Ionization energy, nucleophile

The rate of complex formation of dimethylsilylene with a variety of Lewis bases was found to be close to the diffusion limit in cyclohexane at room temperature.33,34 The results of Yamaji et al.34 indicate that the rate of this reaction is governed not so much by electronic factors such as the HOMO energy of the Lewis base as by steric hindrance around the heteroatom center. In contrast, Baggott et al.35 found a satisfying correlation between rate constants and ionization energies of the nucleophile for the reaction of dimethylsilylene with various oxygen-containing substrates in the gas phase. 

E. Buncel, S. S. Shaik, I.-H. Um, S. Wolfe, J. Am. Chem. Soc. 110, 1275 (1988). A Theoretical Treatment of Nucleophilic Reactivity in Additions to Carbonyl Compounds. Role of the Vertical Ionization Energy. 

The relative nucleophilicity of a carbonyl compound can be estimated from the energy of the oxygen lone-pair orbital, a measure of which is obtained from the n(0) ionization energies (see Table 17.15). On this basis the following reactivity towards Lewis acids is predicted ... 

Chemical reactivity is typically site-specific. For example, certain sites may be more susceptible to electrophilic attack, others to nucleophilic. It could be argued, therefore, that there is a need for being able to determine the ionization energy as a function of position r in the space of the molecule. We have introduced such a function [10], which is rigorously defined within the framework of the Hartree-Fock molecular orbital model by eq. (4) ... 

Theoretical Treatment of Nucleophilic Reactivity in Additions to Carbonyl Compounds. Role of the Vertical Ionization Energy. 

Transposition of substituents takes place from aromatic compounds based on ortho effects from hydroxyphenyl ketones via assumed nucleophilic attack on the carbonyl carbon atom of the phenoxide site to give a tight tetravalent intermediate that promptly decomposes through benzyne neutral release and formation of a carboxylate anion (Scheme 17.19a). This reaction is hindered from meta- and para-substituted phenols. Alternatively, radical alkane loss is also observed that can be rationalized by considering the formation of an ion-neutral complex (Scheme 17.19b) comprised of quinone-like and alkylide groups. The relatively low ionization energy allows the generation of odd-electron quinone-like species and the elimination of the alkane radical (Scheme 17.19b). 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Because carbocations are key intermediates in many nucleophilic substitution reactions, it is important to develop a grasp of their structural properties and the effect substituents have on stability. The critical step in the ionization mechanism of nucleophilic substitution is the generation of the tricoordinate carbocation intermediate. For this mechanism to operate, it is essential that this species not be prohibitively high in energy. Carbocations are inherently high-energy species. The ionization of r-butyl chloride is endothermic by 153kcal/mol in the gas phase. ... 

The first step is a slow ionization of the substrate and is the rate-determining step. The second is a rapid reaction between the intermediate carbocation and the nucleophile. The ionization is always assisted by the solvent, since the energy necessary to break the bond is largely recovered by solvation of R" " and of X. For example, the ionization of f-BuCl to f-Bu" and Cl" in the gas phase without a solvent requires ISOkcalmol" (630kJmol" ). In the absence of a solvent such a process simply would not take place, except at very high temperatures. In water, this... 

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