Question

Explain the preparation of alkenes from vicinal dihalides with an example.

Answer 1

Hint: To solve this question we must know what vicinal dihalides contain. These are compounds containing two halogen atoms present on two adjacent carbon atoms.

Complete answer:
Vicinal dihalides undergo treatment of zinc metal to form an alkene. The treatment of vicinal dihalides with zinc metal will help in removal of a molecule from \[{\text{Zn}}{{\text{X}}_{\text{2}}}\]thereby resulting in the formation of the alkene. The reaction is an example of dehalogenation.
Example:
\[\begin{gathered}
  {\text{C}}{{\text{H}}_{\text{2}}}{\text{Br - C}}{{\text{H}}_{\text{2}}}{\text{Br + Zn}} \to {\text{C}}{{\text{H}}_{\text{2}}}{\text{ = C}}{{\text{H}}_{\text{2}}}{\text{ + ZnB}}{{\text{r}}_{\text{2}}} \\
  {\text{C}}{{\text{H}}_{\text{3}}}{\text{CHBr - C}}{{\text{H}}_{\text{2}}}{\text{Br + Zn }} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{CH = C}}{{\text{H}}_{\text{2}}}{\text{ + ZnB}}{{\text{r}}_{\text{2}}} \\
\end{gathered} \]

Additional Information:
Vicinal dihalides are also known as Geminal dihalides. In our language we can say, the brothers from two different mothers. In vicinal dihalides the same halogen atom is attached with adjacent carbon atoms of the same compound. These are compounds that have halogens on adjacent carbons, and are prepared by the reaction between a halogen and an alkene. The simplest example is the reaction between ethylene and chlorine to give 1,2-dichloroethane.

Note: Alkenes belong to the family of hydrocarbons. They contain a double bond between the carbon atoms. They are also known as olefins, are organic compounds that consist of carbon and hydrogen atoms with one or more carbon-carbon double bonds in their chemical structure. Alkenes are unsaturated hydrocarbons. They are hydrocarbons because they are made of only carbon and hydrogen atoms, and they are unsaturated because they have one or more double bonds in their chemical structure. The general formula for alkenes is \[{{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}\].