Electrophile-nucleophile reactions, rates

The N+ scale, which is derived from nucleophile/electrophile combination reactions, differs from the nucleophihcity scales generated from rates of Sn2 reactions cf. Table 5-15). The N+ equation, an example of a so-called constant selectivity relationship , has been applied to many other electrophile/nucleophile reactions in solution, although often with only Hmited success. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

Of course, it is important that we have a nucleophile present, but how much we have doesn t matter. So now we can understand the 1 in SnL The rate of the reaction is dependent only on the concentration of the electrophile, and not that of the nucleophile. The rate is dependent on the concentration of only one entity, and the reaction is said to be first order. We signify this by placing a 1 in the name. Of course, this does not mean that you only need the electrophile. You still need the nucleophile for the reaction to happen. You still need two different things (nucleophile and electrophile). The 1 simply means that the rate is not dependent on the concentration of both of them. The rate is dependent on the concentration of only one of them. 

The electron-transfer paradigm for chemical reactivity in Scheme 1 (equation 8) provides a unifying mechanistic basis for various bimolecular reactions via the identification of nucleophiles as electron donors and electrophiles as electron acceptors according to Chart 1. Such a reclassification of either a nucleophile/ electrophile, an anion/cation, a base/acid, or a reductant/oxidant pair under a single donor/acceptor rubric offers a number of advantages previously unavailable, foremost of which is the quantitative prediction of reaction rates by invoking the FERET in equation (104). 

The nucleophilicity of the amine is another factor affecting reactivity, and changes in it have been sometimes responsible for the observed scattering in the Brpnsted plots. The Ritchie equation80 (equation 11) has been applied to a variety of reactions in which nucleophilic addition to, or combination with, an electrophilic center is rate-limiting. 

Substituent effects on the solvomercuration reaction differ markedly from those on many other electrophilic additions and these have been explained by assuming that the formation of the intermediate is often rate limiting in electrophilic additions whereas the reaction of the ionic intermediate with nucleophiles is rate limiting in solvomercuration147. In other words, the solvomercuration involves a fast pre-equilibrium formation of an intermediate, followed by rate-limiting attack of the nucleophile on this species. 

The rate-determining step in this process is the oxidative addition of methyl iodide to 1. Within the operating window of the process the reaction rate is independent of the carbon monoxide pressure and independent of the concentration of methanol. The methyl species 2 formed in reaction (2) cannot be observed under the reaction conditions. The methyl iodide intermediate enables the formation of a methyl rhodium complex methanol is not sufficiently electrophilic to carry out this reaction. As for other nucleophiles, the reaction is much slower with methyl bromide or methyl chloride as the catalyst component. 

Correlation analysis of solvent effects on the heterolysis of p-methoxyneophyl tosyl-ate has been performed by using the Koppel-Palm and Kamlet-Taft equations. The reaction rate is satisfactorily described by the electrophilicity and polarity parameters of solvents, but a possible role for polarizability or nucleophilicity parameters was also examined. 

The relative reactivity expressed by an electrophile. Electrophilicity is measured by the relative rate constants for a particular reaction of different electrophiles for a common substrate. 2. The property of being electrophilic. See also Electrophile Nucleophilicity lUPAC (1979) Pure and Appl. Chem. 51, 1725. 

It is accepted that the acmal nucleophile in the reactions of oximes with OPs is the oximate anion, Pyr+-CH=N-0 , and the availability of the unshared electrons on the a-N neighboring atom enhances reactions that involve nucleophilic displacements at tetravalent OP compounds (known also as the a-effect). In view of the fact that the concentration of the oximate ion depends on the oxime s pATa and on the reaction pH, and since the pKs also reflects the affinity of the oximate ion for the electrophile, such as tetra valent OP, the theoretical relationship between the pATa and the nucleophilicity parameter was analyzed by Wilson and Froede . They proposed that for each type of OP, at a given pH, there is an optimum pK value of an oxime nucleophile that will provide a maximal reaction rate. The dissociation constants of potent reactivators, such as 38-43 (with pA a values of 7.0-8.5), are close to this optimum pK, and can be calculated, at pH = 7.4, from pKg = — log[l//3 — 1] -h 7.4, where is the OP electrophile susceptibility factor, known as the Brpnsted coefficient. If the above relationship holds also for the reactivation kinetics of the tetravalent OP-AChE conjugate (see equation 20), it would be important to estimate the magnitude of the effect of changes in oxime pX a on the rate of reactivation, and to address two questions (a) How do changes in the dissociation constants of oximes affect the rate of reactivation (b) What is the impact of the /3 value, that ranges from 0.1 to 0.9 for the various OPs, on the relationship between the pKg, and the rate of reactivation To this end, Table 3 summarizes some theoretical calculations for the pK. ... 

On one hand, they increase the reaction rate due to an electrophilic assistance for the epoxy ring opening and, on the other, lower the reactivity of the alcoxy anion owing to its solvation and the decrease of its nucleophility. Positive, neutral or even negative effects of the alcohol additives on the reaction rate are governed by the relationship between these two factors. The chain propagation reaction mechanism itself remains trimolecular. 

A comparison between the observed rate constants of electrophilic fluorination and the calculated rates for electron transfer gives, for the first time, kinetic proof that nucleophilic attack at fluorine has to occur in order to explain the high reaction rates. The low yields of fluorinated products under conditions where electron transfer becomes important are an indication that Sn2 and electron transfer are competing and different pathways.10,11... 

Solvent effects on the kinetics and mechanism of unimolecular heterolysis of commercial organohalogen compounds have been investigated.9-11 The reaction rate is satisfactorily correlated by parameters for polarity, electrophilicity, and cohesion of the solvent, whereas the solvent nucleophilicity and polarizability exert no effect. 

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