Dehydrohalogenation alkenes

Removal of the elements of HX, called dehydrohalogenation, is one of the most common methods to introduce a n bond and prepare an alkene. Dehydrohalogenation is an example of P elimination, because it involves loss of elements from two adjacent atoms the a carbon bonded to the leaving group X, and the P carbon adjacent to it. Three curved arrows illus- ... 

An elimination reaction, dehydrohalogenation, can occur for chloro-, bromo- and iodoalkanes. In such a reaction, the halogen, X, from one C atom and a hydrogen from an adjacent C atom are eliminated. A double bond between two carbon atoms is formed the molecule becomes more unsaturated. The net reaction is the transformation of an alkyl halide (or haloalkane) into an alkene. Dehydrohalogenation reactions usually require a strong base such as sodium hydroxide, NaOH. 

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... 

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... 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

In addition to being regioselective dehydrohalogenation of alkyl halides is stereo selective and favors formation of the more stable stereoisomer Usually as m the case of 5 bromononane the trans (or E) alkene is formed m greater amounts than its cis (or Z) stereoisomer... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

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... 

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... 

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

Dehydrohalogenation of the diastereomenc forms of 1 chloro 1 2 diphenylpropane is stereo specific One diastereomer yields (E) 1 2 diphenylpropene and the other yields the Z isomer Which diastereomer yields which alkene" Why" ... 

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... 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Cyclohexyl bromide, for exfflnple, is converted to cyclohexene by sodium ethoxide in ethanol over 60 times faster than cyclohexyl chloride. Iodide is the best leaving group in a dehydrohalogenation reaction, fluoride the poorest. Fluoride is such a poor leaving group that alkyl fluorides are rarely used as starting materials in the preparation of alkenes. 

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. 

Dehydrohalogenation (Sections 7.1, 11.8) The loss of HX from an alkyl halide. Alkyl halides undergo dehydrohalogenation to yield alkenes on treatment with strong base. 

Another route commonly used in the laboratory to produce alkenes is the dehydrohalogenation of haloalkanes, the removal of a hydrogen atom and a halogen atom from neighboring carbon atoms ... 

The dehydrohalogenation of haloalkanes to produce alkenes is always carried out in a nonaqueous solvent, usually ethanol. Give two reasons why water cannot be used as the solvent for the reaction. 

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

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. 

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