Free Radical
A free radical is a highly reactive chemical species containing one or more unpaired electrons in its outermost orbital. Free radicals are typically short-lived and can be derived from atoms, molecules, or ions by homolytic cleavage of a covalent bond. They play key roles in combustion, atmospheric chemistry, polymerisation, and biological oxidative stress, where they can damage DNA, proteins, and lipids.
Key Formula
R-R → 2 R• (homolytic cleavage produces two radicals)
LaTeX: R\text{-}R \xrightarrow{h\nu \text{ or } \Delta} 2\,R^{\bullet}
| Symbol | Meaning | Unit |
|---|---|---|
| R• | Free radical (species with one unpaired electron) | — |
| hν | Photon energy (ultraviolet light) | J |
| Δ | Heat energy | kJ/mol |
Worked Example
Problem
In the free-radical chlorination of methane (CH4 + Cl2 → CH3Cl + HCl), identify the initiation, propagation, and termination steps.
Solution
Initiation: Cl2 → 2 Cl• (homolytic cleavage by UV light) Propagation Step 1: Cl• + CH4 → HCl + CH3• (Cl• abstracts H from methane) Propagation Step 2: CH3• + Cl2 → CH3Cl + Cl• (methyl radical reacts with Cl2, regenerating Cl•) Termination (radical combination): Cl• + Cl• → Cl2 CH3• + Cl• → CH3Cl CH3• + CH3• → C2H6
Answer
Major product is chloromethane (CH3Cl); termination stops chain by consuming radicals without regenerating new ones.
Stability Order of Carbon-Centred Free Radicals
| Radical Type | Example | No. of Alkyl Groups | Relative Stability | Reason |
|---|---|---|---|---|
| Methyl | CH3• | 0 | Least stable | No hyperconjugation or induction |
| Primary | CH3CH2• | 1 | Low stability | Minimal hyperconjugation |
| Secondary | (CH3)2CH• | 2 | Moderate stability | Hyperconjugation from 2 groups |
| Tertiary | (CH3)3C• | 3 | High stability | Maximum hyperconjugation |
| Allylic | CH2=CH-CH2• | resonance | Very high stability | Delocalisation over π system |
| Benzylic | C6H5-CH2• | resonance | Highest (among common) | Aromatic delocalisation |
Interactive Tools
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Related Terms
Markovnikov's Rule
Markovnikov's Rule states that when a protic acid (HX) adds to an unsymmetrical alkene, the hydrogen atom bonds to the carbon that already bears the greater number of hydrogen atoms, while the halide or nucleophile bonds to the more substituted carbon. This regioselectivity arises because the reaction proceeds through the more stable (more substituted) carbocation intermediate. The rule is foundational for predicting the major product of electrophilic addition reactions in organic synthesis.
Polymer
A polymer is a large macromolecule composed of many repeating structural units called monomers, linked together by covalent bonds through a process called polymerisation. Polymers can be natural (e.g., cellulose, proteins, DNA) or synthetic (e.g., polyethylene, nylon, PVC), and their physical properties are governed by chain length, branching, cross-linking, and monomer identity. They are indispensable in modern industry, biology, and materials science.
Condensation Reaction
A condensation reaction is a type of chemical reaction in which two molecules combine to form a larger molecule with the simultaneous loss of a small molecule, most commonly water (H2O) but sometimes methanol, HCl, or ammonia. Condensation reactions are fundamental in the synthesis of polymers, esters, amides, and biological macromolecules including proteins, nucleic acids, and polysaccharides. The reverse process, in which the small molecule is reincorporated to break the bond, is called hydrolysis.
The term 'radical' comes from Latin 'radix' (root), originally used in chemistry to describe a group of atoms that remains intact during reactions. 'Free radical' was introduced to distinguish species existing independently rather than as part of a compound; the concept was fully established by Moses Gomberg in 1900 with the discovery of the triphenylmethyl radical.