Preparation of Alkenes

Preparation of Alkenes by Partial Reduction

Alkynes on partial reduction with H2 in the presence of suitable catalysts yield alkenes. Reduction of alkynes in the presence of palladised charcoal partially deactivated with poison such as PbCO3, S or quinoline predominantly gives cis-alkenes. This catalyst is known as Lindlar's catalyst. In contrast, Na in liquid ammonia is used to obtain trans isomers.

Preparation of cis Alkanes
Preparation of trans Alkanes

Preparation of Alkenes from Alkyl Halides

Haloalkanes, also known as alkyl halides, on heating with a strong base such as alcoholic solution of potassium hydroxide eliminate one molecule of halogen acid (HCl, HBr or HI) to form alkenes. This reaction is known as dehydrohalogenation.

Preparation of Alkenes from alkyl halides : dehydrohalogenation reaction

Since in this reaction, hydrogen is removed from beta carbon, this reaction is also known as β-elimination. Some important points regarding this reaction such as Saytzeff rule are discussed in Elimination reactions in haloalkanes.

β-carbon : Carbon atom next to the carbon to which halogen is attached is known as β-carbon. Hydrogen atoms attached to β-carbon are termed as β-hydrogens.

Preparation of Alkenes from Vicinal Dihalides

In vicinal dihalides, two halogens are attached to adjacent carbon atoms. Vicinal dihalides on reaction with Zn lose a molecule of ZnX2 to form alkenes.

Preparation of alkenes from vicinal dihalides
Preparation of alkenes from vicinal dihalides

Preparation of Alkenes from Alcohols

Alcohols containing a β-hydrogen on heating with concentrated sulphuric acid (H2SO4) lose a molecule of water to form alkenes. Due to the elimination of water and use of acid, this reaction is known as acidic dehydration of alcohols. Since a hydrogen atom is eliminated from β-carbon, this reaction is an example of β-elimination.

Preparation of alkenes from alcohols

Alternatively, the vapours of alcohols are passed over heated alumina at 623K-633K to form alkenes with the elimination of one molecule of water.


Revision includes earlier concepts

Let's do some revision. The reactions used in the answer are the ones that we have learned so far. To find the answer, all you need to do is click the 'Answer' button; however, it is not recommended. Try to put some effort before clicking the answer button.

Conversion : Prepare ethane from ethyl chloride.

Do not forget the reactions. We will need them to do conversions in later chapters.