Methods Of Preparation Of Alkenes In The Laboratory


Alkenes belong to the family of hydrocarbons. They contain a double bond between the carbon atoms. The general formula for alkenes is CnH2n. Alkenes preparation can be done by various methods. Explore different methods of preparation of alkenes.

From alkynes: Alkynes are used for the preparation. Alkyne to alkene conversion is carried out by the reduction of alkynes with hydrogen in the presence palladised charcoal. The charcoal used is moderately deactivated with the help of quinoline or sulphur compounds. This reaction results in the formation of alkenes. Palladised charcoal which is halfway deactivated is called as Lindlar’s catalyst. The alkenes obtained from the above reaction have cis geometry. In order to form trans alkenes, alkynes are made to undergo reduction with sodium in liquid ammonia.

Preparation Of Alkenes

Preparation of Alkenes from Alkynes

From alkyl halides: Alkenes are obtained by heating alkyl halides with alcoholic potash. Alcoholic potash is obtained by dissolving potassium hydroxide in alcohol. In this reaction dehydrohalogenation takes place i.e. a single molecule of halogen acid is removed. The rate of reaction depends upon the alkyl group and the nature of the halogen group attached.

Preparation Of Alkenes

Preparation of Alkenes from Alkyl Halides

From vicinal halides: Vicinal dihalides can be defined as the dihalides in which two adjacent carbon atoms are attached to two halogens. When such dihalides react with zinc metal, they lose halogen molecules which result in the formation of alkenes. Such a reaction of preparation of alkenes from Vicinal dihalides is known as dehalogenation.

Preparation Of Alkenes

Preparation of Alkenes from Vicinal Halides

From alcohols: Alcohols reacts with concentrated sulphuric acid which results in the formation of alkenes due to the elimination of a water molecule. As water molecule is removed in this reaction, it is called as acidic dehydration of alcohol and the dehydrating agent is concentrated sulphuric acid.

Preparation Of Alkenes

Preparation of Alkenes from alcohol

These were few methods for preparation of alkenes in a laboratory. Join Byju’s  and clear all doubts by our expert mentors and lecturers.

Practise This Question

For alkenes, generally boiling point of Cis form is higher than trans form - True or False?