Dissociation of ethanes

Relationships between Models and Actual Reactions. The dissociation of ethane to give methyl radicals corresponds most closely to Model... 

Thus in the free radical several valency bond structures arc possible in addition to the normal resonance structures of benzene which are still present in /, and resonance amongst all these states will lead to an increase in the stability of the free radical. Consequently, for the dissociation of dibenzyl less energy is required than for the dissociation of ethane because... 

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,... 

The earliest studies of the photolysis of ethane suggested that the dissociation of ethane to two methyl radicals and to an ethyl radical and a hydrogen atom were the primary steps of the process. The more recent work has shown that the photolysis is much more complicated, with several primary steps involved. As a basis for discussion the following reaction scheme is used. 

The method FIND-ALL-PATHWAYS begins by calling the composite operators that constitute the domain of free-radical operations on the substrates specified by the keyword argument -.substrates. These operators loop over the substrate list and evaluate properties specified by Kfree-radical- Homolytic dissociation of ethane is initiated when FEASIBLE-P, a compound predicate method of (Kmitiation... 

Many statistical models have been applied to reaction (3.1), and it might be considered a test case for theoretical treatments of the rate constant. The process inverse to (3.1), the dissociation of ethane, has also been extensively studied experimentally25,26 and theoretically.116,2 2,27 The theoretical predictions for the rate of dissociation are, of course, quite sensitive to the value of the bond dissociation energy. On the other hand, recombination rates depend only weakly on that quantity. In the present review, attention is focused on the prediction of the recombination rate using the transition state theory outlined in Section IIC. First, the high-pressure limit of kr, denoted by kK, is considered, particularly its temperature dependence. This is followed by a brief description of some results for the pressure dependence of kr and for the dissociation of a vibrationally excited C2H6 molecule. 

Dissociation of Ethanes to Give Trivalent Carbon Free Radi... 

DISSOCIATION OF ETHANES TO GIVE TRIVALENT CARBON FREE... 

The presence of hydrocarbon impurities has been shown to affect the oxidation of carbon monoxide [22] and the decomposition of carbon dioxide [23]. It has been reported that the dissociations of ethane [24] and butane [25] at the elevated temperatures and typical densities of shock tube experiments are in the low pressure region with activation energies that are much less than their respective high pressure limit values. The reaction of p.p.m. levels of hydrogen atoms with the molecule under investigation can result in a low apparent energy for dissociation due to the increased importance of abstraction steps. 

In the more complex dissociation of ethane into two methyl radicals, there are 12 conserved and 6 transitional modes. The conserved modes are associated with the CH stretch and bending motions of the two methyl fragments. The fate of the conserved and disappearing vibrational modes is shown in table 7.4. The 6 transitional modes for... 

In the dissociation of ethanal,2-diol, CH(0H)2CH=0 to C (OH)2CH=0 + H, We observe a very small, (only 75.4 kcal mol ) RC—energy in the dihydroxy C—bond. This can be explained with resonance of the carbon radical with a carbonyl group. We have calculated this energy with the G3 method to verify the data and find a similar value, 75.1 kcal mol. We have not observed this effect with species containing only one hydroxy (OH) and the carbonyl and look to study on simple dihydroxy alkanes to fiirther, understand this effect of a second OH. 

Apart from the original use of equation (53) by Bunker and Pattengill, there have been recent applications to several systems. It has been applied to kinetics of dissociation of ethane and recombination of methyl radicals. 

Dissociation of Ethane and Recombination ot hfefliyi Radicals.—The reaction system CiHc 2CH3 is particularly suited for detailed investigations. It may be considered to be the simplest fission of the alkyl C—C bmd. Its rate has oftmi been taken as a reference for other radical reactions. Also, this reaction has been a testing ground for various theoretical model calculations for quite a long time. 

H. von Koch, Dissociation of ethane molecule ions formed in charge exchange collisions with positive ions. Ion-molecule reactions of ethane, Arkiv Fysik 28, 559-574 (1965). 

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. 

Heating of ZnZSM-5 zeolite with adsorbed ethane at 423 K results in heterolytic dissociation of ethane with the formation of a Zn-ethyl fragment and that the bridging hydroxyl group. Prolonged evacuation of the sample with adsorbed ethane at 523 K results in the formation of ethylene and an appearance of a Zn-H bond characterized by the stretching vibration at 1934 cm [26], The reactions described by Eqs. 20.3 and 20.4 were studied and structures of I-VI were calculated. These... 

Fig. 20.26 Heterolytic dissociation of ethane on the hydride form of the model cluster (Fig. 20.25)...

The bond-dissociation energy of a bond in a molecule is the energy required to break that bond alone —that is, to split the molecule into the two parts that were previously connected by that bond. For example, the bond-dissociation energy of the C—C bond in ethane, H3C—CH3, is the enthalpy of dissociation of ethane into two methyl radicals, CH3. 

For instance, the dissociation of ethane into two methyl radicals, p.ff) is due to the vibrational and rotational motion of the methyl fragments. When there is a large increase in rotational motion in the transition state... 

This will be illustrated with numerical examples for the dissociation of ethane and fluoroform. Figure 4.12a shows the computed geometries of ethane as reactant in its transition state and its dissociated products (Gilbert and Smith, 1990). In the transition state the carbon-carbon bond length is significantly increased and the CH3 groups rotate almost freely. Typical pre-exponential factors are in the order of lo -io sec-. ... 

G. Transition state theory for unimolecular reactions. In the high-pressure limit one can assume that the energy-rich species A has reached thermal equi-libriiun. (a) Verify the TST result for the rate of unimolecular dissociation k(T) = (ytBr/A)(gV0exp(— o)wheregis the partition function forAand 0 is the partition function for the transition state, (b) This result looks just like the TST expression for the bimolecular thermal reaction rate constant. But this cannot be. A imimolecular reaction rate constant has different dimensions from a bimolecidar one. Resolve this dilemma, (c) The thermal dissociation of ethane. 

H. F. Winters, Dissociation of ethane by electron impact, Chem. Phys. 36 353 (1979). 

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