Question

In which of the following, homolytic bond fission takes place?
A. Hydrolysis of ethyl chloride
B. Nitration of benzene
C. Addition of HBr to benzene
D. Free radical chlorination of methane.

Answer 1

Hint: Homolytic fission is the distribution of the electrons when the bond is broken down. Due to the homolytic fission, the free radicals are formed as a product. Chlorination is the process in which the chlorine is added to a substance by the homolytic cleavage of the bond.

Complete step by step answer:
- Firstly, we have to understand the chlorination of methane.
- This reaction is an example of a photochemical reaction also because the reaction takes place in the presence of sunlight.
- When methane and chlorine react with each other in the presence of light or ultraviolet radiation it yields chloromethane.
- Here, the substitution of hydrogen by chlorine takes place, also known as substitution reaction and the process is called chlorination of methane.
- The mechanism of the reaction is known as the free radical substitution, in which the first step is to break the bond of chlorine through homolytic fission:
\[\text{C}{{\text{l}}_{2}}\to \ 2\text{Cl}\centerdot \]
- Now, the propagation of chain takes place by the substitution of hydrogen by chlorine atom i.e.
\[\text{C}{{\text{H}}_{4}}\text{ + Cl}\centerdot \text{ }\to \text{ C}{{\text{H}}_{3}}\centerdot \ \text{+ HCl}\]
\[\text{C}{{\text{H}}_{3}}\centerdot \text{ + C}{{\text{l}}_{2}}\ \to \text{ C}{{\text{H}}_{3}}\text{Cl + Cl}\centerdot \]
- The last step is the chain termination reaction, in which the process ends and the free radicals are removed by the system.
\[\text{2Cl}\centerdot \ \to \ \text{C}{{\text{l}}_{2}}\]
\[\text{C}{{\text{H}}_{3}}\centerdot \text{ + Cl}\centerdot \text{ }\to \text{ C}{{\text{H}}_{3}}\text{Cl}\]
\[\text{C}{{\text{H}}_{3}}\centerdot \text{ + C}{{\text{H}}_{3}}\centerdot \text{ }\to \text{ C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{3}}\]
- Heterolytic fission is the distribution of electrons when the covalent bond is broken such that the more electronegative atom gets the electrons.
- Hydrolysis of ethyl chloride, nitration of benzene and addition of HBr to the benzene consist of heterolytic fission.

Therefore, homolytic fission takes place in (D) the free radical chlorination of methane.

Note: After the end of the heterolytic fission, the cation and anion are formed as a result because the transfer of bond pair of electron takes place only towards the one atom forming it anion with a negative charge whereas one who doesn't get any electron is a cation with a positive charge.