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Ion-Molecule Reactions in the Condensed-Phase Radiolysis of Hydrocarbon Mixtures, 11. Cyclopentane and Cyclohexane’a P. Ausloos, A. A. Scala,Ib and S. G. Lias Contribution f r o m the Radiation Chemistry Section, National Bureau of Standards, Washington, D. C. 20234. Received March 13,1967 Abstract: The radiolysis of c-CsHloand c-CSHl2,as well as of equimolar c-CsHlo-c-CsDlo and c-C~%~-C-C~DI? mixtures, has been investigated in the presence of 0.5 to 5 mole % (CD,),, C2D2,C2D4,or CD3CDCD2. Isotopic analysis
of the products formed in the radiolysis of cycloalkane-(CD2), mixtures points to the occurrence of the following and (b) C,HZ,+ (CD& + C,H?,-l+ ion-molecule reactions: (a) C,H2,+ (CD2)3+ C,H2,-2+ CDsHCD2CD2H CDeHCDzCDz.The formation of n-propyl radicals in reaction b is confirmed by the production of CD~CDZCD~H as a major species in the radiolysis of C,D2,-(CD2),-H2S mixtures. The ion-molecule character of the reactions is further confirmed by the facts that addition of a known parent ion interceptor such as HnSreduces G(propane), while addition of an electron scavenger such as CCI, raises G(propane). When cyclopentane is the solvent, the H2-transfer reaction (a) is of major importance. The most efficient H2-acceptor molecule for the cyclopentane (1 : 1) mixtures are irradiated in the presparent ion is C2D1,and the least efficient is C2D2. When c-CsHlo-c-C5Dlo ence of deuterated H2-acceptormolecules such as C2Dl or C:D2, the ratio of products of reaction a, C2D4H&D6 is greater than unity, indicating the occurrence of charge-transfer processes favoring the formation or C2DaH2/C2D4, of c-C5Hlo+: (c) c-C5Dlo+ c-CsHlo+ c-CSHlo+ c-CsDlo. A decrease in the mole per cent of the C2D4or CZD? from 6 to 2% raises C2D,H2/C2D, or CzDzH2/CzD4 from 1.7 to 2.6, demonstrating that relatively high concentrations of Hz-acceptor molecules can interfere with the charge-transfer mechanism. When cyclohexane is the solvent, reaction b predominates over reaction a. A plot of the square root of the (CD& concentration cs. G(propane) yields a straight line which goes through the origin, demonstrating the difficulty of obtaining chemical evidence for the presence of free ions.
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n the first paper of this series,2 we presented experimental evidence for the occurrence in the liquidphase radiolysis of an Hz-transfer reaction of the type
+ C m t H m j l +CnH*n++ C m ’ H m J r + 2
CnH2n+?+
(1)
where CnH2n+2+ is the parent ion of 2-methylbutane or 3-methylpentane, and C , ~ H , ~ is ~ acetylene, ethylene, propylene, cyclopropane, 1-butene, or 2-butene. It was demonstrated that the efficiency of reaction 1 depended on the structure of C m ~ H , ~ ~Of. the abovelisted compounds, cyclopropane was shown t o be the most efficient H2-acceptor molecule, and 2-butene, the least efficient. I n the present communication, we intend to examine the occurrence of the Hz-transfer reaction in the radiolysis of cyclopentane and cyclohexane liquids containing various concentrations of cyclopropane or unsaturated hydrocarbons as solutes. It has already been demonstrated3 in the course of radiolysis and photoionization studies of cyclopentane and cyclohexane that, in the gas phase, the parent cycloalkane ion (CnHsn+) participates in the Hz-transfer reaction
+ CmjHmrT*CnH2n-zC + Cm,Hm’,+n
CnH*n+
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(2)
In these gas-phase studies, propylene was shown to be the most efficient Hz-acceptor molecule (C,,H,
