Free Radical Halogenation and Radical Stability in Organic Chemistry

Bond Cleavage: Homolytic vs. Heterolytic

Homolytic Cleavage

Homolytic cleavage occurs when a covalent bond breaks and each atom retains one of the shared electrons, forming two radicals. This process is often initiated by heat or light.

• Definition: The breaking of a bond so that each fragment retains one electron, producing two radicals.

• Example: $Br_2 \xrightarrow{\Delta \text{ or } h\nu} Br^\cdot + Br^\cdot$

• Radicals: Highly reactive species with an unpaired electron.

Heterolytic Cleavage

Heterolytic cleavage occurs when a covalent bond breaks and both electrons go to one atom, forming a cation and an anion.

• Definition: The breaking of a bond so that one fragment retains both electrons, producing ions.

• Example: $A-B \rightarrow A^+ + B^-$

• Application: In alkyl halides, the halogen often acquires the bonded electrons due to its higher electronegativity.

Free Radical Halogenation Mechanism

General Reaction

Alkanes react with halogens (Cl2, Br2) under heat or light to form haloalkanes via a free radical mechanism.

• Example: $CH_4 + Cl_2 \xrightarrow{\Delta \text{ or } h\nu} CH_3Cl + HCl$

Mechanistic Steps

1. Initiation: Formation of radicals by homolytic cleavage of a halogen molecule.

   • $Cl_2 \xrightarrow{\Delta \text{ or } h\nu} Cl^\cdot + Cl^\cdot$

2. Propagation: Radicals react with stable molecules to form new radicals and products.

   • $CH_4 + Cl^\cdot \rightarrow CH_3^\cdot + HCl$

   • $CH_3^\cdot + Cl_2 \rightarrow CH_3Cl + Cl^\cdot$

3. Termination: Two radicals combine to form a stable molecule, ending the chain reaction.

   • $Cl^\cdot + Cl^\cdot \rightarrow Cl_2$

   • $CH_3^\cdot + Cl^\cdot \rightarrow CH_3Cl$

   • $CH_3^\cdot + CH_3^\cdot \rightarrow CH_3-CH_3$

Examples of Free Radical Halogenation

• Chlorination: $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$

• Bromination: $CH_3CH_2CH_3 + Br_2 \rightarrow$ (major product: bromine at most substituted carbon)

Product Selectivity and Major/Minor Products

Factors Affecting Product Distribution

• Radical Stability: More substituted radicals (tertiary > secondary > primary) are more stable and thus more likely to form.

• Halogen Reactivity: Bromine is more selective than chlorine, favoring formation of the most stable radical.

Relative Rates of Radical Formation

Halogen	3°	2°	1°
Cl	5	3.8	1
Br	1600	82	1

Additional info: These values indicate that bromination is much more selective for tertiary hydrogens than chlorination.

Calculating Product Percentages

• For a molecule with 2 secondary and 6 primary hydrogens:

  • Secondary: $2 \times 3.8 = 7.6$

  • Primary: $6 \times 1 = 6$

  • Total: $7.6 + 6 = 13.6$

  • Secondary product: $\frac{7.6}{13.6} \times 100 = 55.9\%$

  • Primary product: $\frac{6}{13.6} \times 100 = 44.1\%$

Radical Stability and Trends

Order of Stability

• Radicals: $3^\circ > 2^\circ > 1^\circ > CH_3^\cdot$

• Carbocations: Same trend as radicals (electrophilic, electron-deficient).

• Carbanions: Opposite trend (nucleophilic, electron-rich).

Potential Energy Diagram

The activation energy for hydrogen abstraction decreases as the stability of the resulting radical increases. The reaction is more exothermic for primary hydrogens.

• 1° radical: highest activation energy

• 2° radical: intermediate

• 3° radical: lowest activation energy

Halogen Reactivity and Selectivity

Halogen	Reactivity	Selectivity
F2	Very high	Low
Cl2	High	Moderate
Br2	Low	High
I2	Very low	Very low

Additional info: Chlorine is most commonly used due to its balance of reactivity and selectivity.

Applications: Identifying Products

• Halogenation of alkanes produces mixtures of products, with the major product determined by radical stability and halogen selectivity.

• For cyclic alkanes, bromination under light yields the brominated product at the most substituted carbon.

• In some cases, only one product is possible, so no major/minor distinction is made.

Summary Table: Mechanistic Steps in Free Radical Halogenation

Step	Process	Example
Initiation	Formation of radicals	$Cl_2 \xrightarrow{\Delta \text{ or } h\nu} Cl^\cdot + Cl^\cdot$
Propagation	Radical reacts with molecule to form new radical	$CH_4 + Cl^\cdot \rightarrow CH_3^\cdot + HCl$
Termination	Two radicals combine to form stable molecule	$CH_3^\cdot + Cl^\cdot \rightarrow CH_3Cl$

Key Terms

• Radical: An atom or molecule with an unpaired electron.

• Homolytic cleavage: Bond breaking where each atom retains one electron.

• Heterolytic cleavage: Bond breaking where one atom retains both electrons.

• Initiation: Step where radicals are first formed.

• Propagation: Steps where radicals react to form new radicals and products.

• Termination: Step where radicals combine to form stable molecules.