Unimolecular decomposition concerted

The most exciting application of bond order indices concerns the description of chemical reactions involving the simultaneous change of several bonds. An example is the unimolecular decomposition of ethanol, which can happen at high temperature or IR multiphoton excitation of the molecule. Out of the possible dissociation channels, the lowest barrier characterizes the concerted water loss of the molecule, yielding ethene and H20 [30]. 

The kinetics of formation of ketene and acetic acid on thermal unimolecular decomposition of acetic anhydride at 750-980 K have been reported and used to reevaluate the Arrhenius equation as k = 10122exp(—145 kJ mol l/RT) s-1 for the temperature range 470-980 K.54 Results of ah initio MO calculations suggest that the reaction proceeds by concerted elimination through a six-centre transition state, with potential barrier height 156 kJ mol-1. 

A theoretical investigation showed that the most favourable unimolecular decomposition path of primary fluorozonide is a concerted cleavage to carbonyl oxide and formyl fluoride. The secondary fluorozonide decomposition takes place most readily in a stepwise manner initiated by the 0-0 bond rupture.153 DFT calculations have shown that ozone-difluoroethylene reactions are initiated by the formation of van der Waals complexes and then yield primary ozonides, which rapidly open to carbonyl oxide compounds. The formation of primary ozonide has been predicted to be the rate-controlling step of the oxidation process.154... 

The mechanism of the decomposition reaction of 5-methoxy- 1,2,3,4-thiatriazole to dinitrogen sulfide and methoxy-nitrile was studied by the DFT method at the CCSD(T)//MP2/6-31+G level of theory <2003JOC6049>. The calculations indicated that this is a concerted retro-[2+3]-dipolar cycloaddition process with an activation energy of 28.9 kcal mol 1 and a reaction energy of 1.9 kcal mol. This unimolecular decomposition is favored due to the entropy gain (25.8 eu) involved in the overall reaction (Scheme 1 and Table 2). 

Figure 3. Energetics of unimolecular decomposition mechanisms in RDX obtained using the ReaxFF (full lines with filled symbols) and with QM (dashed lines with open symbols). Circles represent the sequential HONO elimination, triangles show the decomposition process following homolytic N-N bond breaking (NO2 elimination), and diamonds represent the concerted ring-opening pathway.

Also, in 2007, El-Nahas et al. reported a computational study of the thermochemistry and decomposition kinetics of MB and its C5H10O2 isomer, ethyl propanoate (EP). For each compound, they identified six unimolecular decomposition paths, both molecular and homolytic, finding that concerted six-center eliminations were the lowest energy paths ... 

Two extreme mechanisms have been proposed for the unimolecular dioxetane decomposition the concerted mechanism , whereby cleavage of the peroxide and the ring C—C bond occurs simultaneously, and the biradical mechanism whereby the initial cleavage of the 0—0 bond leads to the formation of a 1,4-dioxy biradical whose subsequent C—C bond cleavage leads to the formation of the two carbonyl fragments (Scheme 8). Although the biradical mechanism adequately explains the activation parameters obtained for most of the dioxetanes smdied, it appears not to be the appropriate mechanistic model for the rationalization of singlet and triplet quanmm yields. Therefore, an intermediate mechanism has been proposed, whereby the C—C and 0—0 bonds cleave in a concerted, but not simultaneous, manner (Scheme 8) . 

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

The gas-phase unimolecular pyrolysis of 2,4-dimethylpentane-2,4-diol has been found to occur by eliminative formation of acetone, isobutene, and H20 via a concerted six-membered cyclic transition state (Scheme 10).69 Single-pulse shock tube studies of the eliminative decomposition of ethoxy compounds have also been reported.70... 

Concerted Decomposition. Both unimolecular and bimolecular reactions involving only molecular species (no free radicals or ions) may play a role in certain aspects of coal conversion. In some types of concerted reactions, however, such as reactions involving C-C or C-0 bond breaking, the significance of such reactions appears to be drastically limited by the lack of suitable molecular structures. 

The decomposition of dichlorosilacyclopent-3-ene has been proved to be unimolecular by the study of inert dilution. The mass spectral study suggested that the decomposition was concerted, and, in the opinion of the investigators, it corresponded formally to the retro Diels-Alder reaction 13, 29). Thus the reaction in Eq. (38) was believed to proceed via a concerted 1,4-cycloaddition. 

Elementary uni- and bimolecular reactions will necessarily show first- and second-order kinetic behaviour, but the reverse is not necessarily true a first-order reaction may not be unimolecular and a second-order reaction may not be bimolecular. For example, we considered the decomposition of dibenzylmercury in Chapter 1, in which the mechanism could either be elementary, giving a mercury atom and a 1,2-diphenylethane molecule directly (reaction 2.13a), or the reaction could be complex, with a slow initial homolysis of a carbon-mercury bond, followed by rapid further reactions to give the products (reaction 2.13b). Similarly for the Cope rearrangement of diene 2 to diene 4, the reaction could be elementary, with a concerted cyclic movement of electrons (reaction 2.14a), or might involve a di-radical intermediate 3 which rapidly reacted further to give the observed product 4 (reaction 2.14b). Both these mechanisms would lead to first-order kinetics, so the establishment of first-order kinetic behaviour for both these reaction schemes does not establish the... 

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