Reaction mechanisms bimolecular

Under these circumstances there is no need to postulate unimolaiular rate determining reactions. Instead, the bimolecular reaction mechanism (c) discussed above would explain all experimental observations much better. The reaction could proceed via a sulfurane-like molecule which after pseudorotation at the central atom could dissociate to the observed products ... 

The hydroxymethylphosphonium ion first formed changes into mono-hydroxymethylphosphine by releasing a proton. This phosphine reacts further in the same way as phosphine itself until finally the quarternary phosphonium ion is formed. For a bimolecular reaction mechanism, the first stage must be assumed to be the formation of a carbonium ion from the aldehyde molecule and a proton. This ion then reacts with phosphine. 

Figure 2 shows that there is also no positive deviation of the rate constant for reaction with water in a correlation of log k for the reactions of oxygen nucleophiles with the monoanions of phosphorylated y-picoline and with methyl dinitrophenyl phosphate (27). Methyl dinitrophenyl phosphate reacts with RO by a concerted bimolecular substitution. The fit of the rate constant for water to this correlation is consistent with a concerted bimolecular reaction mechanism of water with phosphorylated y-picoline. 

The lack of dependence of the reaction rate for polycondensation on the extent of reaction (to a first approximation) allows a simple bimolecular reaction mechanism to be employed. Noting, from the previous section, that the reaction mechanism for simple esterification reactions (Scheme 1.1) had as the rate-determining step the second reaction, namely... 

Depending upon the substituents, a number of cleavage patterns occur in isoxazolidines, and so as the fragments. The molecular ions are detectable. The MIKE technique on the structure of the products aided deduction of the substituent effect on the bimolecular reaction mechanism of isoxazolidinium salt with LiAlH4 <83JHC1207>. 

Application of equation (4) to the organic cocktail oxidation profiles, given in Fig. 6(a), yields straight line plots. Two of these kinetic plots for current densities of 0.14 and 0.40 A cm are presented in Fig. 8. The linear relationships obtained demonstrate that pseudo first-order kinetics are obeyed. This relationship directly supports the heterogeneous bimolecular reaction mechanism proposed for the electrochemical oxidation of organics in the electrochemical reactor. The slopes of the linear plots yield the pseudo first-order rate constants, which are summarized in Table 2 for each value of the current density (i.e. the anodic electrode potential) used. It can be seen from the table that, with increasing current... 

Diffusion-Controlled Encounter. Elementary bimolecular reaction mechanisms require diffiisional encovmter before the reaction. If the intrinsic kinetics are fast, and/or the viscosity of the solution is high, diffusion-controlled encounter may occur. In a homogeneous medium, a rate constant /jdiff can be evaluated which reflects the effective bulk-averaged rate constant associated with bimolecular encounters (45). Diffusional bimolecular encounter should be considered in the appropriate context. If Areact is the intrinsic bimolecular rate constant and djff is the differential rate constant defined above, then the observed rate constant for the bimolecular reaction is given by equation (11) (46). The limiting cases of this equation can be readily identified that is when the rate constant is very large, the observed rate constant corresponds to the diffusional rate constant. 

Modern theoretical and experimental studies have revealed additional details of the reaction kinetics. The results of these studies surest that the low temperature thermal reaction proceeds by both the direct bimolecular reaction of hydrogen molecules with vibrationally excited iodine molecules H2 + l2(hi v) —> HI + HI and the termolecular reaction of hydrogen molecules with iodine atoms H2 + I + I—>HI- - HI. The direct bimolecular reaction mechanism and the termolecular reaction mechanism were among those suggested by Bodenstein one hundred years ago. 

The presence of the cyclic dimer in the initial reaction products lends some support for the bimolecular reaction mechanism shown in Figure 7. The postulated formation of a blcyclic structure, involving Intramolecular nucleophilic addition of the carboxylate ion to the lactam carbonyl group and addition of a proton, appears to be the initial reaction in either mechanism. This postulate is supported by the observation that neither polymerization nor rearrangement occurred when the corresponding ester lactams rather them the carboxy lactams were employed. 

This product distribution is temperature dependent. If the temperature is raised, the amount of coupled product t-butylpentafluorobenzene decreases with respect to the other reaction products, and the ratio of isobutane and isobutene approaches 1 1. The fact that the rate of reaction depends on the concentration of hexafluorobenzene is suggestive of a bimolecular reaction mechanism, rather than a... 

Similarly, a bimolecular reaction mechanism via /C4+2 or /L3+3 types, rather than a chain-growth mechanism, seems to be in operation as well for AlMe3-mediated one-pot cyclohexameiization of 17a that affords 18a (Fig. 9.11 and Table 9.9). 

A bimolecular reaction mechanism generating diradical intermediate species is proposed and its implications are discussed. 

---