Bond-fission Reactions

Bond-fission ReactkHis.—Tables S and 6 give rate constants for bond-fission reactions and the reverse recombinations, from recent investigations. We do not wish to comment on all of these in detail. The recent work on HNOa decomposition and the reverse recombination OH -l- NO2 is remarkable because of the 

One may hope that this important reaction will soon be as well understood as the recombination of methyl radicals. 

An interesting type of recombination reaction in excited electronic states is the excimer formation in the mercury-ammonia system. By measuring the relaxation times for the decay of the excimer emission, Call ear and co-workers were able to obtain the rate constants for the reaction  

Decompositions ot benzyl acetate , benzyl benzoate and a series of allyl acetate esters have been studied by the toluene carrier flow technique. First-order kinetics for reactant disappearance or for CO2 formation were assumed to apply to the initial bond rupture processes 

Kinetic results are given in Table 9. The toluene carrier technique often yields low values of the activation energies and 4-factors. Results for the benzyl esters, however, appear to be reasonably good. Note that the estimated reaction enthalpies compare favorably with the observed activation energies and that -factors are reasonable (AS 5 cal.deg mole ). Since only absolute decomposition rate coefficients were reported for the allyl esters, we have estimated the Arrhenius parameters on the assumption that the -factors were all 10 sec The activation energies so obtained are reasonable, since they compare quite favorably to those estimated by group additivities and the accepted heats of formation of the product radicals (Column 3, Table 9). 

In addition to the normal decomposition products of the allyl esters, about 3 % acetic acid was also observed . This product probably came from the normal ester elimination, viz. 

Values which follow have been calculated from rate coefficients at 485 °C assuming log A = = 14.7. 

This is about 9 kcal.mole higher than the ethyl acetate elimination activation energy, but is nevertheless reasonable since the vinyl (C-H) bond which is broken in the allyl ester reaction is close to 15 kcal.mole stronger than the sp (C-H)bond in ethyl acetate. 

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The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

Homolytic Bond Fission Reactions. The homolytic fission of a chemical bond... 

It is clear that if homo lytic bond fission reactions are going to be useful, they must have almost perfect quantum yields and a Ag° between 100 and 150 kJ mol l. One reaction which might work is... 

Scheme 7.44 Monoruthenium complex-catalyzed bond fission reactions of propargylic ethers.

This transformation was first explored by treatment of l-bromo-4-(cyanomethyl)pentacyclo-[4.3.0.02 5.03-8.04-7]nonan-9-one ethylene acetal (61) with lithium diisopropylamide in tetrahy-drofuran at 0 °C, which resulted in almost quantitative yield of an inseparable mixture of two alkenes to which the structures l-bromo-4-cyanomethyltricyclo[4.2.1.02,5]nona-3,7-dien-9-one ethylene acetal (66) and l-bromo-4-(cyanomethylene)tricyclo[4.2.1.02,5]non-7-en-9-one ethylene acetal (67) were assigned.170,171 As illustrated below, the overall cage-degradation reaction can be mechanistically represented by the stepwise C —C bond fission reactions, whose driving force can be attributed to the apparently substantial reduction in cage constraint. 

Brouwer, L., Cobos, C.J., Troe, J., Dubai, H.-R., and Crim, F.F. (1987). Specific rate constants k(E, J) and product state distributions in simple bond fission reactions. II. Application to HOOH —> OH+OH, J. Chem. Phys. 86, 6171-6182. 

Reisler, H. and Wittig, C. (1992). State-resolved simple bond-fission reactions Experiment and theory, in Advances in Chemical Kinetics and Dynamics, ed. J.A. Barker (JAI Press, Greenwich). 

Whereas the chain propagation reactions are in radical balance, and the bond fission reaction creates two free radicals from the parent molecule, radical recombination and disproportionation consume two free radicals in the formation of molecular species. The kinetics of this step are essentially collision controlled, as logioA = 8.5-10 is quite reasonable. For reasonably sized hydrocarbon free radicals, E is essentially zero. 

Above 383 K, the favored decomposition pathway produces NO2 in a homolytic bond fission reaction ... 

Three carboxylic acid bond fission reactions have been studied in the gas phase those of phenylacetic acid diphenylacetic acid , and peracetic acid . They were all studied by the toluene carrier technique and are therefore subject to the usual errors. In the phenyl and diphenylacetic acid decompositions, viz. 

ADJUSTED PARAMETERS FOR CARBOXYLIC ACID BOND FISSION REACTIONS... 

There is no evidence in any of the gas phase systems for initial multiple bond rupture (i.e., fragmentation reactions). Because of the low reaction temperatures, the alkoxy radical intermediates of the bond fission reactions (or radicals resulting from alkoxy radicals) are mainly involved in radical-radical termination processes ( 0) rather than participating in hydrogen abstraction from the parent peroxide E oi 6-8). Thus it has been commonly believed that the peroxide decompositions were classic examples of free radical non-chain processes. Identification of the rate coefficients and the overall decomposition Arrhenius parameters with the initial peroxide bond fission kinetics were therefore made. However, recent studies indicate that free radical sensitized decompositions of some peroxides do occur, and that the low Arrhenius parameters obtained in many of the early studies (rates measured by simple manometric techniques) were undoubtedly a result of competitive chain processes. The possible importance of free radical reactions in peroxide decompositions is illustrated below with specific regard to the dimethyl peroxide decomposition. 

Resonance stiffening in the transition state seems to lower activation entropies by about 3 eu per resonance interactions. Therefore the following ranges of A-factors are reasonable in simple bond fission reactions. 

Many bond fission reactions have been studied using the toluene carrier technique. 

Thompson, Raff, and co-workers have carried out several gas-phase studies of disilane dissociation (140,141) and bond fission reactions (142,143). 

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