Preparation of Alkene from Alcohols, Alkyl Halide and Alkynes

Alkenes are prepared by alcohols, alkly halides, alkynes and more. Different reagents and physical conditions should be maintained according to thee using compound. Preparation of alkene is a lesson of organic chemistry in grade 12.



Preparing Alkenes

  1. Heating alkyl halides with alcoholic potassium hydroxide (KOH).
  2. Preparation of alkene from alkyne.
  3. Preparation of alkene from alcohols


Preparing alkene by Alkyl halides with alcoholic potassium hydroxide (KOH).

When alkyl halide compound is heated with alcoholic potassium hydroxide, an alkene is formed as the product. A H-X molecule is removed from alkyl halide to give alkene. Heated KOH is acted as an alkaline and remove a hydrogen atom which is linked to a carbon atom which is adjoin to the halogen linked to the carbon atom.

preparation of alkene from alkyl halides with alcoholic KOH

Preparing alkene from alkyne

Alkynes are reduced to alkenes using Linda catalyst ( alladium- charcoal catalyst partially deactivated with poison like sulfur compounds or quinoline ) and hydrogen gas (H2).

If alkyne reacts with hydrogen gas in the presence of Pt or Pd or Ni catalyst, alkane is given, not alkene.

preparation of alkenes from alkynes

Preparing alkene from alcohols

When alcohols are heated with a dehydrator, a water molecule is eliminated from alcohol molecule and alkene is formed as the product. As a dehydrator one of the following substance can be used.

  • Concentrated sulfuric acid
  • Alumina (Al2O3)
  • Phosphoric acid
  • Diphosphorus pentoxide (P2O5)

Example: When ethanol is heated with concentrated sulfuric, ethene acid is formed.


alkynes preparing by alcohol and concentrated
                     sulfuric acid


Questions




If there is no dehydrator to dehydrate an alcohol to alkene, what are the alternative ways of preparating an alkene?

If there is no direct way, you have to find an alternative method. It may be longer than usual one.

Prepare an alkyl halide from alcohol using a suitable reagent like PCl3 or PBr3. Then, alkyl halide is heated with alcoholic KOH to produce alkene.



Can I prepare an alkene by ethanol?

Ethanol is an alcohol which contains two carbon atoms. Therefore, you can prepare ethene which is the simplest alkene from ethanol. You need an dehydrator to convert ethanol to ethene.



In the preparation of an alkene from 1-propanol, what will be the physical changes?

1-propanol is a liquid at room temperature. When you prepare propene (an alkene with three carbon atoms) by heating 1-propanol with a dehydrator, a physical state change can be seen. Because, propene is a gas.



method of preparation of alkenes

  • Heating alkyl halides with alcoholic potassium hydroxide (KOH) or sodium hydroxide.
  • Preparation of alkene from alkyne by reacting with hydrogen gas in the presence of platinum catalyst and Lindlar catalyst.
  • Preparation of alkene from alcohols by heating with a dehydrator.


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