Unimolecular fission

Troe J 1983 Specific rate constants k(E, J) for unimolecular bond fissions J. Chem. Phys. 79 6017-29... 

The treatment of these simple associations directly follows that of the simple fission reactions discussed previously. For example, these reactions proceed via the formation of a loose transition state and without an activation energy barrier. The rates and rate parameters of simple associations can be determined either directly, by the application of bimolecular TST, or from their reverse, simple unimolecular fission reactions, through the use of the principle of microscopic reversibility. 

Again the analysis of these reactions can be undertaken either directly by the use of TST or by considering their reverse, the complex fission reactions discussed earlier. Since unimolecular reactions were treated in considerable detail before, we will not repeat related issues here. 

Many association reactions, as well as their reverse unimolecular decompositions, exhibit rate parameters that depend both on temperature and pressure, i.e., density, at process conditions. This is particularly the case for molecules with fewer than 10 atoms, because these small species do not have enough vibrational and rotational degrees of freedom to retain the energy imparted to or liberated within the species. Under these conditions, energy transfer rates affect product distributions. Consequently, the treatment of association reactions, in general, would be different than that of the fission reactions. 

The unimolecular reaction of the ion aggregate follows a similar course and the intermediate faces the same three possibilities for reaction. The rate of bond fission will not necessarily be the same as that of the free ion because the solvation environment has changed. We see this effect in the ion pair-catalyzed solvolytic reactions (7). In addition, since the reagent Y is in position before the five-coordinate intermediate is formed, the path by which X re-enters the coordination shell becomes less probable as a result of more effective competition by Y, and the rate is increased. 

Many of the esters which are hydrolyzed by the AalI mechanism in acid are also hydrolyzed with alkyl-oxygen fission under neutral condi-tions60,67 74 75 84 85 88 89. These reactions have the high enthalpies and entropies of activation characteristic of unimolecular reactions, and involve the ionization of (usually) tertiary alkyl esters, to the carbonium ion and a carboxylate anion in the rate-determining step, viz. 

The reverse reactivity is noted in the acid-catalyzed hydrolysis of the esters. Pyrrole-3-carboxylic esters are hydrolyzed upon dissolution in concentrated sulfuric acid and subsequent dilution with ice. Evidence has been presented indicating that unimolecular acyl—O fission forms the resonance-stabilized pyrrolyl acylium ion (B-77MI30505). 

A or B acid or base catalysis AC or AL acyl-oxygen or alkyl-oxygen fission 1 or 2 unimolecular or bimolecular. E10B designates unimolecular elimination through the conjugate base. 

The decomposition mechanism for DADNE can be expected to be quite different from that for RDX and HMX. Even though DADNE has the same stoichiometry as RDX and HMX, structurally it bears little similarity to these molecules. Thus, possible reaction pathways involving unimolecular decomposition of these compounds would differ from those studied in DADNE. Instead of C-N02, N-N02 bond dissociation would occur, and the ring geometries would present the possibility of symmetric ring fission. 

The initial decomposition chemistry involves unimolecular reactions. This was the conclusion of the first gas-phase kinetics study [84] and has been repeatedly confirmed by subsequent bulb and shock-tube experiments [85, 86]. That first study used shock heating to induce thermal decomposition [84], The data were interpreted in terms of simple C-N bond fission to give CH2 and N02. A more extensive and definitive shock-tube study was reported by Zhang and Bauer in 1997 [85]. Zhang and Bauer presented a detailed kinetics model based on 99 chemical reactions that reproduced their own data and that of other shock-tube experiments [84, 86]. An interesting conclusion is that about 40% of the nitromethane is lost in secondary reactions. 

The activation mechanism of phosphosulfate linkages (P—O —S)has been studied to understand the chemistry of biological sulfate-transfer reactions of phosphosul-fates of adenosine (APS and PAPS). Several phosphosul-fates were prepared and subjected to several nucleophilic reactions including hydrolysis. In general, phosphosulfates are stable in neutral aqueous mediay but become labile under acidic conditions, resulting in selective S—O fission. This S—O fission appears to occur by unimolecular elimination of sulfur trioxide, which can react with a nucleophilic acceptor, leading to a sulfate-transfer reaction. This process can be accelerated by Mg2+ ion when the solvent is of low water content. Under neutral conditions, divalent metal ions also were found to catalyze nucleophilic reactions, but these occurred on phosphorus to result in exclusive P-O fission. 

Reactions of 2-Pyridylmethylphosphosulfate (23). As illustrated in Figure 16, two modes of S—O fission are conceivable for the reaction of 2-pyridylmethylphosphosulfate (PMPS) (a) A divalent metal ion may bridge pyridyl and phosphoryl groups, leaving the sulfate group free. Such chelation would lower the pKa of the leaving phosphoryl group and assist S—O fission by either unimolecular or... 

Secondly, the rate coefficients of unimolecular bond fissions and of bimolecular combinations depends, not only on the temperature, but also on the concentrations of the species which are not chemically transformed by the elementary process under consideration, but which play a role in energy transfer processes. Various theoretical treatments of this effect have been suggested (see, for example, refs. 1—15). 

In this mechanism, pH is the organic compound which decomposes thermally, YH another compound with a labile H atom, m and 3H are the main reaction products (if chains are long), p- is a chain carrier free radical which can decompose by an unimolecular fission, whereas chain carriers of the / type cannot decompose and can only react in bimol-ecular processes. It is assumed, at least as a first approximation, that the radicals Y are thermally stable, i.e. their decomposition by unimolecular fission can be neglected. Notice that transfer processes, intermediary between initiation and propagation processes are not written indeed, these processes can be neglected if chains are long, i.e. at low temperature. 

Let us give some examples of the various types of free radical involved in hydrocarbon pyrolyses. H- and CH3- radicals are / type radicals, since they cannot decompose by C—C or C—H bond breaking. Alkyl radicals (except for H-, CH3-) are p- type radicals, since they can decompose by C—C or C—H bond unimolecular fissions. Ally lie radicals are Y type radicals, since they are stabilized by resonance. 

At high temperatures, both simplifications and complications of the above mechanism occur. Bimolecular initiation processes (involving at least one unsaturated molecule) cannot be excluded (see, for example, ref. 15). Transfer processes must be included since chains are no longer long. H abstraction from alkenes generates not only allylic type radicals, but also vinylic type radicals. As the temperature increases, allylic type radicals become thermally unstable. As the activation energy of unimolecular fissions of radicals is much higher than that of bimolecular processes such as metatheses, when the temperature increases the relative concentration of the p- radicals, compared with that of the thermally stable / and Y- radicals, decreases. Therefore, termination processes involve mainly / radicals (except for H- radicals, because they are very reactive and recombine in a third-order process) and Y-radicals. Finally, the addition of the most concentrated / and Y- radicals to unsaturated molecules can play a role, because this process is followed by a very fast unimolecular fission. For reasons of size limitation, the addition of radicals (e.g. H- and CH3-) will mainly be considered. Of course, the above a priori hypotheses about relative radical concentrations or reaction rates must be checked a posteriori, when numerical calculations have been carried out. 

Propargyl cations may also be generated by heterolytic fission of the C—X bond of allenyl derivatives (178). Early attempts to solvolyse chloride 178 (R = H R = Me) in alcoholic solution, even in the presence of silver nitrate, did not furnish reliable conclusions concerning the possibility of unimolecular solvolysis through the propargyl cation (Pudovik, 1951 Hennion and Maloney, 1951). The apparently slow... 

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