Elimination reaction - Preparation of alkenes from alkyl halides

From Alkyl Halides :
Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol,say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of b-elimination reaction, since hydrogen atom is eliminated from the b carbon atom (carbon atom next to the carbon to which halogen is attached).
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 elimination.
Nature of halogen atom and the alkyl group determine rate of the reaction. It is observed that for halogens, the rate is: iodine > bromine > chlorine, while for alkyl groups it is : tert > secondary > primary.
Sodium methoxide (NaOCH3) is a suitable base and is used in methyl alcohol. Potassium hydroxide in ethyl alcohol is another base–solvent combination often employed in the dehydrohalogenation of alkyl halides. Potassium tert -butoxide [KOC(CH3)3] is the preferred base when the alkyl halide is primary; it is used in either tert -butyl alcohol or dimethyl sulfoxide as solvent.
The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule; beta-elimination predominates in the direction that leads to the more highly substituted alkene.
In addition to being regioselective, dehydrohalogenation of alkyl halides is stereoselective and favors formation of the more stable stereoisomer. Usually, as in the case of 5-bromononane, the trans (or E) alkene is formed in greater amounts than its cis (or Z) stereoisomer.
From vicinal dihalides:   
Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides. Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX2 to form an alkene. This reaction is known as dehalogenation

CH2Br−CH2Br + Zn  --> CH2=CH2 +ZnBr2 


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