Question

In which of the following, homolytic bond fission takes place (1) Alkaline hydrolysis of ethyl chloride (2) Addition of \(\mathrm{HBr}\) to double bond (3) Photochlorination of methane (4) Nitration of benzene

Short answer

Photochlorination of methane involves homolytic bond fission.

Step-by-step solution

- Define Homolytic Bond Fission

In homolytic bond fission, a covalent bond breaks such that each of the bonded atoms takes away one of the electrons from the bond, resulting in the formation of two free radicals. This process typically requires energy in the form of heat or light.

- Analyze Alkaline Hydrolysis of Ethyl Chloride

Alkaline hydrolysis of ethyl chloride involves the breaking of a C-Cl bond in the presence of a base, forming ethanol and chloride ion. This process takes place via heterolytic bond fission, not homolytic bond fission.

- Review Addition of \(\text{HBr}\) to Double Bond

The addition of \(\text{HBr}\) to a double bond proceeds via the formation of carbocations, which involves heterolytic bond breaking. Therefore, it is not an example of homolytic bond fission.

- Examine Photochlorination of Methane

In the photochlorination of methane, chlorine molecules are exposed to UV light, causing the Cl-Cl bond to break homolytically. This results in the formation of two chlorine radicals, which then react with methane. This is a clear example of homolytic bond fission.

- Look into Nitration of Benzene

Nitration of benzene involves the generation of a nitronium ion (NO2+) and its electrophilic addition to benzene, usually followed by retraction of hydrogen via heterolytic bond fission. This is not an example of homolytic bond fission.

- Conclusion

Based on the analysis, photochlorination of methane is the reaction where homolytic bond fission takes place.

Key concepts

Free Radicals

Free radicals are highly reactive species with unpaired electrons. When a covalent bond undergoes homolytic bond fission, it produces two free radicals, each with one unpaired electron.
Homolytic bond fission typically requires energy input from light or heat to break the bond evenly.
Examples include:

• Breaking Cl-Cl bond in chlorination reactions using UV light.
• Formation of hydroxyl radicals (OH•) in water oxidation processes.

Free radicals are essential intermediates in many organic reactions, leading to new chemical products.

Photochemistry

Photochemistry is the study of chemical reactions that occur due to the absorption of light. Light provides the energy necessary for homolytic bond fission, creating free radicals.
A common example is the photochlorination of methane:

• UV light breaks Cl-Cl bonds homolytically, forming chlorine radicals.
• Chlorine radicals react with methane to create methyl radicals and hydrogen chloride.

This process illustrates the role of light in driving chemical reactions and forming reactive intermediates.

Organic Reactions

Organic reactions involve the modification of organic compounds through various mechanisms. Homolytic bond fission and free radical formation are crucial mechanisms in organic chemistry.
Examples of reactions driven by organic free radicals include:

• Radical substitution reactions such as halogenation of alkanes.
• Polymerization processes where monomers form long chain polymers via radical intermediates.

Understanding homolytic bond fission helps in predicting the behavior of radicals and designing efficient synthetic pathways.