General Features of Nucleophilic Substitution

The remainder of Chapter 7 is devoted to a discussion of the substitution reactions of alkyl halides. Elimination reactions are discussed in Chapter 8. 

Because the.se substitution reactions involve electron-rich nucleophiles, they are called nucleophilic substitution reactions. Examples are shown in Equations [l]-[3]. Nucleophilic substitutions are Lewis acid—base reactions. The nucleophile donates its electron pair, the alkyl halide (Lewis acid) accepts it, and the C-X bond is heterolytically cleaved. Curved arrow notation can be used to show the movement of electron pairs, as shown in Equation [3]. 

Negatively charged nucleophiles like OH and SH are used as salts with LT, Na% or K counterions to balance charge. The identity of the cation is usually inconsequential, and therefore it is often omitted from the chemical equation. 

When a neutral nucleophile is used, the substitution product bears a positive charge. Note that all atoms originally bonded to the nucleophile stay bonded to it after substitution occurs. 

All three CH3 groups stay bonded to the N atom in the given example. 

The reaction of alkyl halides with NH3 to form amines (RNH2) Is discussed In Chapter 25. 

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Now that you know something about the general features of nucleophilic substitution, you can begin to understand the mechanism. 

Nomenclature Physical properties Interesting alkyl halides The polar carbon-halogen bond General features of nucleophilic substitution The leaving group The nucleophile Possible mechanisms for nucleophilic substitution Two mechanisms for nucleophilic substitution The S 2 mechanism Application Useful Snj2 reactions... 

The synthesis of nitro dyes is relatively simple, a feature which accounts to a certain extent for their low cost. The synthesis, illustrated in Scheme 6.5 for compounds 140 and 141, generally involves a nucleophilic substitution reaction between an aromatic amine and a chloronitroaromatic compound. The synthesis of C. I. Disperse Yellow 14 (140) involves the reaction of aniline with l-chloro-2,4-dinitroaniline while compound 141 is prepared by reacting aniline (2 mol) with compound 144 (1 mol). 

The oxidation of carbonyl compounds can be achieved with hypervalent iodine reagents quite easily. A general feature of these reactions is the electrophilic attack of the hypervalent iodine reagent at the a-carbon atom of a carbonyl group and a review on this chemistry has been published recently [6]. This leads to hypervalent iodine intermediates of type 55. These phenyliodinated intermediates are quite unstable and a variety of subsequent reactions are possible. Intermediates 55, Scheme 24, can be considered as umpoled substrates regarding the reactivity of the a-position of the initial carbonyl compounds. Major processes are the substitution by a nucleophile (see Sect. 3.5.1 Functionalization in the a-Position) or the introduction of a carbon-carbon double bond (see Sect. 3.5.2 Introduction of an a,/ -Unsaturation). 

The reaction of (CH3)3CBr with CH3COO is an example of the second mechanism for nucleophilic substitution, the S l mechanism. What are the general features of this mechanism ... 

Figure 24.6 compares the general reaction for nucleophilic substitution of an ester with the Claisen reaction. Sample Problem 24.3 reinforces the basic features of the Claisen reaction. 

A close look at the nature of a nucleophile will emphasize that it shares common features with a Lewis hase (see Chapter 18). Indeed, a nucleophilic species can act as such a base if the reaction conditions are appropriate - it can remove a proton (H ion) from a halc enoalkane and thereby initiate an elimination reaction. In this type of reaction HX is eliminated from the halogenoalkane and an alkene is produced. It is essential to realize that, given the similarity of the reagents involved, the two processes of nucleophilic substitution and elimination are generally in competition with each other. If a primary halogenoalkane is reacted with aqueous alkali (OH (aq)) then the substitution reaction we have discussed earlier is favoured. However, if ethanolic alkali (OH (ethanol)) is used, then the elimination reaction is favoured. 

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