Novel Deviations From the 'Mixture Law' in the Radiolysis of Hydrocarbon Mixtures

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A mechanism is suggested for the radiolysis of hydrocarbons. Both ions and excited ~nole-cules are produced in the initial excitation. Ion-electron recombination leads to the formation of excited molecules which have similar properties to thosc formed initially; all subsequent decon~position proceeds through these excited molecules. 111 ~nixtures of hydrocarbons, ions and n~olecules may undergo positive charge exchange and additives reduce the hydrogen yield by capturing electrons or reacting with the positive ion. T h e mechanism provides a ratio~lalization for a variety of observatio~ls. This paper presents a mechanism for part of the radiolytic decomposition of hydrocarbons in which electron capture and charge exchange play a dominant role. There is no novelty in postulating these reactions a s mechanistic steps. The significance of electron capture has been discussed on inany occasions by I-Iamill and his colleagues (see, for instance, refs. 1 , 2, 3). Charge exchange was proposed by Hardxirick (4) as a inechanisn~ for the reactivity transfer observed in mixtures of aliphatic hydrocarbons and has also been suggested by Hamill. The crux of this proposed mechanism lies in the combination of two hypotheses, first, that the initial act of excitation leads to the formation of both ions and excited molecules and secondly, that, in pure hydrocarbons, the recapture of the electron by the positive ion also leads to the formation of excited inolecules which are, for practical purposes, identical to these forined in the initial excitation. The mechanism provides an explanation for a variety of phenomena; viz., the rapid inhibition of part but not all of the hydrogen yield by solutes, reactivity transfer in mixtures of aliphatic and alicyclic hydrocarboils, and the effects of additives on the isotopic composition of radiolytic hydrogen produced from lnixtures containing deuterated hydrocarbons. Tlze iWeclzanism( 1 ) Radiolysis of a hydrocarbon A produces, initially, positive ions, an equivalent nuinber of electrons, and excited, un-ionized molecules A*.The yields of ions and excited inolecules are comparable and probably 3/100 eV absorbed.( 2 ) The recapture of the electron by the positive ion produces an excited species which is, for the purposes of the present discussion, identical with A* produced in [lb].

Section: Charge Exchange Processesmentioning

confidence: 97%

Charge Transfer and Electron Capture in the Radiolysis of Aliphatic Hydrocarbons

Dyne¹

1965

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“…If one takes into account the scatter of the data, it appears that between 100 and 200" C g(HD) increases by, a t the most, 10%; a 20% increase would have been readily apparent. Part of the 1111 yield is for~iiecl in reaction [5], wl~icli is in co~ilpetition with reaction [3]; this conipetition is liltely to be temperature dependent. The rest of the I-ID comes fro111 reaction [GI, wliich (in dilute solutio~ls of the deuterated hydrocarbon) is not involved in any co~ilpetitive reactions.…”

Section: Cghi?-cgdg Mivt~lresmentioning

confidence: 99%

“…The rest of the I-ID comes fro111 reaction [GI, wliich (in dilute solutio~ls of the deuterated hydrocarbon) is not involved in any co~ilpetitive reactions. If reaction [GI contributed significantly to tlie I-ID yield, then the temperature depende~lce of reactions [3] and [5] ~vould be maslted. Wiberg (7) has estimated that the ~n a x i~n u~l i effect due to the difference between C-1-1 and C-D bond-dissociation energies would lower kH/kD (-k,/k5) by about 25y0 between 100 and 200" C. If about half the I-ID yield came through reaction [GI, then the observed, very small, temperature coefficient is in accord with Wiberg's estimate.…”

Section: Cghi?-cgdg Mivt~lresmentioning

confidence: 99%

Vapor-Phase Radiolysis of Cyclohexane and Mixtures of Benzene and Cyclohexane

Blachford¹,

Dyne²

1964

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T h e vapor-phase radiolysis o f cyclohexane and benzene-cyclohexane mixtures has been studied with isotopic tracer techniques using CsDlr and CGDG. Benzene does not quench the molecular detachment o f hydrogen, CGDI? D! +. CGDIo, but interacts with the secondorder processes giving I-ID in CGD1?-Cr,H1? mixtures. T h~s ~nteraction is not H-atom scavenging and an ionic mechanism is preferred. T h e radiolysis o f all these systenls is very different from their liquid-phase radiolysis.'The low radiolytic yields in liquid benzene-cyclohexane mixtures are a classic example of 'protection'. T h e interactions causing these low yields are physical; they do not involve the ma1;ing or breaking of chemical bonds. This has been shown in two ways: by complete product analysis (1) and by techniques using isotopically labelled hydrocarbons (2). These isotopic techniques have revealed similar interactions in alkane mixtures (3) which are, however, absent in the vapor phase (4). As a result, we expect the vapor-phase radiolysis of benzene-cyclohexane inixtures to show marked differences froin the liquid-phase radiolysis.We have applied our isotopic techniques t o the vapor-phase radiolysis. In the liquid, benzene quenches both the first-order process, which gives D2 by molecular detachment from CGDl?, and the second-order processes which give HD in C G H~~-C G D~~ mixtures. I n the vapor, however, the D2 yield is reduced much more sloivly than would be expected if I-I-atom scavenging were occurring and so we favor an ionic mechanism for this interaction. Physical interactions quenching the first-order processes are absent. 1;isher 'Spectranalyzed' cyclohesane and benzene and Merck benzene-dG and benzene-free cyclohexane-d,z were used as received. T o minimize air contamination the sa~nple vessels (volume about 330 m l ) were evacuated t o 5x10-"mm Hg at 500' C for at least 24 hours before being filled and irradiated. Even so, the gaseous radiolytic products contained about 3% air. T h e vessels were sealed t o a grease-and mercury-free preparation line; the sample (about 2 1111 o f liquid) was degassed b y repeated freezing, pumping, and thawing and was then distilled into the irradiation vessel. During irradiation in our Co60 source, t h e vessels were kept at a constant temperature ( 2~2 ' C ) in a n electric furnace. Using the techniques already described ( 4 ) , we found t h e dose rate t o be 4.S7XlOL6 ev/sec g cyclohexane; the dose in most experiments was 3.61 X 101%v/g organic vapor.After

“…This effect was first observed in alkane solutions of cyclohexane-dl2 (C6D12) where the changes in reactivity were detected through the variation in the Dz and H D content of the radiolytic hydrogen (1). For example, the yield of Dz from a dilute solution of C6D12 in cyclopentane is about five times greater than that from a comparable solution of C6D12 in cyclohexane.…”

Section: Introductionmentioning

confidence: 97%

Deviations From the "Mixture Law" in the Radiolysis of Hydrocarbon Mixtures: Ii. Yields of Liquid Products in Mixtures of Cyclohexane and Cyclopentane

Dyne¹,

Denhartog²

1963

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The radiolytic yields of olefins and dilners have been measured in mixtures of cyclohexane and cyclope~lta~le. While the di~neric products ((C6H11)2, CsH11.CjHs) 'show a bias towards C j groups, the olefins show a bias towards Cs groups; the yield of cyclohexene a t low concentrations of cyclohexane is more than twice the maximum predicted by the "mixture law". These results confirm earlier isotopic work which showed a higher reactivity of cyclohexane-dlz in cq-clohexane-dIz -cyclopentane mixtures relative to that in cyclohexane-dlz -cyclohexane. The results also confirm the preferential change of identity of cyclohexyl radicals to cyclopentyl radicals, which leads to a predominance of Cg groups in the ditner.