington, D. C. 20234. Received January 7, 1970 t-Butyl ions, generated by the y irradiation of neopentane, undergo hydride transfer reactions with alkanes having tertiary hydrogen atoms (t-C4Hs+ RH + i-C4H10 R+)to form isobutane and a tertiary carbonium ion as products. By irradiating neopentane in the presence of mixtures of two reactant molecules, one deuterated and one nondeuterated, the relative rates of the hydride transfer reaction for 22 Cs-Cs alkanes were determined with an unusually high accuracy, from the observed ratios of i-CaHsDto i-C4H10. Absolute rate constants for the reactions, based on a determination of the rate of depletion of the isobutane yield when ammonia is added, are given. The rate constants are generally very low [10-11-10-10 cm3/(moleculesec)], but are shown to depend on the heat of reaction and steric factors involving van der Waals interactions between the approaching ion and hydrogen atoms or methyl groups on the carbon atom 0 to the tertiary carbon. The evidence indicates that the hydride transfer reactions of the t-butyl ion have an activation energy, E,, 5 0.15 eV. In another series of experiments, unsaturated hydrocarbons were added to neopentane-isopentane mixtures, and the rates of the condensation reaction of these additives with t-butyl ions were determined by measuring the rate of depletion of the isobutane yield. The rate constants for the condensation reactions are in the range 10d”J-10-9 cma/(molecule sec). Some insight into processes occurringin the radiolysis of neopentane can be gained from these results. It is shown, for example, that in the absence of reactive additives, not more than about 1 4 x of the t-butyl ions undergoes processes resulting in the formation of isobutene. Abstract:
+
+
T
he hydride transfer reactions of small alkyl ions such as C2H5+and C3H7+with alkanes have been studied extensively.2a-c In addition, proton transfer and condensation reactions of these ions with unsaturated hydrocarbons and inorganic molecules such as ammonia are well known.2a-c The t-butyl ion is less reactive than the normal and secondary C1-C4 carbonium ions; the hydride transfer reactions between the t-butyl ion and alkane molecules Y-C4H9+
+ RH1 +i-C4HlO + RH’
(1)
are endothermic for many alkanes, and therefore d o not occur at a rate great enough t o compete with homogeneous or heterogeneous neutralization of the ion. Indeed, in an earlier study from this laboratory of the radiolysis of neopentane, we failed to observe any isobutane from reaction 1 in pure neopentane or in neopentane-cyclohexane mixtures, even though the t-butyl ion is the major fragment ion formed in the dissociation of the neopentane ion rieo-C6H12+ +t-C4H9+
+ CH3
(2)
(M[t-C4H,+]/N+ in the 70-eV mass spectrum, 0.41). Recently, a high-pressure mass spectrometric study 4 appeared in which the reactions of t-butyl ions with olefins were explored. In addition, a rate constant of 10- l 1 cm3/(molecule sec) was determined for the hydride transfer reaction (reaction 1) between the t-butyl ion and 2,2,4-trimethylpentane. Another study concerning the gas-phase radiolysis of neopentaneb has showed that t-butyl ions transfer a proton to ammonia. ( I ) This research was supported by the Atomic Energy Commission. (2) (a) F. W. Lampe, J. L. Franklin, and F. H. Field, Progr. React. Kinet., 1,67 (1961); (b) P. Ausloos, ibid., 5, 113 (1969); (c) 3. H. Futrell
and T. 0. Tiernan, “Fundamental Processes in Radiation Chemistry,” Wiley, New York, N. Y.,1968, p 171 ; (d) F. W. Lampe and F. H. Field, J . Amer. Chem. SOC., 81,3238 (1959). (3) S. G. Lias and P. Ausloos, J . Chem. Phys., 43, 2748 (1965). (4) M. S. B. Munson, J . Amer. Chem. SOC. 90, 83 (1968). (5) T. Miyazaki and S. Shida, Bull. Chem. SOC.Jap., 39,2344 (1966).
Since the earlier study of the radiolysis of neopentane was performed in this laboratory, our sophistication, in both experimental techniques and in the understanding of ion-molecule reaction mechanisms, has increased greatly. We therefore felt that a systematic examination of the ion-molecule reactions of the t-butyl ion might prove fruitful at this time. It was expected, for example, that the rate of the hydride transfer reaction between the t-butyl ion and larger alkanes containing a tertiary hydrogen atom might be measured precisely using the technique of end product analysis in deuterium-labeled mixtures.2b Variations in the observed rates with the structures of the reacting molecules should yield information about the rate controlling factors of such reactions. In addition, a further exploration of the rates of reaction of this ion with unsaturated hydrocarbons and with ammonia would complement the information on these systems in the literature.
Experimental Section All undeuterated hydrocarbons used in this study were American Petroleum Institute standard samples which were of sufficiently high purity that no further purification was necessary. All isotopically labeled materials used in this study were purified by gas chromatography and low-temperature distillation. The neo-C5Dlz contained 8% C5D11H; i-C5D12 contained 6z i-C5D11H. Mass spectrometric analysis indicated that at least 95z of the 2-methyl butane-24 and 3-methyl pentane-3-dl was correctly labeled with the deuterium atom at the tertiary position. The samples were irradiated in the NBS high-intensity “TO y-ray source, which, at the time these experiments were performed, had an intensity of 0.3915 X 10-8 eV sec-1 electron-1. The irradiation and analytical procedures were identical with those described before.3 The relative amounts of different deuterated isobutane products (i-CdH10, &C4H9D,i-C4HsDz? etc.) were determined Inass spectrometrically using standard cracking patterns.
Results Relative rates of reaction of the t-butyl ion with saturated alkanes were determined in this study using the technique of competitive reaction.2b Neopentane ions, Ausloos, Lias J Carbonium Ions in Radiation Chemistry
503 8 Table I.
Isobutane Formed in the Radiolysis of neo-CsH1z-i-CSDlpMixtures in the Presence of Hydrocarbon Additives.
i-C5Dl~ pressure,
tL
Relative yields---------
r
Torr
Additive
CHzDCD(CH3h
11.2 4.0 2.0 3.5 3.0 17.0 0 2.3 15.4 19.7 19.7
None C-CsHiz, 38.8 (CH,),CCHzCH,, 100.4 IZ-CjHiz, 57. 1 !Z-C$Hi4, 38.8 (C,H&CHCH3, 10.0 (C2H&CHCH3, 10.0 (CzHs),CDCH3, 10.0 (CH3)3CCH(CHdz, 2. 3 c-CbH,(CHa), 3 . 0 c-CsHg(CHs), 18.1 (CH~)ZCHCH(CH~)Z, 10.0
0,067 0.14 1.35 0.17 0.080 0.057 0.015 0.10 0.091 0.20 0.15
+
All experiments contain 39 Torr of neo-CsHln and 5
NO or
0 2
radical scavenger.
(CH3)zCD
(CH&CH
kilk3”
1 .oo 1 .oo 1.00 1 .oo 1.00 1 .oo 1.00 1.00 1 .oo 1 .oo 1 .oo
0.066 0.38 0.43 0.68 1.04 1.64 2.10 0.92 4.48 3.53
