Chapter 10: Introduction to Free Radicals and Their Reactions

Introduction to Free Radicals

Definition and Examples

Free radicals are highly reactive chemical species that contain one or more unpaired electrons. They are electrically neutral and play a significant role in many organic reactions, especially those involving alkanes and alkenes.

• Free Radical: A species with an unpaired electron.

• Examples: O2 (dioxygen), NO2 (nitrogen dioxide), NO (nitric oxide).

• Free radicals are often represented with a single dot (•) indicating the unpaired electron.

Structure and Stability of Alkyl Radicals

Electronic Structure

Alkyl radicals are carbon-centered radicals. The geometry and hybridization of the carbon atom bearing the unpaired electron can influence the stability of the radical.

• Planar Methyl Radical (•CH3): The carbon is sp2-hybridized, and the unpaired electron occupies a 2p orbital (120° bond angles).

• Pyramidal Methyl Radical: The carbon is sp3-hybridized (109.5° bond angles), and the unpaired electron is in an sp3 orbital.

• Most alkyl radicals are closer to planar (sp2) geometry, which allows for better delocalization of the unpaired electron.

Classification of Alkyl Radicals

Alkyl radicals are classified similarly to carbocations, based on the number of alkyl groups attached to the radical center:

• Methyl radical: •CH3

• Primary radical: R-CH2•

• Secondary radical: R2CH•

• Tertiary radical: R3C•

Stability of Alkyl Radicals

The stability of alkyl radicals increases with the number of alkyl substituents on the radical center, due to hyperconjugation and inductive effects:

• Order of stability: Methyl < Primary < Secondary < Tertiary

• Reason: Alkyl groups donate electron density, stabilizing the electron-deficient radical center.

Bond Dissociation and Radical Formation

Bond Dissociation

Radicals are commonly formed by homolytic bond cleavage, where each atom in a bond retains one electron:

• Homolytic cleavage: $X:Y \rightarrow X\cdot + Y\cdot$

• Heterolytic cleavage: $X:Y \rightarrow X^+ + Y^-$

Bond Dissociation Enthalpy (BDE)

BDE is the energy required to break a bond homolytically. Lower BDE indicates easier radical formation.

Bond	BDE (kJ/mol)	BDE (kcal/mol)
H–H	436	104
Cl–Cl	243	58
CH3–H	439	105
CH3–CH2	368	88
CH3–Br	293	70
CH3–Cl	350	84
CH3–I	238	57
CH3–O	385	92
CH3–S	272	65
CH3–F	452	108

Additional info: Table values are representative; refer to your textbook for a complete list.

Example: Bond Dissociation in 2-Methylpropane

• Primary C–H bond: $\Delta H^\circ = +422\ \text{kJ/mol}$ (101 kcal/mol)

• Tertiary C–H bond: $\Delta H^\circ = +400\ \text{kJ/mol}$ (95 kcal/mol)

• Conclusion: Tertiary C–H bonds are easier to break, forming more stable radicals.

Free Radical Halogenation of Alkanes

Methane Chlorination

Chlorination of methane is a classic example of free radical halogenation, proceeding via a chain mechanism:

• Overall Reaction: $CH_4 + Cl_2 \rightarrow CH_3Cl + HCl$

• Mechanism:

  1. Initiation: $Cl_2 \rightarrow 2Cl\cdot$ (homolytic cleavage by light or heat)

  2. Propagation:

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

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

  3. Steps 2 and 3 repeat, propagating the chain reaction.

Regioselectivity in Halogenation

Bromination and chlorination of alkanes differ in their selectivity:

• Bromination: Highly regioselective, favoring substitution at the most substituted (tertiary) carbon.

• Chlorination: Less selective, more likely to produce a mixture of products.

Type of Hydrogen	Relative Rate (Br2)	Relative Rate (Cl2)
Tertiary (R3CH)	1640	5.2
Secondary (R2CH2)	82	3.9
Primary (RCH3)	1	1

Example: Bromination of 2-methylpentane yields a major product due to regioselectivity (76% yield for the major isomer).

Free Radical Addition to Alkenes and Alkynes (Peroxide Effect)

Markovnikov vs. Anti-Markovnikov Addition

• Markovnikov Addition: In the absence of peroxides, H–X (X = Cl, Br, I) adds to alkenes so that the hydrogen attaches to the carbon with more hydrogens.

• Anti-Markovnikov Addition: In the presence of peroxides (ROOR), HBr adds to alkenes so that the bromine attaches to the less substituted carbon.

Mechanism: Free-Radical Addition of HBr to 1-Butene

• Initiation: Homolytic cleavage of peroxide forms alkoxy radicals.

• Propagation:

  1. Alkoxy radical abstracts H from HBr, forming a bromine radical.

  2. Bromine radical adds to the alkene, forming the more stable secondary radical.

  3. The new radical abstracts H from another HBr, forming the product and regenerating the bromine radical.

• Steps 2 and 3 repeat, propagating the chain.

Example: 1-Butene + HBr (with ROOR) → 1-Bromobutane (anti-Markovnikov product)

Metal-Ammonia Reduction of Alkynes

Formation of Trans-Alkenes

Reduction of alkynes with sodium in liquid ammonia produces trans-alkenes via a radical mechanism.

• Overall Reaction: $RC \equiv CR' + 2Na + 2NH_3 \rightarrow RC(H)=CH(R') + 2NaNH_2$

• Mechanism:

  1. Electron transfer from sodium to alkyne forms a radical anion.

  2. Protonation by ammonia gives a vinyl radical.

  3. Second electron transfer forms a vinyl anion.

  4. Second protonation yields the trans-alkene.

Synthetic Applications

Example 1: Synthesis of 1-Bromo-2,3,3-trimethylbutane

• Starting material: 2,2,3-trimethylbutane

• Strategy: Use selective bromination at the primary carbon to introduce the bromine atom.

Example 2: Synthesis of trans-1,2-dichlorocyclopentane from cyclopentane

• Strategy: Halogenate cyclopentane to form the dichloro derivative, then use stereoselective methods to obtain the trans isomer.

• Additional info: This may involve free radical halogenation followed by separation of stereoisomers.

Polymerization of Ethylene

Free-Radical Polymerization

Ethylene can undergo free-radical polymerization to form polyethylene, a common synthetic polymer.

• Initiation: Peroxide dissociation forms radicals that add to ethylene.

• Propagation: The radical adds to another ethylene molecule, extending the chain.

• Termination: Two radical chains combine, ending the polymerization.

Equation:

$n\ CH_2=CH_2 \xrightarrow{\text{radical initiator}} -(CH_2-CH_2)-_n$

Example: Free-radical polymerization is used industrially to produce plastics.