Ethyl radical, combination reaction

The results listed in Table 19 have been determined by methods analogous to those used to study methyl and ethyl H-abstraction reactions. The yields of the abstraction product, propane or cyclopropane, must be corrected for the disproportionation reaction. The reference reaction is the radical combination reaction. For n-propyl radical reactions TABLE 20... 

Reaction enthalpies, AHr, and reaction free enthalpies, ACr, at 298.15 K estimated via DFT calculations (LIB3LYP, 6-3lG(d,p)) for the addition of an ethyl radical to ethyl dithiobenzoate, for several radical-radical combination reactions, and for the missing step reactions of the four cross-combination products (4a, 4b, 4d, and 4 ) with an ethyl radical 1. The reaction enthalpy and reaction free enthalpy values are given in units of kj mol . ... 

The use of PbEt4 as an anti-knock agent in petrol depends in part on the ability of the ethyl radicals, generated on its thermal decomposition, to combine with radicals produced in the over-rapid combustion of petroleum hydrocarbons chain reactions which are building up to explosion (knocking) are thus terminated short of this. The complete details of how PbEt4 operates are not known, but there is some evidence that minute Pb02 particles derived from it can also act as chain-stoppers . 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodide, and thus is a good initiator for iodine atom transfer reactions [13,33,34]. Indeed, the ethyl radical, issued from the reaction of triethylborane with molecular oxygen, can abstract an iodine atom from the radical precursor to produce a radical R that enters into the chain process (Scheme 13). The iodine exchange is fast and efficient when R is more stable than the ethyl radical. 

A highly economical production of ethyl chloride combines radical ethane chlorination and ethylene hydrochlorination.185 186 Called the Shell integrated process, it uses the hydrogen chloride produced in the first reaction to carry out the second addition step ... 

In an attempt to explain apparent inconsistencies in the ratio of disproportionation to combination of two ethyl radicals obtained in different investigations, Bradley et al. (13) carried out in 1956 a comparative study of the behavior of ethyl radicals produced by direct photolysis of diethyl mercury and by addition of hydrogen atoms to ethylene. The latter process involved the reactions... 

The ratio of the products G IR/C4Hi0 was believed to be equal to the ratio of the rates of disproportionation and combination of ethyl radicals (kf,/kt). The alternative ethane producing reaction sequence [reactions (7) and (8)], postulated previously by Smith et al. (96), was concluded... 

In the interpretation based on reactions (13)-(22) the observed increase in C2H6/C4Hio is due to combination of (thermalized) ethyl radicals with H atoms. The concentration of H atoms increases at lower pressures as a result of increased decomposition of the hot ethyl radicals and as a result the ratio C2H6/C4Hi0 also increases. It is of interest that Bradley et al. had indications that some propane was formed, which, if true, would suggest that C2H6 and H did interact to some extent at least. With improved analytical techniques Heller and Gordon (56) have been able to measure the small amounts of propane formed under their experimental conditions. 

The reaction of NaPb (and other sodium-lead alloys) with ethyl chloride undoubtedly proceeds via a free radical sequence, in which the alloy reacts to form sodium chloride and ethyl radicals ethane, ethylene, and butane are by-products formed by combination and disproportionation of the ethyl radicals. Shushunov and his colleagues 230,296) have carried out a detailed investigation of the kinetics of the NaPb-C2H5Cl reaction. The reaction sequence below was proposed for the ethylation reaction ... 

The recombination of bromine atoms can not occur except on collision with a third body but reaction between the more complicated ethyl radicals might be expected without the aid of walls or triple collisions. In any reaction involving the production of free ethyl radicals one might expect to find butane, ethane, etc., but actually very little of these products is found. Apparently in this case radicals of this type are not likely to combine with each other. 

The second reaction is unlikely to be pressure-dependent in contrast to the corresponding reaction for the acetyl radical. The ethyl radicals formed, unlike the methyl radicals, can disproportionate as well as combine ... 

The combination disproportionation ratio is known for this reaction so that from the rate of butane production it is possible to calculate the total rate of this reaction between ethyl radicals. 

Hydrogen is a significantly less important product than ethane reaction (2) therefore occurs more rapidly than (6), so that the ethyl radicals are essentially jS radicals. First-order initiation and pp combination leads to f-order kinetics, in agreement with experiment. Application of the steady-state treatment gives, for the overall rate of disappearance of butane... 

The mechanism of the butane formation was shown to be the combination of ethyl radicals. The yield of butane is reduced to nearly zero by the addition of NO to the reaction mixture. This observation has been confirmed by Ausloos et and by Akimoto et This ruled out the insertion of either CH3CH or C2H4 as the source of butane, since NO would not be expected to inhibit these reactions. Furthermore, the isotopic content of the butanes formed in the photolysis of C2H6-C2D6 mixtures was found to be d, d, d, dj and a trace of ethyl radicals which combine to form the n-butane arise from the addition of hydrogen and deuterium atoms to ethylene formed in the photolysis. 

The formation of propane was attributed to combination of methyl and ethyl radicals. Obi and Tanaka have reexamined the formation of propane in the photolysis of deuterated ethane at 1470 A, and have confirmed on the basis of the isotopic distribution of the propanes formed that it is the combination of radicals that produces propane. The alternative suggestion would be the formation of propane via insertion of CH2 into ethane. Both studies have ruled this out on the basis that the propane/methane ratio is greater than unity, a possibility which is not allowed if one methane is formed for each methylene as in reaction (3). The fact that propane/methane is greater than unity forces one to conclude that at 1470 A a primary process giving methyl radicals, reaction (14), must represent a small but significant fraction of the total primary processes, in order that methyl radicals are present to combine with the ethyl radicals. 

The radiolysis of methane in the solid phase has been examined in several studies. ESR studies by Smaller and Matheson and by Wall eta/. have shown that methyl radicals and hydrogen atoms are formed in nearly equal quantities and that Gchj = 0.9. A product analysis in a study by Ausloos et shows that hydrogen and ethane are almost the only products of solid methane radiolysis at 20 or 77 °K. It seems that ethane is formed by both methylene insertion and by methyl radical combination, while hydrogen is formed by direct elimination and by bimolecular processes. A small quantity of ethylene formed at 20 °K is absent at 77 °K this has been attributed to the reaction of hydrogen atoms with ethylene at 77 °K (but not at 20 °K) to give ethyl radicals and finally higher products which are observed in increased yield at 77 °K. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

Triethylborane in combination with oxygen provides an efficient and useful system for iodine atom abstraction from alkyl iodides and therefore is a good initiator for iodine atom transfer reactions.6 Indeed, the ethyl radical, issuing... 

Since no butane is observed as a photolysis product, and since the accepted value for the ratio of rate constants for disproportionation to combination reactions for ethyl radicals is 0.12 (45), ethyl radical interactions may be ruled out as a source of ethane. Therefore, the ethane probably arises from an abstraction reaction following the initial dissociation,... 

Data listed in Table 19 for H-abstraction reactions of propyl radicals (n-propyl, isopropyl and cyclopropyl) have been recalculated from the original resulte on the basis of log(fe/l mole sec ) = 8.6 for the auto-combination reactions of all three radicals. This is in keeping with the recent experimental rate coefficients for isopropyl combination [264] and for ethyl combination [248, 250]. 

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