Question

How can you prepare alkenes? Give any two methods of preparations of alkenes with equations.

Answer 1

Alkenes are the unsaturated hydrocarbon that contains one double in their structure.

For the preparation of alkenes following two methods are used:

1. Dehydration: Industrially prepared
2. Dehydrohalogenation

Dehydration: Industrial preparation

Reactant used: Ethanol$\mathbf{\left(}{\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{5}}\mathbf{OH}\mathbf{\right)}$, Alumina$\mathbf{\left(}{\mathbf{Al}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{3}}\mathbf{\right)}$

1. In this reaction when Ethanol vapours are passed through a tube that contains Alumina at a temperature of ${300}^{\circ }\mathrm{C}$ then Ethene is formed as the major product.
2. The chemical reaction occurs as follows:$\underset{\left(\mathrm{Ethanol}\right)}{{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{l}\right)}\underset{{350}^{\circ }\mathrm{C}}{\overset{{\mathrm{Al}}_2{\mathrm{O}}_3}{\to }}\underset{\left(\mathrm{Ethene}\right)}{{\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{g}\right)}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Dehydrohalogenation:

Reactant used: Chloroethane$\mathbf{\left(}{\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{5}}\mathbf{Cl}\mathbf{\right)}$, Alcoholic $\mathbf{\left(}\mathbf{KOH}\mathbf{\right)}$

1. In this reaction when Chloroethane is treated with an Alcoholic Potassium hydroxide then we get Ethene as the major product.
2. The alcoholic Potassium hydroxide used here is hot and concentrated in nature.
3. In this reaction Potassium chloride and water is also formed.
4. The chemical reaction is as follows:$\underset{\left(\mathrm{Chloroethane}\right)}{\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Cl}\right)\left(\mathrm{g}\right)}+\underset{(\mathrm{Potassium}\mathrm{hydroxide})}{\mathrm{KOH}\left(\mathrm{s}\right)}\to \underset{\left(\mathrm{Ethene}\right)}{{\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{g}\right)}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\underset{(\mathrm{Potassium}\mathrm{chloride})}{\mathrm{KCl}\left(\mathrm{s}\right)}$

Hence, we successfully get Ethene as the major product by the use of two methods.