Radical Reactions Notes

Radicals - Introduction

• Free radicals form when bonds break homolytically.

• Single-barbed arrows show electron movement.

Radical Structure and Geometry

• Radicals are trigonal planar ($sp^2$ hybridized) or shallow trigonal pyramidal ($sp^3$ hybridized).

Free Radical Stability

• Radicals are neutral but electron-deficient.

• Radical stability trend: same as carbocations (electron-deficient species).

BDE Trend

• Bond strength is inversely proportional to carbon radical stability.

• More stable radical = weaker bond.

Free Radical Resonance

• Radicals are stabilized by resonance delocalization.

• Fishhook arrows indicate resonance forms.

Benzylic Radicals

• More resonance delocalization = more stable radical.

• Benzylic radicals are more stable than allylic radicals.

Radical Mechanisms

• Radical mechanisms follow patterns but don't undergo rearrangement.

• Six key arrow-pushing patterns:

  1. Homolytic cleavage

  2. Addition to a pi bond

  3. Hydrogen abstraction

  4. Halogen abstraction

  5. Elimination

  6. Coupling

Mechanism Steps

• Initiation: radicals formed from non-radical species.

• Propagation: radical reacts with non-radical to form a new radical.

• Termination: two radicals react to form a non-radical.

Chlorination of Methane

• Radical mechanism stages:

  1. Initiation: chlorine radicals created.

  2. Propagation: self-sustaining steps.

  3. Termination: radicals collide/couple.

• Polychlorination Prevention: use excess methane.

Radical Initiators

• Radical initiator: possesses a weak bond that cleaves homolytically with heat or light (dihalides, alkyl peroxides, acyl peroxides).

Radical Inhibitors

• Radical inhibitors: react with radicals, preventing chain initiation/propagation (Oxygen, Hydroquinone).

• Radical reactions slow down in the presence of oxygen.

Halogenation Thermodynamics

• Fluorination: too exothermic (impractical).

• Iodination: endothermic (doesn't occur).

• Chlorination & Bromination: exothermic (favored).

Halogenation Selectivity

• Bromination: slower, more selective than chlorination.

Halogenation Regioselectivity

• Chlorine: less discriminant.

• Bromine: more regioselective (explained by the Hammond Postulate).

Halogenation Stereochemistry

• Halogenation of alkanes can form a new chiral center.

Allylic Halogenation

• Alkenes undergo H-abstraction at the allylic carbon.

Allylic Halogenation with NBS

• NBS: used as a source of $Br$ radicals to avoid alkene addition.

Anti-Markovnikov Addition of HBr

• Peroxides initiate radical mechanism of addition.

Radical Polymerization

• Proceeds through a radical chain mechanism.

• Chain Branching: forms flexible plastic.

Synthetic Utility of Halogenation

• Functionalizing unreactive alkanes.