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Free Radicals in Organic Chemistry | chemca

Organic Chemistry

Free Radicals

Understanding neutral reaction intermediates with unpaired electrons.

By chemca Team • Updated Jan 2026

A Free Radical is a neutral chemical species (atom or group of atoms) that possesses an odd or unpaired electron. They are highly reactive, short-lived intermediates formed by the homolytic cleavage of a covalent bond.

1. Formation: Homolytic Fission

When a covalent bond breaks in such a way that each atom retains one of the shared electrons, two free radicals are formed.

$$ A : B \xrightarrow{\text{Heat/Light/Peroxide}} A^\bullet + B^\bullet $$

Favorable Conditions (HELPR):

Heat (High Temperature)
Electricity
Light (UV radiation, $h\nu$)
Peroxides (Initiators like $R-O-O-R$)
Radicals

2. Structure and Properties

Valence Electrons: The central carbon has 7 valence electrons (an odd number).
Nature: Electron-deficient (Electrophile), Neutral (No charge), Paramagnetic (due to unpaired electron).
Hybridization: Typically $sp^2$ hybridized.
Geometry: Trigonal Planar (The unpaired electron resides in the unhybridized p-orbital perpendicular to the plane).

3. Stability of Alkyl Free Radicals

The stability order is determined by Hyperconjugation and Resonance. Electron-donating groups stabilize radicals by delocalizing the odd electron density.

A. Order of Stability

Benzyl $\approx$ Allyl $>$ $3^\circ$ (Tertiary) $>$ $2^\circ$ (Secondary) $>$ $1^\circ$ (Primary) $>$ Methyl ($\dot{C}H_3$)

B. Reasons for Stability

Hyperconjugation: The interaction of $\sigma$-electrons of adjacent C-H bonds with the half-filled p-orbital. More $\alpha$-hydrogens mean more hyperconjugative structures and greater stability.
Example: $3^\circ$ Radical has 9 $\alpha$-H, Methyl has 0.
Resonance: In Allylic ($CH_2=CH-\dot{C}H_2$) and Benzylic radicals, the unpaired electron is delocalized over the $\pi$-system, providing immense stability.

4. Key Reactions Involving Radicals

1. Free Radical Substitution

Example: Halogenation of Alkanes in UV light.

$CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$

2. Anti-Markovnikov Addition

Addition of HBr to alkenes in the presence of Peroxide (Kharasch Effect).

3. Wurtz Reaction

Coupling of two alkyl halides using Sodium metal involves a radical mechanism.

4. Polymerization

Many polymers (like Polythene, Teflon) are formed via free radical addition polymerization.

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