Unit 13: More Radical Chemistry – Free Radical Reactions in Organic Chemistry

Free Radical Chemistry in Organic Synthesis

Introduction to Free Radical Reactions

Free radical reactions are a fundamental class of organic transformations involving species with unpaired electrons. These reactions are crucial for understanding halogenation of alkanes, polymerization, and various addition mechanisms. This unit focuses on the mechanisms, selectivity, and applications of free radical chemistry, especially in the context of halogenation and polymer formation.

Tracking Electrons with Arrows

Arrow Notation in Mechanisms

• Single-barbed arrow (fishhook): Indicates the movement of a single electron, typical in radical mechanisms.

• Double-barbed arrow: Represents the movement of an electron pair, as in polar mechanisms.

• Arrows must clearly show the origin and destination of electrons.

Example: In the homolytic cleavage of a C–H bond, one electron goes to the carbon, the other to the hydrogen, generating two radicals.

Free Radical Halogenation of Alkanes

Chlorination and Bromination

• General Reaction: Reaction of an alkane with Cl2 or Br2 under light (hv) leads to substitution of H by Cl or Br.

• Example: $\ce{CH3CH2CH3 + Cl2 ->[hv] CH3CHClCH3 + HCl}$

• Excess halogen can lead to multiple substitutions.

Mechanism of Free Radical Substitution

1. Initiation: Homolytic cleavage of Cl2 or Br2 forms two halogen radicals. $\ce{Cl2 ->[hv] 2Cl\cdot}$

2. Propagation: Halogen radical abstracts a hydrogen atom from the alkane, forming an alkyl radical and HX. $\ce{R-H + Cl\cdot -> R\cdot + HCl}$ Alkyl radical reacts with Cl2 to regenerate Cl radical. $\ce{R\cdot + Cl2 -> RCl + Cl\cdot}$

3. Termination: Combination of two radicals to form a stable molecule. $\ce{Cl\cdot + Cl\cdot -> Cl2}$ $\ce{R\cdot + Cl\cdot -> RCl}$ $\ce{R\cdot + R\cdot -> R-R}$

Product Distribution and Selectivity

• Different types of hydrogens (primary, secondary, tertiary) have different reactivities.

• Chlorination is less selective; bromination is much more selective for the most substituted (tertiary) hydrogens.

Hydrogen Type	Relative Reactivity (Chlorination)
Methyl (1°)	1
Methylene (2°)	3.8
Methine (3°)	5.0
Benzylic/Allylic	Very high

Additional info: Actual ratios may vary; bromination is even more selective for tertiary hydrogens.

Kinetics vs. Thermodynamics in Radical Reactions

Energy Profiles

• Kinetic control: Product distribution determined by the lowest activation energy pathway; forms fastest.

• Thermodynamic control: Product distribution determined by the most stable product; forms under equilibrium conditions.

Example: At low temperature, the kinetic product dominates; at high temperature or long reaction time, the thermodynamic product may predominate.

Stability of Carbon Radicals

Order of Stability

• 3° (tertiary) > 2° (secondary) > 1° (primary) > methyl

• Stabilized by resonance (e.g., allylic, benzylic radicals)

Comparison: Chlorination vs. Bromination

Reactivity and Selectivity

• Chlorination: More reactive, less selective.

• Bromination: Less reactive, more selective (prefers tertiary hydrogens).

• Iodination: Not reactive enough for radical substitution.

• Fluorination: Too reactive, often explosive.

Free Radical Addition to Alkenes: Hydrobromination

Anti-Markovnikov Addition of HBr

• In the presence of peroxides, HBr adds to alkenes via a free radical mechanism, giving anti-Markovnikov products.

• Mechanism involves initiation (formation of Br radical), propagation (addition to alkene), and termination.

• Only HBr works; HCl and HI are not favorable due to thermodynamics.

Equations:

• $\ce{ROOR ->[heat/light] 2RO\cdot}$

• $\ce{RO\cdot + HBr -> ROH + Br\cdot}$

• $\ce{Br\cdot + CH2=CH2 -> CH2CH2Br\cdot}$

• $\ce{CH2CH2Br\cdot + HBr -> CH3CH2Br + Br\cdot}$

Properties of Carbon Intermediates

Species	Stability Order	Hybridization	Rearrangement
Carbocation	3° > 2° > 1°	sp2	Yes
Radical	3° > 2° > 1°	sp2	No
Carbanion	1° > 2° > 3°	sp3	No

Additional info: Resonance stabilizes all intermediates, but the order of stability remains as above.

Special Cases in Radical Chemistry

Benzylic and Allylic Hydrogens

• Benzylic and allylic hydrogens are especially reactive due to resonance stabilization of the resulting radicals.

• Selective halogenation can be achieved using NBS (N-bromosuccinimide) and dilute Br2 in CCl4 with light or peroxide.

Mechanism for Allylic Bromination (NBS)

1. Initiation: Formation of Br radicals.

2. Propagation: Abstraction of allylic H, formation of allylic radical, reaction with Br2.

3. Termination: Radical recombination or disproportionation.

Radical Polymerization

Free Radical Addition Polymers

• Alkenes can undergo radical polymerization to form addition polymers such as polyethylene, polypropylene, and polystyrene.

• Initiation: Formation of radical initiator (e.g., benzoyl peroxide, AIBN).

• Propagation: Successive addition of monomer units to the growing radical chain.

• Termination: Combination or disproportionation of radical chains.

Example: Polymerization of ethylene to polyethylene: $\ce{n CH2=CH2 ->[-\text{radical initiator}] (CH2CH2)_n}$

Radical Initiators

Common Initiators

• Heat: Hydrogen peroxide (H2O2), benzoyl peroxide

• Light: t-Butyl peroxide

• Oxygen: AIBN (azobisisobutyronitrile)

These compounds decompose to generate radicals that initiate chain reactions.

Safety and Environmental Aspects

Ether Autoxidation

• Ethereal solvents can form explosive peroxides upon standing in air.

• Should be tested for peroxides before use and not stored for long periods.

Cross-Linking in Oils

• Unsaturated oils (vegetable, olive, sesame) are susceptible to radical-induced cross-linking, leading to spoilage.

• Mineral oil (saturated) is stable and does not undergo such reactions.

Polymer Production and Recycling

Global Impact

• Plastic production has increased exponentially, with over 400 million metric tons produced annually.

• Recycling rates are low, especially in the US, leading to environmental concerns.

Additional info: Most plastics are addition polymers; condensation polymers (e.g., polyesters, polyamides) are also important.