Alkenes By dehydrohalogenation

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Synthesis of an alkene by dehydrohalogenation is almost always better achieved by an E2 reaction ... 

Alkyl halides are converted into alkenes by dehydrohalogenation elimination of the elements of hydrogen halide. Dehydrohalogenation involves removal of the halogen atom together with a hydrogen atom from a carbon adjacent to the one... 

A bridgehead alkene. Japanese chemists have reported the first example of formation of a bridgehead alkene by dehydrohalogenation. Thus the bromide... 

The best reaction conditions to use when synthesizing an alkene by dehydrohalogenation are those that promote an E2 mechanism. 

Purpose To demonstrate the formation of alkenes by dehydrohalogenation and assess product distributions using different bases. 

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. 

Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide It IS one of the most useful methods for preparing alkenes by p elimination... 

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... 

Dehydrohalogenation (Section 5 14) Reaction in which an alkyl halide on being treated with a base such as sodium ethoxide is converted to an alkene by loss of a proton from one carbon and the halogen from the adjacent carbon... 

Sulfonyl imides (78) are, like sulfenes, prepared by dehydrohalogenation of the corresponding sulfonyl chlorides (79) (usually called sulfamoyl chlorides). Like sulfenes, they take part in [2 + 2] and [4 + 2] cycloaddition reactions with electron-rich alkenes or with 1,3-dienes, yielding 1,2-thia-zetidine 1,1-dioxides (80)104 or dihydro-1,2-thiazines (81),105 respectively. 

Predict all the alkenes that would be formed by dehydrohalogenation of the following halides with sodium ethoxide In ethanol and identify the major alkene ... 

The needed alkene is prepared by dehydrohalogenation and then brominated. 

The necessary alkene, styrene, is available by dehydrohalogenation of the given starting material, 1-phenylethyl bromide. 

Because 1,1-dihalocyclopropanes are so readily available by carbene addition to alkenes, their dehydrohalogenation to 1-halocyclopropenes provides, in principle, one of the most attractive routes to functionalised cyclopropenes. However, most early studies of the reaction did not lead to the cyclopropenes themselves, but to products of their further reaction. The main problems arise when the 1-halocyclopropene (9) can undergo prototropic shifts by removal of a proton from C 2 or C3, or when the base used is also a good nucleophile and addition to the cyclopropene can occur ... 

Vicinal dibromides (two bromines on adjacent carbon atoms) are converted to alkenes by reduction with iodide ion in acetone. This debromination is rarely an important synthetic reaction, because the most likely origin of a vicinal dibromide is frombromi-nation of an alkene (Section 8-10). We discuss this reaction with dehydrohalogenation because the mechanisms are similar. 

The trans double bond in the target molecule is a product of reduction of a triple bond with Li in NH3.The alkyne was formed by an alkylation of a terminal alkyne with bromomethane. The terminal alkyne was synthesized from the starting alkene by bromination, followed by dehydrohalogenation. 

Although formation of the diaUcyl peroxide is shown in the prototype reaction (Scheme 9), hydroperoxides, alcohols, aldehydes, and acids also have been isolated. The extent of these secondary paths depends on the choice of solvent and reaction conditions. Secondary and tertiary halides also give substantial quantities of alkenes from dehydrohalogenation by... 

Dehydrohalogenatioiu Alkyl halides can be converted into alkenes by solid potassium 1-butoxide and a catalytic amount of 18-crown-6. Petroleum ether with a suitable boiling point range is used as solvent. No reaction is observed in the absence of a crown ether other bases (KOH, K2CO3) are ineffective. The method is particularly useful for dehydrohalogenation of primary alkyl halides, which are converted into ethers by potassium f-butoxide under usual conditions. ... 

Dehydrohalogenation of vicinal dihalides is particularly useful since the dihalides themselves are readily obtained from the corresponding alkenes by addition of halogen. This amounts to conversion—by several steps—of a double bond into a triple bond. 

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