Unimolecular reactions reaction pathway

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

Formation of products in paraffin cracking reactions over acidic zeolites can proceed via both unimolecular and bimolecular pathways [4], Based on the analysis of the kinetic rate equations it was suggested that the intrinsic acidity shows better correlation with the intrinsic rate constant (kinl) of the unimolecular hexane cracking than with the apparent rate constant (kapp= k K, where K is the constant of adsorption equilibrium). In... 

Reaction pathways apparently analogous to d and f of Eq. (26) yield a mixture of propylene and cyclopropane. Only when photochemical activation was employed were the major products olefins derived from metathesis-decomposition of the metallocycle. The failure to form metathesis olefins under moderate conditions is significant. It may be that either unimolecular dissociation of the olefin from the complex (in the absence of excess olefin to restabilize the carbene) is energetically unfavored, or the metallocyclobutane structure in the equilibrium given by steps a and b in Eq. (26) is highly stabilized and favored. These results... 

The disappearance of nitroxides may also occur by unimolecular reactions. A good example is found with t-butoxy t-butyl nitroxide, known to be a relatively short-lived species, for which three fragmentation pathways are possible (1 la,b,c). Loss of butoxyl radicals (path a) was shown to be reversible (Perkins and Roberts, 1974), and, in the absence of any substrate which... 

The above discussion shows that several possible pathways for the interconversion of sulfur rinp exist. However, none of these alone can explain all the experimental observations. It therefore seems likely that several of them are effective simultaneously. Unimolecular dissociation reactions as discussed under (a) and (d) will dominate at high temperatures due to the increase in entropy. At lower temperatures, however, bimolecular reactions like the dimerization (c) may be most important, at least in case of the small rings (Sg, S, Sg) whose unimolecular dissociation is strongly endothermic. Larger rings will probably decompose according to mechanism (d), which in a way is the reversal of the dimerization (c). 

Equation (25) accounts for the NTC range observed for the ignition of ethane. Essentially, these reactions are refinements of the Semenov mechanism, since unimolecular reactions are important pathways in the oxidation of ethane. 

Fig. 10 Potential unimolecular reaction pathways for phenylperoxy radical. Adapted from Reference 129b.

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

The theoretical analysis of chemical activation reactions is similar to the Lindemann theory of unimolecular and association reactions. There are a number of competing reaction pathways. Depending on total pressure, concentrations of the participating species, and temperature, the outcome of the competition can change. 

Fig. 10.5 Reaction pathways in the QRRK analysis of unimolecular reactions.

The nature of the solvent and the structures of the substrate, nucleophile, and leaving group all help determine whether a nucleophilic displacement proceeds by a unimolecular or bimolecular pathway. They also all affect the rate of reaction. 

In contrast, applying frontier orbital theory to unimolecular reactions like electrocyclic reactions and sigmatropic rearrangements is inherently contrived, since we have artificially to treat a single molecule as having separate components, in order to have any frontier orbitals at all. Furthermore, frontier orbital theory does not explain why the barrier to forbidden reactions is so high—whenever it has been measured, the transition structure for the forbidden pathway has been 40 kJ mol-1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. 

The MNDO method has been employed405 to study the reaction pathway and to optimize the structures of reactant, product, and transition state of the acid-catalysed rearrangement of 1,2-propylene glycol, and the unimolecular dehydration of protonated a,co-diols in the gas phase has been examined406 by tandem mass spectrometric experiments. It has been shown that the reaction of l,2-diarylcyclopropane-l,2-diols (342) with acids yields primarily the a,//-unsaturated ketones (343) in which the aryl... 

Activation energies for unimolecular 1,3-hydrogen shifts connecting ketones and enols are prohibitive, so that thermodynamically unstable enols can survive indefinitely in the gas phase or in dry, aprotic solvents. Ketones are weak carbon acids and oxygen bases enols are oxygen acids and carbon bases. In aqueous solution, keto-enol tautomerization proceeds by proton transfer involving solvent water. In the absence of buffers, three reaction pathways compete, as shown in Scheme 2. 

One of the most common reasons for lowyields is an incomplete reaction. Rates of organic reactions can vary enormously, some are complete in a few seconds whereas rates of others are measured on a geological timescale. Consequently, to ensure that the problem of low yields is not simply due to low reactivity, reaction conditions should be such that some or all of the starting material does actually react. If none of the desired product is obtained, but similar reactions of related compounds are successful, the mechanistic implications should be considered. This situation has been referred to as Limitation of Reaction, and several examples have been given [32 ] the Hofmann rearrangement, for example, does not proceed for secondary amides (RCONHR ) because the intermediate anion 28 cannot form (Scheme 2.11). Sometimes, a substrate for a mechanistic investigation may be chosen deliberately to exclude particular reaction pathways for example, unimolecular substitution reactions of 1-adamantyl derivatives have been studied in detail in the knowledge that rear-side nucleophilic attack and elimination are not possible and hence not complications (see Section 2.7.1). 

Fig. 15 Reaction pathway for the synthesis of cyclic polystyrene by unimolecular process...

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. 

Sumpter and Thompson [70] used DMNA as a prototypical nitramine in one of the earliest molecular dynamics simulations of gas-phase decompositions via competing pathways. The studies focused on practical aspects of simulating unimolecular reactions in large molecules (e.g., the influence of the details of the potential energy surface) and the fundamental dynamics (e.g., IVR) on the decomposition reactions. They carried out simulations using various models for the potential energy surfaces and for various initial energy distributions. 

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