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IB DP Chemistry – R3.3.1 Radicals – Study Notes

IB DP Chemistry - R3.3.1 Radicals - Study Notes - New Syllabus - 2026, 2027 & 2028

IB DP Chemistry – R3.3.1 Radicals – Study Notes – New Syllabus

IITian Academy excellent Introduction to the Proton transfer reactions – Study Notes and effective strategies will help you prepare for your IB DP Chemistry  exam.

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Reactivity 3.3.1 — Radicals: Identification, Representation, and Reactivity

Reactivity 3.3.1 — Radicals: Identification, Representation, and Reactivity

A radical is a molecular or atomic species that contains an unpaired electron. This unpaired electron is typically represented as a single dot (•) in structural formulas.

Key Characteristics of Radicals

  • Radicals are electrically neutral species – they are not ions.
  • The presence of an unpaired electron makes radicals highly reactive.
  • Radicals are often formed through homolytic bond fission, where a covalent bond breaks evenly and each atom retains one electron.
  • Radicals are usually intermediates in chemical reactions, especially in combustion, polymerization, and halogenation mechanisms.

Homolytic Fission: The Source of Radicals

When a covalent bond breaks homolytically, each atom involved in the bond takes one electron from the shared pair:

\( \text{Cl}_2 \rightarrow \cdot\text{Cl} + \cdot\text{Cl} \)

This process is often initiated by UV light or heat and is common in reactions involving halogens or hydrocarbons.

Representation of Radicals

Radicals are denoted by placing a single dot (•) next to the chemical symbol:

  • \( \cdot \text{Cl} \) — a chlorine radical
  • \( \cdot \text{CH}_3 \) — a methyl radical
  • \( \cdot \text{OH} \) — a hydroxyl radical
  • \( \cdot \text{NO}_2 \) — a nitrogen dioxide radical

Example 

Chlorine molecules dissociate under UV light to form two chlorine radicals. 

This is the initiation step in the halogenation of alkanes.Explain.

▶️Answer/Explanation

\( \text{Cl}_2 \xrightarrow{\text{UV}} \cdot \text{Cl} + \cdot \text{Cl} \)

This is homolytic fission of the Cl–Cl bond. Each chlorine atom gets one electron, forming two highly reactive radicals.

Example 

A chlorine radical reacts with methane to abstract a hydrogen atom, forming a methyl radical.Explain.

▶️Answer/Explanation

\( \cdot \text{Cl} + \text{CH}_4 \rightarrow \text{HCl} + \cdot \text{CH}_3 \) The chlorine radical removes a hydrogen atom, forming HCl and a methyl radical.

The \( \cdot \text{CH}_3 \) can now propagate the chain reaction by reacting with another \( \text{Cl}_2 \) molecule.

Example

The hydroxyl radical plays a key role in atmospheric reactions, especially in the breakdown of pollutants.Explain.

▶️Answer/Explanation

\( \cdot \text{OH} \) reacts with NO, VOCs, and other compounds to produce secondary pollutants such as ozone in photochemical smog.

For example: \( \cdot \text{OH} + \text{CO} \rightarrow \cdot \text{HOCO} \) The hydroxyl radical initiates oxidation in the atmosphere.

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