Alkenes, dehydration dehydrohalogenation

In this chapter we described methods for the synthesis of alkenes using dehydrohalogenation, dehydration of alcohols, and reduction of alkynes. We also introduced the alkylation of alkynide anions as a method for forming new carbon-carbon bonds, and we introduced retrosynthetic analysis as a means of logically planning an organic synthesis. 

In Chapter 9, we saw that we can prepare alkenes by dehydrohalogenation. The dehydration of alcohols also gives alkenes, but this reaction occurs with rearrangement of the carbocation, and gives mixtures of products having different carbon skeletons. The elimination of a hydrogen hahde from an alkyl halide is a complex process. We must consider both rcgiochemistry and stereoelectronic effects. These effects are related to the mechanism of the reaction, which may be either E2 or El. 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

Alkenylbenzenes are prepared by the various methods described m Chapter 5 for the preparation of alkenes dehydrogenation dehydration and dehydrohalogenation... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

In dehydration and dehydrohalogenation the preferential order for removal of an H is 3°>2°> 1 (Saytzeff rule). We can say the poor get poorer. This order obtains because the more R s on the C==C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is... 

Isoxazoles and their partially or fully saturated analogs have mainly been prepared, both in solution and on insoluble supports, by 1,3-dipolar cycloadditions of nitrile oxides or nitrones to alkenes or alkynes (Figure 15.10). Nitrile oxides can be generated in situ on insoluble supports by dehydration of nitroalkanes with isocyanates, or by conversion of aldehyde-derived oximes into a-chlorooximes and dehydrohalogenation of the latter. Nitrile oxides react smoothly with a wide variety of alkenes and alkynes to yield the corresponding isoxazoles. A less convergent approach to isoxazoles is the cyclocondensation of hydroxylamine with 1,3-dicarbonyl compounds or a,[3-unsatu-rated ketones. 

Alkenes are a central functional group in oiganic chemistry. Alkenes are easily prepared by elimination reactions such as dehydrohalogenation and dehydration. Because their n bond is easily broken, they undergo many addition reactions to prepare a variety of useful compounds. 

The most important of these methods of preparation—since they are the most generally applicable—are the dehydrohalogenation of alkyl halides and the dehydration of alcohols. Both methods suffer from the disadvantage that, where the structure permits, hydrogen can be eliminated from the carbon on either side of the carbon bearing the —X or OH this frequently produces isomers. Since the isomerism usually involves only the position of the double bond, it is not important in the cases where we plan to convert the alkene into an alkane. 

The formation of 2-butene from //-butyl alcohol illustrates a characteristic of dehydration that is not shared by dehydrohalogenalion the double bond can be formed at a position remote from the carbon originally holding the —OH group. This characteristic is accounted for later (Sec. 5.22). It is chiefly because. of the greater certainty as to where the double bond will appear that dehydrohalogenation is often preferred over dehydration as a method of making alkenes. 

Here, as in dehydrohalogenation, the preferred alkene is the more highly substituted one, that is, the more stable one (Sec. 6.4). In dehydration, die more stable aikene is the preferred product. 

We Have seen (Secs. 5.14, 5.23) that the stability of alkenes determines orientation in dehydrohalogenation and dehydration. 

Dehydrohalogenation of l-phenyI-2-chloropropane, or dehydration of l-phenyl-2-propanol, could yield two products l-phenylpropene or 3-phenyl-propene. Actually, only the first of these products is obtained. We saw earlier (Secs. 5.14 and 5.23) that where isomeric alkenes can be formed by elimination, the... 

Alkenes are prepared from alcohols either by direct dehydration or by de-hydrohalogenation of intermediate alkyl halides to avoid rearrangement we often select dehydrohalogenation of halides even though this route involves an extra step. (Or, sometimes better, we use elimination from alkyl sulfonates.)... 

The two most common alkene-forming elimination reactions are dehy-drohalogenation—the loss of HX from an alkyl halide—and dehydration—the loss of water from an alcohol. Dehydrohalogenation usually occurs by reaction of an alkyl halide with strong base, such as potassium hydroxide. For example, bromocyclohexane yields cyclohexene when treated with KOH in ethanol solution ... 

Methods for the preparation of alkenes generally involve elimination reactions, such as dehydrohalogenation, the elimination of HX from an alkyl halide, and dehydration, the elimination of water from an alcohol. 

Dehydrohalogenation of Alkyl Halides Dehalogenation of 12-Dihalides Dehydration of Alcohols Alkenes from Ethers... 

Elimination Reactions to Produce Alkenes Section 4.5A-B Examples a and c are dehydrohalogenation reactions and the others are dehydrations. The predominant product, when more than one product is possible, is the most highly substituted alkene. The most substituted alkene is the one with the most carbons directly connected to the carbons of the carbon-carbon double bond. 

Common variations are the use of an alcohol, which dehydrates in situj or of a halide which dehydrohalogenates to give the alkene. 

We can synthesize alkenes by processes such as dehydrogenation of alkanes, dehydrohalogenation of alkyl halides, and dehydration of alcohols. 

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