Nucleophiles ionization mechanism

The Stvfl mechanism is an ionization mechanism The nucleophile does not participate until after the rate determining step has taken place Thus the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions pro ceedmg by the 8 2 pathway How the structure of the alkyl halide affects the rate of Stvfl reactions is the topic of the next section... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

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. 

Because the nucleophile is intimately involved in the rate-determining step, not only will the rate depend on its concentration, but the nature of the nucleophile will be very important in determining the rate of the reaction. This is in marked contrast to the ionization mechanism, in wiiich the identity and concentration of the nucleophile do not affect the rate of the reaction. 

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. ... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Ejfect ofSolvent. In addition to the solvent effects on certain SeI reactions, mentioned earlier (p. 764), solvents can influence the mechanism that is preferred. As with nucleophilic substitution (p. 448), an increase in solvent polarity increases the possibility of an ionizing mechanism, in this case SeI, in comparison with the second-order mechanisms, which do not involve ions. As previously mentioned (p. 763), the solvent can also exert an influence between the Se2 (front or back) and SeI mechanisms in that the rates of Se2 mechanisms should be increased by an increase in solvent polarity, while Sni mechanisms are much less affected. 

The rate of solvolysis of the phosphinic chlorides (83a, b) in trifluoro-acetic acid and aqueous acetone (the composition of the latter solvent being chosen such that the rate of S nI solvolysis of Bu Cl was the same in both) have been examined to assess the possible operation of an 5 n1(P) ionization mechanism. In the more nucleophilic aqueous acetone solvent,... 

SN1. The substrate is a secondary halide and may react by either mechanism. The nucleophile (CH3OH) is relatively weak and also polar, favoring the ionization mechanism. 

The ionization mechanism has several distinguishing features. The ionization step is rate determining and the reaction exhibits first-order kinetics, with the rate of decomposition of the reactant being independent of the concentration and identity of the nucleophile. The symbol assigned to this mechanism is Sjyl, for substitution, nucleophilic, unimolecular. 

Studies of the stereochemistry are a powerful tool for investigation of nucleophilic substitution reactions. Direct displacement reactions by the Sjv2(lim) mechanism are expected to result in complete inversion of configuration. The stereochemical outcome of the ionization mechanism is less predictable, because it depends on whether reaction occurs via an ion pair intermediate or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of ion pair recombination. 

The critical step in the ionization mechanism for nucleophilic substitution is the generation of the carbocation intermediate. For this mechanism to operate, it is essential... 

The product of a substitution reaction that follows the limiting Sf 2 mechanism is determined by the identity of the nucleophile. The nucleophile replaces the leaving group and product mixtures are obtained only if there is competition from several nucleophiles. Product mixtures from ionization mechanisms are often more complex. For many carbocations there are two competing processes that lead to other products elimination and rearrangement. We discuss rearrangements in the next section. Here we consider the competition between substitution and elimination under solvolysis conditions. We return to another aspect of this competition in Section 5.10, when base-mediated elimination is considered. 

The limiting cases of nucleophilic substitution have been described as the ionization mechanism (SnI, substitution-nucleophilic-unimolecular) and the direct displacement mechanism (8 2, substitution-nucleophilic-bimolecular Gleave et al., 1935). The S l and Sn2 mechanisms describe the extremes in nucleophilic substitution reactions. Pure SnI and Sn2 reaction mechanisms, however, are rarely observed. More often a mix of these reaction mechanisms are occurring simultaneously. 

The carbocation may react with an electron-rich species (neutral or anionic), that is, with a nucleophile (known as Sj l) to give the stable compound. The carbon-halogen bond breaks heterolytically without any assistance from the nucleophile, forming a carbocation. The carbocation then reacts with the nucleophile to form the substitution product, that is, an ionization mechanism (Scheme 2.12). 

The ionization mechanism for nucleophilic substitution proceeds by ratedetermining heterolytic dissociation of the substrate to a tricoordinate carbocation... 

The distinctive feature of any ionization mechanism for nucleophilic substitution is the generation of a tricoordinate carbocation in the rate-determining step. It is essential, then, that such a species not be prohibitively high in energy. The production of carbonium ions in the gas phase is a particularly unfavorable process. The heat of formation of (CH3)3C (tert-butyl cation) is +169 kcal/mol, compared with -32 kcal/mol for (CH3)3CH. The reaction... 

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