Monday, January 28, 2008

Revision Alkenes - Methods of Preparation

alkyl halides
dihalogen derivatives (dihalides)
potassium salts of dicarboxylic acids

1. Partial reduction of alkynes
Alkynes can be reduced to alkenes using palladium-charcoal catalyst in catalytic hydrogenation (addition of hydrogen). The reduction (addition of hydrogen) can also be done using sodium in liquid ammonia.

2. Dehydrohalogenation of alkyl halides.
From alkyl halides, an atom of hydrogen and an atom of halogen are removed by treating it with alcoholic KOH.

As a hydrogen atom and a halogen atom are removed from the molecule, the reaction is called dehydrohalogenation.

3. Dehalogenation of vicinal dihalides (dihalogen compounds)
Vicinal dihalides are converted to alkenes by heating with zinc dust in ethyl alcohol.

4. Dehydration of alcohols
Alcohols are heated with sulphuric acid or phosphoric acid at about 443 K. A H2O molecule gets removed from the alcohol giving alkene.

The reaction occurs in multiple steps.

5. Kolbe's electrolytic method (alkenes from salts dicarboxylic acids)
The electrolysis of potassium salts of dicarboxylic acids gives alkenes.

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