Organic Halogen Compounds Substitution and Elimination Reactions

Organic Halogen Compounds Substitution and Elimination Reactions 

Alkyl halides react with nucleophiles, reagents that can supply an electron pair to form a covalent bond, to give a product in which the nucleophile takes the place of the halogen. Table 6.1 gives fifteen examples of such nucleophilic substitution reactions, which can be used to convert alkyl halides to alcohols, ethers, esters, amines, thiols, alkyl cyanides, or acetylenes. 

Nucleophilic substitution may occur by two mechanisms. The SN2 mechanism is a one-step process. Its rate depends on the concentrations of substrate and nucleophile. If the halogen-bearing carbon is stereogenic, substitution occurs with inversion of configuration. The reaction is fastest for primary halides and slowest for tertiary halides. 

The SN1 mechanism is a two-step process. In the first step, the alkyl halide ionizes to a carbocation and a halide ion. In the second, fast step, the carbocation combines with the nucleophile. The overall rate is independent of nucleophile concentration. If the halogenbearing carbon is stereogenic, substitution occurs with racemization. The reaction is fastest for tertiary halides and slowest for primary halides. The two mechanisms are compared in Table 6.2. 

Elimination reactions often compete with substitution. They involve elimination of the halogen and a hydrogen from adjacent carbons to form an alkene. Like substitution, they occur by two main mechanisms. The E2 mechanism is a one-step process. The nucleophile acts as a base to remove the adjacent proton. The preferred form of the transition state is planar, with the hydrogen and the leaving group in an anti conformation. The E1 mechanism has the same first step as the SN1 mechanism. The resulting carbocation then loses a proton from a carbon atom adjacent to the positive carbon to form the alkene. 

Chlorine- and bromine-containing natural products have been isolated from various species that live in the sea—sponges, moiiusks, and other ocean creatures that adapted to their environment by metaboiizing inorganic chlorides and bromides that are prevalent there. With these exceptions, most organic halogen compounds are creatures of the laboratory. 

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Chapter 6 Organic Halogen Compounds Substitution and Elimination Reactions... 

In this and the next chapter, we ll be discussing the chemistry of alkyl halides—compounds that have a halogen atom bonded to a saturated, s/> -hybridized carbon atom. We ll begin in this chapter with a look at how to name and prepare alkyl halides, and we ll see several of their reactions. Then in the following chapter, we ll make a detailed study of the substitution and elimination reactions of alkyl halides—two of the most important and well-studied reaction types in organic chemistry. 

The organization is fairly classical, with some exceptions. After an introductory chapter on bonding, isomerism, and an overview of the subject (Chapter 1), the next three chapters treat saturated, unsaturated, and aromatic hydrocarbons in sequence. The concept of reaction mechanism is presented early, and examples are included in virtually all subsequent chapters. Stereoisomerism is also introduced early, briefly in Chapters 2 and 3, and then given separate attention in a fuU chapter (Chapter 5). Halogenated compounds are used in Chapter 6 as a vehicle for introducing aliphatic substitution and elimination mechanisms and dynamic stereochemistry. 

We shall then study the haloalkanes, our first example of compounds containing a functional group—the carbon-halogen bond. The haloalkanes participate in two types of organic reactions substitution and elimination (Chapters 6 and 7). In a substitution reaction, one halogen atom may be replaced by another in an elimination process, adjacent atoms may be removed from a molecule to generate a double bond. 

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