Alkene Synthesis by Elimination of Alkyl Halides

Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. In Sections 6-17 through 6-21 we saw how dehydrohalogenation can take place by the El and E2 mechanisms. The second-order elimination (E2) is usually better for synthetic purposes because the El has more competing reactions. 

Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poorSf42 substrate. E2 dehydrohalogenation takes place in one step, in which a strong base abstracts a piroton from one carbon atom as the leaving group leaves the adjacent carbon. 

E2 elimination takes place by a concerted one-step reaction. 

A strong base abstracts a proton on a carbon next to the one bearing a halogen. The leaving group (halide) leaves simultaneously. 

EXAMPLE E2 elimination of /-butyl bromide with sodium hydroxide. Na+ QH 

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With a regioselectivity opposite to that of the Zaitsev rule, the Hofmann elimination is sometimes used in synthesis to prepare alkenes not accessible by dehydrohalogenation of alkyl halides. This application decreased in importance once the Wittig reaction (Section 17.12) became established as a synthetic method. Similarly, most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods. 

Dehydrogenation of alkanes is not a practical laboratory synthesis for the vast majority of alkenes The principal methods by which alkenes are prepared m the labo ratory are two other (3 eliminations the dehydration of alcohols and the dehydrohalo genation of alkyl halides A discussion of these two methods makes up the remainder of this chapter... 

The selection of reagents is governed by availability, cost, and, more importantly, the possible intrusion of side reactions. Thus in the above example, the action of the strongly basic ethoxide ion on t-butyl bromide would give rise to extensive alkene formation on the other hand little or no elimination would occur by the alternative reaction route. In general therefore, secondary or tertiary alkyl groups can only be incorporated into ethers by the Williamson synthesis by way of the corresponding alkoxide ions in reaction with a primary halide. 

The primary, secondary, and tertiary iodoalkanes are carbonylated and coupled with 9-alkyl-9-BBN. A variety of functional groups in both iodoalkanes and 9-R-9-BBN are tolerated, as is evident in the synthesis of several functionalized ketones having acetal, nitrile, and carbomethoxy groups (Table 7.12) [8]. The side reactions of alkyl halides with -hydrogen results by (3-hydrogen elimination to the alkene, and the isomerization of alkyl groups are not serious and limit to less than 10%. 

Another method for the synthesis of epoxides is through the use of halo-hydrins, prepared by electrophilic addition of HO—X to alkenes (Section 7.3). When halohydrins are treated with base, HX is eliminated and an epoxide is produced by an intramolecular Williamson ether synthesis. That is, the nucleophilic alkoxide ion and the electrophilic alkyl halide are in the same molecule. 

Since sodium acetylide is the salt of the extremely weak acid, acetylene, the acetylide ion is an extremely strong base, stronger in fact than hydroxide ion. In our discussion of the synthesis of alkenes from alkyl halides (Sec. 5.13), we saw that the basic hydroxide ion causes elimination by abstracting a hydrogen ion. It is not surprising that the even more basic acetylide ion can also cause elimination. 

As we already know (Secs. 5.12 and 8.12), alkyl halides undergo not only substitution but also elimination, a reaction that is important in the synthesis of alkenes. Both elimination and substitution are brought about by basic reagents, and hence there must always be competition between the two reactions. We shall be interested to see how this competition is affected by such factors as the structure of the halide or the particular nucleophilic reagent used. 

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