Displacements at saturated carbon

We consider first the Sn2 type of process. (In some important Sn2 reactions the solvent may function as the nucleophile. We will treat solvent nucleophilicity as a separate topic in Chapter 8.) Basicity toward the proton, that is, the pKa of the conjugate acid of the nucleophile, has been found to be less successful as a model property for reactions at saturated carbon than for nucleophilic acyl transfers, although basicity must have some relationship to nucleophilicity. Bordwell et al. have demonstrated very satisfactory Brjinsted-type plots for nucleophilic displacements at saturated carbon when the basicities and reactivities are measured in polar aprotic solvents like dimethylsulfoxide. The problem of establishing such simple correlations in hydroxylic solvents lies in the varying solvation stabilization within a reaction series in H-bond donor solvents. 

These experiments tell us that a displacement (substitution) has occurred at the carbonyl carbon atom, and rule out the alternative displacement at saturated carbon. 

Figure 3.2 More O Ferrall-Jencks Diagram for nucleophilic displacements at saturated carbon.

The chlorine kinetic isotope effect in nucleophilic displacement at saturated carbon in para-substituted benzyl chlorides, with thiolate and analogous oxygen nucleophiles, has been examined . The reactions proceed via a concerted transition state. 

Unlike Sn2 displacements at saturated carbon, which proceed in a single step of synchronous bond-breaking and bond-formation, these substitutions have been discussed most frequently in terms of an intermediate complex, formed by addition of the nucleophile to the carbon atom undergoing substitution and converting that carbon atom to one with its substituents arranged in a tetrahedral configuration. For the reaction of 2,4-dinitrochlorobenzene and methoxide ion the intermediate complex would have a structure of the type indicated by II. It is, at this time, pertinent to consider in detail the nature of the evidence for such an intermediate. 

The Sn2 direct displacement reaction at saturated carbon can be represented as Y + R3CX R3CY + X ... 

Streitwieser, A, jr. Solvolytic displacement reactions at saturated carbon atoms. Chem. Reviews 56, 675 (1956). 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms Box 13.1 The Concept of Hard and Soft Lewis Acids and Bases (HSAB) Illustrative Example 13.2 Some More Reactions Involving Methyl Bromide Illustrative Example 13.3 1,2-Dibromoethane in the Hypolimnion of the Lower Mystic Lake, Massachusetts Polyhalogenated Alkanes — Elimination Mechanisms... 

Nucleophilic Displacement of Halogens at Saturated Carbon Atoms... 

A. Streitwieser, Solvolytic Displacement Reactions at Saturated Carbon Atoms," Chem. Rev. 56, 571 (1956). 

Since most SH2 displacements occur at univalent atoms, tests of stereochemistry at the reaction center are ordinarily not possible. Substitutions do nevertheless occur at saturated carbons in highly strained rings.131 Halogen atoms attack cyclopropane (Equation 9.76) and other strained cyclic compounds,... 

The displacement of chromium(III) from pentaaquo-Y-pyridiomethyl-chromium(III) ions by mercury(I) follows second-order kinetics, and has been suggested43 to involve a direct attack of the binuclear Hg22+ ion at saturated carbon through mechanism SE2(open), viz. 

Electron transfer substitution at saturated carbon has been performed through photochemical initiation involving a charge transfer complex between the nucleophilic anion and a neutral substrate in HMPA, nitrite anion was displaced by azide on a,p-dinitrocumene [217]. 

Mechanism of electrophilic displacements of metals at saturated carbon On the importance of coordination in electrophilic substitution reactions of organometallic compounds... 

Before we go any further we are going to look in a bit more detail at these two mechanisms because they allow us to explain and predict many aspects of substitution reactions. The evidence that convinced chemists that there are two different mechanisms for substitution at saturated carbon is kinetic it relates to the rate of reactions such as the displacement of bromide by hydroxide, as shown in the margin. 

Nucleophilic displacement may be effected at sp as well as at saturated carbon, although in the former case, the reaction is probably an addition-elimination sequence. Benzoyl chloride, for example, reacts with sodium cyanide under liquid-liquid phase transfer conditions to yield benzoyl cyanide according to equation 7.4 [16]. Several examples of this reaction are given in Table 7.2. The preparation of aroyl cyanides... 

A type of reaction that has probably received more detailed study than any other—largely due to the monumental work of Ingold and his school—is nucleophilic substitution at a saturated carbon atom the classical displacement reaction exemplified by the conversion of an alkyl halide into an alcohol by the action of aqueous base ... 

In this discussion of nucleophilic displacement at a saturated carbon atom, interest has tended to centre on attack by nucleophilic anions Nu e, especially eOH, on polarised neutral species, especially alkyl halides, +R—Hala . In fact this general type of displacement is extremely common involving, in addition to the above, attack by non-charged nucleophiles Nu- on polarised neutral species,... 

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