Ethane, dissociation reaction

Heavier aldehydes behave similarly, eg ethanal dissociates to methane, carbon oxide, and two different sets of free radicals. The reaction mode depends on the... 

Unimolecular reactions are in principle the simplest types of chemical reaction that can occur in the gas phase because the reaction formally involves only one molecule. There are several different kinds of unimolecular reaction, but most can be considered either to be dissociation reactions, such as the dissociation of ethane to methyl radicals,... 

Figure 4. Proportional depletion by reaction with energy-dependent k2 E) and according to the Lindemann-Hinshelwood model (constant kz). The curve has been calculated at 999 K for ethane dissociation at the transition pressure.

We shall consider in this review mainly the information to be obtained from ka on unimolecular dynamics. We shall in this context first review the ethane dissociation-methyl radical recombination system, to show what can be done for such a well-studied model reaction and what could eventually be done for other reactions as well. We shall then give a short summary for the most recent results on various classes of unimolecular reactions. We are afraid that this enumeration is not complete, and we apologize for any important omissions. Mechanistic and physical organic aspects, which have previously been extensively reviewed, will not be discussed. 

PI2.4 At high temperatures, ethane dissociates via the reaction (ethane cracker)... 

The reaction shown above for the steam reforming of methatie led to die formation of a mixture of CO and H2, die so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which medrane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. 

Ethane, C2H , dissociates into methyl radicals by a first-order reaction at elevated temperatures. If 250. mg of ethane is confined to a 500.-mL reaction vessel and heated to 700°C, what is the initial rate of ethane decomposition if k = 5.5 X 10 4 s-1 in the rate law (for the rate of dissociation of C2H6) ... 

Ethane, C2H6, dissociates into methyl radicals at 700.°C with a rate constant k = 5.5 X 10-4 s 1. Determine the rate constant at 800.°C, given that the activation energy of the reaction is 384 kj-mol-1. 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

The reaction with oxygen has been used to measure the rate of dissociation of the ethane.7... 

The dissociation is the slow step that fixes the rate of the over-all oxidation reaction, but only if an inhibitor is added to remove the intermediate triphenylmethylperoxy radicals. Otherwise they contribute to the disappearance of the ethane by attacking it directly rather than waiting for additional free triphenylmethyl radicals. 

The product of the latter reaction, diphenylbifluorenyl, is almost entirely undissociated in spite of the ease with which the radical is formed in ways other than the dissociation of the ethane.25... 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The rate of dissociation has been measured by oxygen uptake in the presence of an inhibitor of chain reactions as in the case of hexaaryl-ethanes. Since the uptake of oxygen obeys the same kinetic law, it is a reasonable extrapolation to suppose that here too the rate-determining step is a dissociation into radicals. When one of the phenyl groups in triphenylmethyl is replaced by a cyclohexyl group, the rate of dissociation of the ethane is reduced by a factor of 170.38 Some dissociation rate parameters are given in Tables III A and B. 

Barnett and co-workers recently reported that it might be possible to utilize hydrocarbons directly in SOFC with Ni-based anodes. " ° First, with methane. they observed that there is a narrow temperature window, between 550 and 650 °C. in which carbon is not as stable. The equilibrium constant for methane dissociation to carbon and Hz is strongly shifted to methane below 650 °C. and the equilibrium constant for the Boudouard reaction, the disproportionation of CO to carbon and COz, is shifted to CO above 550 °C. Therefore, in this temperature range, they reported that it is possible to operate the cell in a stable manner. (However, a subsequent report by this group showed that there is no stable operating window for ethane due to the fact that carbon formation from ethane is shifted to lower temperatures. ) In more recent work, this group has suggested that, even when carbon does form on Ni-based anodes, it may be possible to remove this carbon as fast as it forms if the flux from the electrolyte is sufficient to remove carbon faster than it is formed.Observations by Weber et al. have confirmed the possibility of stable operation in methane. Similarly, Kendall et al. showed that dilution of methane with COz caused a shift in the reaction mechanism that allowed for more stable operation. 

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