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Preparation of Alkenes

Alkenes can be prepared on a small scale by starting from alcohols or alkyl halides.

Dehydration of Alcohols

This gives alkenes. Alcohols can by dehydrated by two different ways, namely with concentrated sulphuric acid and with the help of a catalyst.
  1. Sulphuric acid removes a molecule of water from alcohols to give alkenes.
For example, ethyl alcohol is converted into ethylene by slowly adding concentrated sulphuric acid to ethyl alcohol. Ethyl hydrogen sulphate (CH3.CH2.OSO3H) is formed. Sulphuric acid absorbs water that is formed during this reaction.

CH3.CH2.OH + H2SO4 CH3CH2. O.SO3H + H2O

At 440 K, ethyl hydrogen sulphate decomposes to ethylene (ethene) and sulphuric acid. The regenerated sulphuric acid acts as a catalyst.
  1. Catalytic dehydration is carried out by passing vapours of alcohol over heated alumina (Al2O3) or kaolin at 620 K.


Alkyl halides lose a molecule of halogen acid (HX) when heated with alcoholic alkali (NaOH or KOH). This process of removal of halogen acid (HX) is called dehydrohalogenation.
For example, ethyl bromide on heating with alcoholic KOH gives ethene.

Dehalogenation of Vicinal Dihalides

A vicinal dilhalide is a compound in which two halogen atoms are linked to adjacent carbon atoms. Alkane is obtained when vicinal dihalide is heated with zinc dust.

For example, ethylene bromide on heating with zinc dust gives ethylene,

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