Electrophilic center definition

An electrophilic center is an electron-deficient atom that is capable of accepting a pair of electrons. Notice that this definition is very similar to the definition for a Lewis acid. In fact, the terms electrophile and Lewis acid are synonymous. 

In order to gain an insight into the mechanism on the basis of the slope of a Type A correlation requires a more complicated procedure. Consider the Hammett equation. The usual statement that electrophilic reactions exhibit negative slopes and nucleophilic ones positive slopes may not be true, especially when the values of the slopes are low. The correct interpretation has to take the reference process into account, for example, the dissociation equilibrium of substituted benzoic acids at 25°C in water for which the slope was taken, by definition, as unity (p = 1). The precise characterization of the process under study is therefore that it is more or less nucleophilic than the reference process. However, one also must consider the possible influence of temperature on the value of the slope when the catalytic reaction has been studied under elevated temperatures there is disagreement in the literature over the extent of this influence (cf. 20,39). The sign and value of the slope also depend on the solvent. The situation is similar or a little more complex with the Taft equation, in which the separation of the molecule into the substituent, link, and reaction center may be arbitrary and may strongly influence the values of the slopes obtained. This problem has been discussed by Criado (33) with respect to catalytic reactions. 

Numerous cases of C-H bond activation are known, for which the reaction mechanism cannot be definitely attributed to either typical oxidative addition to a nucleophilic metal center or to an electrophilic substitution at an electron-deficient metal ion. In some cases the mechanism is not at all clear. 

Studies of the reactions of flavan-3-ols and particularly those of catechin have been central to the elucidation of the structure and development of uses for the condensed tannins. This work, initiated by Freudenberg and his colleagues at Heidelberg in the 1920s [see Weinges et al. (377) for a thorough review], continues to be an important aspect of condensed tannin chemistry. A wide range of electrophilic aromatic substitution reactions has been examined to obtain definitive evidence for the location of substitution (i.e. C-6 or C-8) of proanthocyanidins and to establish the influence of steric hindrance on the relative reactivity of these nucleophilic centers in flavan-3-ols. 

By definition, Lewis acid catalysts involve a metal center as an electron pair acceptor that accepts the electron pair from a nucleophile. This property makes them effective in many organic reactions and indispensable for the production of a large category of chemicals from simple alkylated compounds to complicated polymers or pharmaceuticals [6]. Also, the differences between the energy levels of the highest occupied molecular orbital (HOMO) of the reactant (nucleophile) and the lowest unoccupied molecular orbital (LUMO) of the Lewis acid (electrophile) make Lewis acid catalysis more complicated than the corresponding Br0nsted catalysis [7]. 

The parameters E and N can be used to compare the reactivity of compounds and even to predict the possibility that a reaction occurs. The parameter E represents the electrophiUcity parameter, which is defined by the use of a reference electrophilic compound. The values of E for many carbenium ions have been determined [6]. Contrary to common sense, which considers the carbenium ion to be intractable, it is possible to classify several carbenium ions that differ in elec-trophihdty by more than 16 orders of magnitude. Their stability is strictly connected to the presence of aryl, alkenyl, or alkynyl groups directly connected to the carbenium ionic center. For these reasons, sometimes the phrase n activated alcohol [7] is encountered for this type of compounds. The tables compiled by Mayr (Figure 26.2) offers a more readable and precise definition for the stability and reactivity of carbenium ions. Carbenium compounds with E < 0 (benzhy-dryhum ion) are almost impossible to isolate and to store without their rapid decomposition [8]. 

For this study, we use our extended database [11] of 47 radical systems, so 12 more than our previously published gas-phase radical electrophilicity scale [1], including C-, N-, O- and S-centered radicals, as well as some halogens, thus comprising a representative set of radicals for applications in organic chemistry. The structures can be retrieved from the Supporting Information. In order to compute the electrophilicity index, Parr s definition was apphed to the solution phase as shown in Eq. 1, using lEF-PCM and—in the case of water—COSMO as the implicit solvation models. Five solvents were chosen, for which the static dielectric constant covers the entire range of nonpolar to polar solvents n-hexane = 1.8819), dichloromethane (Sr — 8.9300), 2-propanol = 19.2640), acetonitrile (Sr = 35.6880) and water = 78.3553). 

In electrophilic covalent catalysis, the enzyme withdraws electrons from the reaction center, thereby activating the substrate. The distinction between electrophilic and nucleophilic catalysis is not always easy or profitable, since electrophilic attack is often preceded by a step in which the catalyst acts as a nucleophile to bind to the reactive center on the substrate. Further, electrophilic catalysis in one direction is likely to be nucleophilic in the other. Often, the definition depends upon whether the step presumed to be decisive is nucleophilic or electrophilic. 

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