Radical Chlorination

Overview

• Involves chlorination of alkanes via a radical mechanism.
• Key steps: Initiation, Propagation, Termination.

Initiation Step

• A photon of UV light promotes homolysis.
• Cl2 molecule dissociates into two chlorine radicals.
• Single covalent species transforms into two radical species.

Propagation Step

• Chlorine radical interacts with alkanes.
• Unstable radicals propagate further radicals.
• Example: Methane → Methyl Radical + HCl.
• Methyl radical interacts with Cl2, forming chlorinated product.
• Can lead to further chlorinated products (e.g., dichlorinated, trichlorinated).

Termination Step

• Any two radicals can collide, forming covalent compounds.

Details of Radical Mechanisms

• Planar, sp2 hybridized carbons may form racemic mixtures.
• Carbon radicals with different groups can form stereocenters.

Regioselectivity in Radical Halogenation

Bromination

• More regioselective.
• Example: Bromination of isobutane favors tertiary bromoalkane.

Chlorination

• Less regioselective.
• Generates mixture of tertiary and primary chloroalkanes.

Stability Differences

• Bromine radicals are more stable than chlorine radicals.
• Larger atomic size allows bromine radicals to follow lower energy pathways.
• Chlorine radicals are less stable and extract any available hydrogen.

Conclusion

• Radical mechanisms involve initiation, propagation, termination steps.
• Bromination is more regioselective due to stability differences between halogen radicals.

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