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Alkenes: Preparation And Physical Properties

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Alkenes: Preparation And Physical Properties - Lesson Summary

Different methods for the preparation of Alkenes.

Hydrogenation:
Alkenes can be prepared from alkynes by partial catalytic hydrogenation. During this reaction, addition of a hydrogen molecule takes place across the triple bond of the alkyne. The two catalysts that are commonly used are palladised charcoal which is partially deactivated using sulphur compounds like barium sulphate or quinoline known as Lindlar’s catalyst and other catalyst is sodium in liquid ammonia.


Ex: Propyne on reduction with palladised charcoal or sodium in liquid ammonia forms propene.

If Lindlar’s catalyst is used the alkene obtained shows cis geometry. If Sodium in liquid ammonia is used the alkene obtained shows Trans geometry.
Ex: 2- Butyne on reduction with palladised charcoal in presence of quinoline, forms cis-2-butene. And on reduction with Na in the presence of liquid NH 3 forms trans-2-butene.




Dehydrohalogenation of alkyl halides:
When an alkyl halide is heated in presence of   alcoholic potash gives an alkene. As hydrogen and halogen atoms are removed in the form of halogen acid from alkyl halide the reaction is also known as dehydrohalogenation reaction.


Ex: Chloroethane on heating with alcoholic potassium hydroxide it forms ethene. During this reaction the hydrogen and halogen atoms on adjacent carbon atoms of chloroethane are eliminated in the form of hydrochloric acid.

In a dehydrohalogenation reaction if there are two possible positions where the double bond can be formed then the product obtained is governed by Saytzeff’s Rule.

Dehalogenation of vicinal dihalides:
In Vicinal dihalides the two halogen atoms are attached to two adjacent carbon atoms. When such dihalides are treated with zinc metal, the zinc atom removes the two halogen atoms forming zinc halide leaving behind an alkene in the process. This reaction is also called dehalogenation.



Ex: Di-bromoethane when treated with zinc forms ethene.

Acidic dehydration of alcohol:
When heated with concentrated sulphuric acid the –OH group of the alcohol and one hydrogen atom from the adjacent carbon atom are eliminated in the form of a water molecule to form an alkene.

Ex: Dehydration reaction of ethanol on heating with concentrated sulphuric acid to form ethene.


Physical properties of alkenes:
Alkenes are colourless hydrocarbons. Except for ethene, all the other alkenes are odourless as well. The first three members of the alkene homologous series are gases, the fourth to fourteenth are liquids and all members above 15 th are solids. Being non-polar compounds the alkenes are insoluble in water but soluble in non-polar solvents like benzene and petroleum ether.

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