Ethane formation

Another pathway of ethane formation is the disproportionation equivalent to reactions 23 and 40. 

The photochemistry of Titan s atmosphere can be summarized as follows the unsaturated compounds are formed from HCN and C2H2, which is derived from CH4. Methane decomposition leads to further ethane formation. 

Since deuterium addition to ethylene yields C2H4D2, both of the last steps are irreversible and (13) can be taken as the rate-determining step. Accordingly, if all prior steps are at equilibrium, we can write for the rate R of ethane formation ... 

The rate of ethane formation in ethanol dehyration (C2H5OH — C2H4 + H20) on the Nb dimer/Si02 catalyst is accelerated 1.7 times when C2H5OH is replaced by C2D5OH. The substitution of the OH hydrogen with deuterium gives no effect on the rate. Explain the inverse isotope effect. 

The rate of ethane formation was used to obtain kinetic data on this reaction. The effect of coordination on the reaction is primarily to vary the substrate and perhaps retard the reaction somewhat by a slight depletion of electron density from the ortho position. 

The chief evidence for reaction (38) was that upon addition of 02 to a few millimeters pressure to the reaction mixture ethane formation was almost entirely eliminated whereas methane and ethylene formation were much less drastically reduced, the assumption being that 02 would suppress methyl reactions but not methylene reactions (see Sec. IV-C). 

When a solution of 19 and excess HCN was warmed to 25°C, rapid formation of propionitrile was observed along with a small amount of ethane. Ethane formation is accompanied by irreversible oxidation of 19 (vCN = 2152 cm-1) to nickel dicyanide complexes, which precipitate from solution and give a new broad IR band at 2170 cm . Product propionitrile (in CH2C12) appears at 2252 cm-1 with a shoulder at 2240 cm 1 due to (C2H5CN)NiL3. Most likely, this oxidation arises by attack of HCN on the 16-electron intermediate 19. 

While copper favours ethylene formation, iron and cobalt, when a sufficently high temperature is reached, favour ethane formation accompanied in one case by benzene hydrocarbons and in the other case by liquid paraffins. 

This can in pan be answered because Pt/alumina, e,g. EUROPT-3 (and Pt/Re/alumina) has also been studied [7]. In n-butane hydrogenolysis on Pt/alumina the accumulation of carbonaceous deposits on the catalyst surface suppressed ethane formation (i.e. relative to that of propane formation (i.e. S3), Thus for Pt/alutnina sites responsible for central C-C bond scission in n-butane may be selectively deactivated, e.g. at 603K sample S2 S3... 

A little work on structure-insensitive reactions has been reported [18]. Both catalysts were very active for ethene hydrogenation, and rapid deactivation occurred even at 176 K. Ethyne and 1,3-butadiene react in a more controlled manner study of ethyne hydrogenation using both l4C-labeled ethyne and ethene showed that ethane formation took place directly from adsorbed ethyne, without the intervention of gas-phase ethene. 

It is possible that some acetate radicals are formed by the direct discharge of the ions as, it will be seen shortly, is the case in non-aqueous solutions but an additional mechanism must be introduced, such as the one proposed above, to account for the influence of electrode material, catalysts for hydrogen peroxide decomposition, etc. It is significant that the anodes at which there is no Kolbe reaction consist of substances that are either themselves catalysts, or which become oxidized to compounds that are catalysts, for hydrogen peroxide decomposition. By diverting the hydroxyl radicals or the peroxide into an alternative path, viz., oxygen evolution, the efficiency of ethane formation is diminished. Under these conditions, as well as when access of acetate ions to the anode is prevented by the presence of foreign anions, the reactions mentioned above presumably do not occur, but instead peracetic acid is probably formed, thus,... 

Ethane formation in the ethylene oxide flame might then arise in a similar sequence, viz. 

They also reported that the reaction was of the first order. However, the work of Echols and Pease, Purnell and Quinn and Sagert and Laidler has shown that the order is quite close to three halves. Purnell and Quinn made a careful study of the rates of formation of the individual products of reaction for the ethane formation, for example, they find an order of f and an activation energy of 58.3 kcal.mole . ... 

Reduction in the quantum yield for the formation of methane resulting from the addition of inert foreign gases such as helium, neon, argon, nitrogen and carbon dioxide supports this suggestion of a hot radical mechanism, as does the observation by Harris and Willard that methane formation is enhanced at short wavelengths (1849 A). Souffle et have also proposed some ethane formation from the reaction of hot radicals by... 

Of the three chain terminating steps, they considered the ethane formation to be the most important one, and were able to deduce the overall reaction order of observed experimentally. 

Assuming ethane to be the main chain termination product, the rate of the initiation can be deduced from the rate of C2H6 formation. Eusuf and Laidler found the rate of ethane formation to be proportional to the concentration of acetaldehyde, concluding that the initiation reaction is second order. Reinvestigation of the reaction gave a first-order dependence of the ethane formation on the aldehyde concentration at high pressures, while an appreciable falling off could be observed at low pressures Come et found the rate of ethane... 

Since the order of the initiation reaction is nearly unity at high pressures, and is pressure dependent at low pressures, one expects the order of the termination step to be around 2, becoming pressure dependent at lower pressures. According to the results of Laidler and Liu 5,159 pressure dependence of ethane formation... 

McNesby et al could not detect methyl ethyl ketone at 500 °C, but found ethane and acetonyl acetone among the products. If ethane formation was the main termination step the decomposition should be order, while termination by the formation of acetonyl acetone gives an overall order of between 1 and i, depending on the relative rates of the elementary reactions. Experimental results clearly show that, at low pressures, the reaction order is f rather than 1. 

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 . 

Finally it is to be noted that allyl alcohol does not completely suppress ethane formation, which indicates that ethane may also be formed in a non-radical process of some sort. 

Radical scavengers suppressed almost entirely the formation of CjHe and CO. Ethane could not be detected, and the formation of CO was also strongly diminished in the presence of butadiene at 100 Iodine suppressed ethane formation in the experiments of Pitts and Blacet at both 3130 and 2654 A wavelengths at a temperature of 100 °C the same was found by Benson and Forbes at 140 °C and 2537 A with a flow system. In the experiments with added HI at 3130 A, no ethane formation could be observed over the temperature range 126-295 C, and the amount of CO formed below 235 °C did not exceed the limits of experimental error A few torr of HBr almost entirely suppressed the formation of C2H6 around 3130 A and at 150 °C ". The distribution of isotopic species in the ethane sample obtained in the photolysis of mixtures of ordinary and deutero-acetone verified the free-radical origin of this product . 

The quantum yields of acetone consumption and ethane formation decrease with increasing intensity. The dependence of the C2H6/CO ratio on light intensity can be demonstrated in a very wide intensity range if the results of the mercury-sensitized photolysis are also taken into consideration (Table 16). An increase in the quantmn yield of biacetyl formation with increasing intensity is apparent from the results of Herr and Noyes, and has also been confirmed by the direct measurements of Howland and Noyes. 

Of the numerous poisoning experiments that indicate heterogeneity of catalyst surfaces, reference may be made first to a study of selective poisoning made by Russell et al. (83,84). Russell and Ghering found that the rate of the hydrogenation of ethylene over reduced copper was not markedly affected by nitrous oxide but that the rate of ethane formation decreased rapidly as soon as the adsorbed nitrous oxide was decomposed. 

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