Ethane formation from methyl radicals

The kinetics of the formation of ethane from methyl radicals in the flash photolysis of acetone vapour,72 the interaction of tin states of alkanones with... 

The work of Bell and Kistiakowsky (1962) provides a possible analogy to this situation. They found the dominant mode of formation of methyl radicals from the reaction between excited methylene and methane to be, first, insertion of methylene to give excited ethane, and then decay to give two methyl radicals. Hydrogen abstraction by methylene to give two methyl radicals was ruled out. The details of methyne formation may involve a very short-lived excited intermediate formed by attack of the carbon atom essentially normal to a carbon-hydrogen bond, followed by rapid scission of this bond before appreciable equilibration of the energy has taken place. 

The simplest way to account for these reactions is to postulate the obvious intermediate, CIONO, to calculate its rate of formation just as one would calculate the rate of formation of ethane from methyl radicals, and to calculate its rate of dissociation to the products just as one would calculate the rate of dissociation of ethane to (say) ethyl radicals and hydrogen atoms. These reactions, therefore, are just a generalisation of the standard chemical activation process [74.L], but in which the stabilisation product is not observed. 

The observed product distribution indicates that Reaction 4 consumes virtually all methyl radicals. The formation of ethyl radicals is rapidly enhanced by electrostatic fields, clearly evident from the large increase in the yields of butane and ethane. These result predominantly from Reactions 5a and 5b. 

Thus methyl radicals are consumed by other methyl radicals to form ethane, which must then be oxidized. The characteristics of the oxidation of ethane and the higher-order aliphatics are substantially different from those of methane (see Section HI). For this reason, methane should not be used to typify hydrocarbon oxidation processes in combustion experiments. Generally, a third body is not written for reaction (3.85) since the ethane molecule s numerous internal degrees of freedom can redistribute the energy created by the formation of the new bond. 

An example of radical coupling foUowing hydrogen abstraction by excited nitro-ethane from cyclohexane or diethyl ether in solution has also been reported Formation of -methyl-N-arylnitrones is observed during photoreduction (via electron transfer) of sterically hindered nitrobenzenes in triethylamine 39) ... 

An excited product is to be expected from radical-radical combination. The ethane produced in the recombination of methyl radicals must contain the net of the energy released by the carbon-carbon bond formation and the change in configurations of the methyl group. Unless stabilized, this hot molecule will revert to the reactants, viz. 

Norton and co-workers 102) briefly noted that photolysis of Os(CH3)2(CO)4 in hexane solution led to the formation methane but produced no ethane. Presumably, methane derives from photoinduced homolysis of an osmium-methyl bond and scavenging of hydrogen by the resultant methyl radical. 

Recent investigations on ethane formation in the photolysis of acetaldehyde indicate that decomposition into methyl and formyl radicals occurs from the triplet state which is also removed by first-order internal conversion and, to some extent, by second-order deactivation. In the mercury-photosensitized reaction methyl radicals are formed by direct dissociation of the excited aldehyde molecules, as well as by collision of excited mercury atoms . 

The bond dissociation energies in Table 1.2 (p. 21) show that 104 kcal of energy is needed to form vinyl radicals from a mole of ethylene, as compared with 98 kcal for formation of ethyl radicals from ethane. Relative to the hydrocarbon from which each is formed, then, the vinyl radical contains more energy and is less stable than a primary radical, and about the same as a methyl radical. 

Djega-Mariadassou et al. [16] reported the occurrence of incongruent melting and the formation of an unidentified phase during the decomposition of zinc formate. Additional products identified were methane, ethane, acetone and methyl acetate. From the results of qualitative analyses, it was concluded that reaction proceeds by several routes involving radical formation, shown [bracketed] ... 

We now turn to the energetics of 2-carbon species and commence with the saturated ethane derivatives. As a further desire for brevity, we will only consider the parent ethane and hexafluoroethane. The C-C bond strengths in ethane and hexafluoroethane are taken here to be the enthalpy of the homolysis reactions (equations 1 wherein X - H and F respectively, where the heats of formation, AHfP(q), of the ethane and of the methyl radicals are from ref. 3). 

In the presence of ethane, the methyl radical CH3 is sooner or later going to undergo a reactive collision with an ethane molecule it has little opportunity to do otherwise. This will occasionally result in the transfer of a hydrogen atom from an ethane molecule to the methyl radical, since the other possible reaction, the transfer of a whole methyl group, if it occurs, does not produce distinguishable products. This chain transfer reaction therefore involves the breaking and the formation of a C-H bond and is almost thermo-neutral. 

In another work, Wentrup (this time in collaboration with the Tarczay s group, from Budapest), explored the photochemistiy of dimethylcarba-moyl azide in argon matrices. Broadband irradiation of the compound, using both xenon and mercuiy lamps, was found to lead to the sequential formation of dimethylamino isocyanate (MeaN-NCO), 1,1-dimethyldia-zene (Me2N=N), and ethane, via consecutive N2, CO and N2 elimination reactions (Fig. 35). Ethane formation was ascribed to recombination of methyl radicals. The obtained photoproducts were identified with the aid of quantum chemical calculations. This was the first experimental... 

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