NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS

We already have described one very important type of substitution reaction, the halogenation of alkanes (Section 4-4), in which a hydrogen atom is replaced by a halogen atom (X = H, Y = halogen). The chlorination of 2,2-dimethylpropane is an example  

Reactions of this type proceed by radical-chain mechanisms in which the bonds are broken and formed by atoms or radicals as reactive intermediates. This 

A specific substitution reaction of this type is that of chloromethane with hydroxide ion to form methanol  

In this chapter, we shall discuss substitution reactions that proceed by ionic or polar mechanisms in which the bonds cleave heterolytically. We also will discuss the mechanistically related elimination reactions that result in the formation of carbon-carbon multiple bonds  

These reactions often are influenced profoundly by seemingly minor variations in the structure of the reactants, in the solvent, or in the temperature. It is our purpose to show how these reactions can be understood and how they can be used to prepare other useful organic compounds. But first it will be helpful to introduce the concepts of nucleophilic and electrophilic reagenfs, and to consider the AH values for heterolytic bond breaking. 

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Conversion to p-toluenesulfonate esters (Section 8.14) Alcohols react with p-toluenesulfonyl chloride to give p-toluenesulfonate esters. Sulfonate esters are reactive substrates for nucleophilic substitution and elimination reactions. The p-toluenesulfonate group is often abbreviated —OTs. 

Ionic Reactions—Nucleophilic Substitution and Elimination Reactions of Alkyl Halides... 

In this chapter we review published results of studies of the kinetics and products of stepwise nucleophilic substitution and elimination reactions of alkyl derivatives, and we present a small amount of unpublished data from our laboratory. Our review of the literature is selective rather than comprehensive, and focuses on work that provides interesting insight into the factors that control the rate constant ratio ks/kp for partitioning of carbocations, and that provides an understanding of how the absolute rate constants ks and kp that constitute this ratio change with changing carbocation structure. 

It was expected that values of ks/kp for partitioning of [1+] could be obtained from the yields of the products of acid-catalyzed reactions of [l]-OH and [2]. However, significantly different relative yields of these products are obtained from the perchloric acid-catalyzed reactions of [l]-OH and [2] in several mixed alcohol/water solvents.21 This demonstrates that the nucleophilic substitution and elimination reactions of these two substrates do not proceed through identical tertiary carbocation intermediates (Scheme 4). The observed... 

Table 1 includes partitioning data only for carbocations that are sufficiently stable to form in the nucleophilic aqueous/organic solvents used in these experiments. For example, it is not possible to obtain values of ks for reaction of secondary aliphatic carbocations in water and other nucleophilic solvents, because the chemical barrier to ks is smaller than that for a bond vibration.83 The vanishingly small barriers for reaction of secondary carbocations with nucleophilic solvents results in enforced concerted mechanisms2-3 for the nucleophilic substitution and elimination reactions of secondary derivatives in largely aqueous solvents.83-84... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Rate constants and products of the nucleophilic substitution and elimination reactions of l-(4-metlioxyphenyl)-3-methyl-3-butyl derivatives 4-McOCf,H4CH2CMc2X (X = Cl, OH, O CCfiFs) have been determined in mostly aqueous solvents.153 The absolute rate constant for reaction of the corresponding tertiary carbocation in 50 50 (v/v) TFE-water was estimated as 3.5 x 1012 s 1. 

Examples of the solvent-dependent competition between nucleophilic substitution and / -elimination reactions [i.e. SnI versus Ei and Sn2 versus E2) have already been given in Section 5.3.1 [cf. Table 5-7). A nice example of a dichotomic y9-elimination reaction, which can proceed via an Ei or E2 mechanism depending on the solvent used, is shown in Eq. (5-140a) cf. also Eqs. (5-20) and (5-21) in Section 5.3.1. The thermolysis of the potassium salt of racemic 2,3-dibromo-l-phenylpropanoic acid (A), prepared by bromine addition to ( )-cinnamic acid, yields, in polar solvents [e.g. water), apart from carbon dioxide and potassium bromide, the ( )-isomer of l-bromo-2-phenylethene, while in solvents with low or intermediate polarity e.g. butanone) it yields the (Z)-isomer [851]. 

We shall look rather closely at both nucleophilic substitution and elimination reactions of the alkyl halides, for they provide a particularly good illustration of the effect of structure on reactivity, and of the methods that may be used to determine mechanisms of reactions. 

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