Question

Write the chemical equation for the preparation of ethene from below,
1. Alkyl halides
2. Vicinal dihalide

Answer 1

Hint: First we will treat halides with a good base which will result in the substitution of the halide group and the formation of alcohol molecules. After that we will treat the product with strong acid which will lead to the dehydration of alcohol to yield alkenes.

Preparation of ethane from Alkyl halides -
When an alkyl halide (ethyl chloride) is treated with alcoholic KOH (good base), $\beta$ - elimination results in the production of ethene by dehydration of alcohol.
When ethyl alcohol is heated with conc. ${H_2}S{O_4}$ at $170^\circ C$, dehydration of alcohol takes place and ethene is formed.
(this will lead to formation of alcohol molecules)

Dehydration of Alcohol to yield Alkenes -
To synthesize alkenes it is done by dehydration of alcohol which undergoes ${E_1}$ or ${E_2}$ mechanism to lose water resulting in the formation of double bonds.
The dehydration reaction of alcohol to generate alkene proceeds by heating the alcohol in the presence of a strong acid, such as ${H_2}S{O_4}$ ,${H_3}P{O_4}$ ,at high temperatures.
$C{H_3}C{H_2}OH\xrightarrow{{Acid,\vartriangle }}C{H_2} = C{H_2} + H\mathop O\limits_{ \bullet \bullet }^{ \bullet \bullet } H$
Or
Ethyl alcohol may also be converted into ethane by passing vapours of ethyl alcohol over catalyst $\left( {A{l_2}{O_3},{H_3}P{O_4}/A{l_2}{O_3}} \right)$ .The reaction is carried out at $350^\circ C$ in the presence of $A{l_2}{O_3}$ and at $250^\circ C$ when ${H_3}P{O_4}/A{l_2}{O_3}$ is used
${C_2}{H_5}OH \to {C_2}{H_4} + {H_2}O$

Preparation of ethene by Vicinal dihalide
When Vicinal dihalide is heated with the zinc dust, two halide atoms are removed from the compound and ethane is formed.
$C{H_2}Cl - C{H_2}Cl + Zn \to C{H_2} = C{H_2} + ZnC{l_2}$:

NOTE – The ${H_2}O$ and $KX$ molecules are eliminated while performing the reaction. Hence, we can say that Alkene is prepared from alkyl halide by Elimination reaction. In our reaction, strong base, as well as a strong acid is used to complete the reaction.