Dehydrohalogenation reaction

Dehydrohalogenation reaction:

·        It is also called a β-Elimination reaction and is a type of elimination reaction.

• ·        In this reaction alkyl halide when heated with alcoholic solution of potassium or sodium hydroxide, it undergoes dehydrohalogenation (i.e. removal of hydrogen and halogen) and forms alkene.

• ·        Dehydrohalogenation involves removal of the halogen atom from the α-carbon and a hydrogen atom from the adjacent β-carbon atom.

• ·        The relative reactivity of alkyl halide are I > Br > Cl > F (elimination of F is rarely used.) 

·        Chlorobenzene does not react with potassium hydroxide due to the presence of the benzene ring, which, due to stabilization as a result of aromaticity, does not give conventional elimination, as it would lead to a very unstable benzene intermediate.

• ·        If there is more than one type of β-hydrogens (β1 and β2) in alkyl halide then there is a possibility of formation of more than one alkene. In such case, one of the alkenes is formed as a major product, which can be identified by using Saytzeff’s rule.

Saytzeff’s rule: In a dehydrohalogentaion reaction, the preferred product is that alkene which is formed by removal of the hydrogen from the β-carbon having the fewest hydrogen substitutents.

Example: When 2-bromobutane is heated with alcoholic solution of KOH, it forms but-2-ene (80%) and but-1-ene (20%).

Since there are two types of β carbon (β1 and β2), therefore two alkenes are expected.  Here β2 is having fewer number of hydrogen than β1, so according to Saytzeff’s rule preferred product is formed by the removal of the hydrogen from β2. Thus but-2-ene is obtained as major product.