Ethyl radicals, disproportionation

One of the most striking features of this work was the observation that methyl radicals were produced in the flash photolysis of Pb(C2H6)4 and Hg(C2H6)2. Thrush s interpretation of this finding was that an excited ethyl radical disproportionated with another producing a very highly excited C2H6 molecule. 

Figure 5.23 reprinted with permission from Doubleday C, J Mclver, M Page and T Zielinski. Temperature Dependence of the Trcmsition-State Structure for the Disproportionation of 1 lydrogen Atom with Ethyl Radical. The Journal of the American Chemical Society 107 5800-5801. c,1985 American Chemical Society. 

If other possible termination steps are considered, then it can be inferred that recombination or disproportionation of ethyl radicals, c,H5 is more likely than recombination of C and. Step 5... 

Large volumes of gas are generated, primarily ethane and ethylene, from the disproportionation of the ethyl radicals produced in the reaction of ethylmagnesium bromide with ferric chloride. The reaction should be carried out in an efficient hood, or else a tube should be run from the top of the reflux condenser to a hood. 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

Butenyl (C4H7) mainly disappears by reacting with ethyl radicals. The leftover ethyl radicals either recombine (faster process) or undergo disproportionation (slower process) ... 

The study of the recombination (kc) and the disproportionation (/tdis) of ethyl radicals proved the dependence of the ratio kdis/kc on the solvent [230-232], Stefani [231] found that log( dis/ c) is a linear function of the solubility 8S, which is equal to the square root of the cohesive energy density of the solvents (Dce) ... 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

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... 

To explain these results Bradley et al. (13) postulated that disproportionation of ethyl radicals increased relative to combination when hot ethyl radicals were involved in the process. At higher pressures of ethylene, or neon, the hot radicals were deactivated and C4H10 approached the values observed with thermalized ethyl radicals (for example, in the photolysis of diethyl ketone). 

An alternative explanation can be proposed for the observed pressure dependence of the C2H6/C4Hio ratio. This involves again hot ethyl radicals but not participating directly in disproportionation. The required mechanism is similar to the one adopted by Rabinovitch et al. (85), and, still more recently, by Heller and Gordon (56). This mechanism does not postulate inefficient deactivation of hot radicals and... 

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 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 ... 

CH3CHCHD and not at all from CH2CHCD3. Ethylene is also formed by disproportionation of ethyl radicals produced in primary processes and perhaps by the decomposition of the excited ethyl radical. 

For example, for ethyl radicals combination leads to the formation of butane and disproportionation to ethane and ethylene. 

The l-ethyl-4-carbomethoxypyridinyl radical disproportionation products, aside from the pyridinium ion, probably included ethylamine, and a compound thought to be an ester dialdehyde (Eq. 31). The pH changes noted after radiolysis of buffer-free isopropyl alcohol-water solutions of the pyridinium ions revealed that the initial dihydropyridine 4 H) formed from 4 hydrolyzed and consumed another proton, presumably because of ethylamine. 

It arises from 54 by cyclization and retro-Claisen loss of an ethoxycar-bonyl group. Brettle suggested that 54 was formed from radical disproportionation followed by Michael addition of the carbanion to the resulting ethyl methyleneacetoacetate. Alternatively, 52 could likewise be derived from a-carbonyl cation intermediates formed by one-electron oxidation of 51. 

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. 

The recombination and disproportionation of ethyl-radicals are considered to be the main termination processes, viz. 

The contribution of this step to the overall aldehyde consumption was estimated to be less than 10 %. It must be mentioned, however, that at temperatures where the rate of decomposition and disproportionation of the ethyl radicals is similar a positive curvature is to be expected even without the inclusion of the molecular decomposition process (10). 

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