Radical Reactions Lecture Summary

Overview

• Presenter: Leah from leah4sci.com
• Topic: Radical reactions, focusing on initiation, propagation, and termination
• Additional Resources: More information available at leah4sci.com/radical

What is a Radical?

• A radical is a single or unpaired electron.
• Electrons typically exist in pairs (e.g., in water, H2O, oxygen is bonded to hydrogen and has lone pairs).
• Free radicals are highly reactive due to having unpaired electrons.
• Free radicals can cause damage in biological systems by attacking cells or DNA.

Reaction Mechanisms

• Heterolytic cleavage: Electrons move together as a pair to one atom.
  • Represented by a double-headed arrow.
• Homolytic cleavage: Electrons split with one going to each atom.
  • Represented by a single-headed or fish hook arrow.
  • Both atoms become radicals.

Steps of a Radical Reaction

1. Initiation

• Purpose: Create radicals.
• Example: Cl2 (chlorine gas) can be split by heat or light.
• Mechanism: Homolytic cleavage where each chlorine atom gets one electron, forming two chlorine radicals.
• Recognition: No radicals on the reactant side, radicals appear only in the products.

2. Propagation

• Purpose: Radicals react with stable molecules to create new radicals, propagating the chain reaction.
• Example: Chlorine radical reacts with methane, forming methyl radicals and HCl.
• Mechanism: Radical reacts with a stable molecule, forming a new radical in the products.
• Recognition: One radical in the reactants, one radical in the products.

3. Termination

• Purpose: Two radicals combine to form a stable molecule, ending the reaction.
• Example: Two chlorine radicals form Cl2, or two methyl radicals form ethane.
• Mechanism: Radicals meet and form a stable bond.
• Recognition: Two radicals in reactants, none in products.

Example: Radical Chlorination of Methane

• Initiation: Cl2 breaks into two Cl radicals with heat/light.
• Propagation Step 1: Chlorine radical forms HCl and methyl radical.
• Propagation Step 2: Methyl radical reacts with another Cl2 to form chloromethane and a new Cl radical.
• Termination: Unlikely as radicals typically propagate rather than terminate.

Conclusion

• Radical reactions are characterized by initiation, propagation, and less frequently, termination.
• Understanding radical reactions is crucial for chemistry, especially in organic synthesis and biochemical processes.
• More resources are available on Leah's website for further study.