R3.3
Electron sharing reactions
2 SL (SL 3.3.1-3.3.3)
Unverified
← View on the mapParent topic: What are the mechanisms of chemical change?
Guiding question
Guiding questionWhat happens when a species possesses an unpaired electron?
The one big idea
A radical is a species with an unpaired electron, formed by homolytic fission. Radicals drive chain reactions — initiation, propagation, termination — and explain the substitution of alkanes by halogens under UV light.
What this rests on
Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.
- EditorialS2.2 The covalent modelHomolytic fission and alkane stability rest on strong, essentially non-polar C-C and C-H bonds.
- EditorialS3.2 Functional groups: Classification of organic compoundsAlkane classification (S3.2) is needed before radical substitution (R3.3).
What rests on this
- EditorialR3.4 Electron-pair sharing reactions
Core concepts that must be mastered
- Radicals and their representation(3.3.1)A radical is a molecular entity with an unpaired electron. Radicals are highly reactive because the unpaired electron seeks to pair. You must identify and represent radicals — the dot indicates the unpaired electron: Cl·, ·CH₃, Br·. A radical may be an atom (Cl·), a molecule (NO·, which has an unpaired electron on nitrogen), or an anion (superoxide O₂⁻·) — the defining feature is the unpaired electron, not the charge. This breadth is what the linking question "How is it possible for a radical to be an atom, a molecule, a cation or an anion?" is getting at: radical character is about electron count, not about whether the species is charged or neutral, atomic or molecular. Radicals form in the atmosphere (hydroxyl radical HO· drives tropospheric chemistry), in flames, and in the stratosphere where UV light generates chlorine radicals from CFCs.
- Homolytic fission and the initiation step(3.3.2)Radicals are produced by homolytic fission — the bond breaks symmetrically, with each fragment keeping one electron. This contrasts with heterolytic fission (R3.4), where both electrons go to one fragment. Halogens undergo homolytic fission under UV light or heat: Cl₂(g) → 2Cl·(g). This is the initiation step of a chain reaction. You must explain this with equations, using the single-barbed "fish-hook" arrow for single-electron movement: a half-headed arrow from the bond to each atom shows each taking one electron. The reverse process — two radicals combining to form a bond — is also homolytic in character, and is the termination step (see below). The CFC connection illustrates why bond strength matters: chlorofluorocarbons release chlorine radicals but typically not fluorine radicals because the C–Cl bond is weaker than the C–F bond, so UV light in the stratosphere can break C–Cl but not C–F. This is why CFCs are ozone-depleting — the released Cl· catalyses the destruction of stratospheric O₃.
- Propagation, termination and the substitution of alkanes(3.3.3)Radicals undergo substitution with alkanas, producing a mixture of products. The propagation steps form a chain: Cl· + CH₄ → HCl + ·CH₃; then ·CH₃ + Cl₂ → CH₃Cl + Cl·. The Cl· regenerated in the second step attacks another CH₄, so the chain continues — one initiation event can cause thousands of substitution cycles. The termination steps end the chain when two radicals combine: Cl· + Cl· → Cl₂; ·CH₃ + ·CH₃ → C₂H₆ (ethane); Cl· + ·CH₃ → CH₃Cl. You must explain all three stages with equations. The reaction produces a mixture — CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, plus ethane from termination — which is why radical substitution is synthetically poor. Further substitution occurs because the chlorinated product can itself be attacked by Cl·. The stability of alkanes comes from strong, essentially non-polar C–C and C–H bonds: alkanes are kinetically stable (high activation energy — they do not spontaneously react with halogens in the dark) but thermodynamically unstable relative to their combustion products. This kinetic–thermodynamic distinction explains why alkanes do not spontaneously combust despite a negative ΔG: the activation energy barrier is too high without a spark or UV light. The ozone-depletion mechanism in the stratosphere is a further illustration: Cl· + O₃ → ClO· + O₂, then ClO· + O → Cl· + O₂ — a chain that destroys thousands of ozone molecules per chlorine radical, but cannot break O₂ because the O=O bond is too strong. This is why CFCs break down ozone but not oxygen in the stratosphere.
- How this sub-topic connectsRadical chemistry is the "electron sharing" companion to the electron-transfer (R3.2) and electron-pair-sharing (R3.4) mechanism sub-topics. Homolytic vs heterolytic fission is the key contrast: radicals come from homolytic fission (3.3.2), where each atom keeps one electron, while nucleophiles and electrophiles come from heterolytic fission (R3.4.3), where one atom takes both. The bond-strength question connects to the covalent model (S2.2: CFCs release Cl radicals but not F radicals because the C–Cl bond is weaker than C–F). The kinetic vs thermodynamic stability of alkanes connects to activation energy and rate (R2.2) and to Gibbs energy (R1.4). The mixture of products from radical substitution motivates the search for more selective mechanisms — nucleophilic and electrophilic — covered in R3.4. Bromine water is decolourised by alkenes (electrophilic addition, R3.4) but not by alkanes (radical substitution requires UV light) — this is the standard test for unsaturation.
Quantitative non-negotiables
No quantitative non-negotiables specified.
Common failure modes
M-75 — Radical substitution of methane gives a clean single product.
Confidence: verified
Why it’s wrong: It gives a mixture (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, plus ethane from termination).
Correction: This is why it's synthetically poor.
What "HL standard" actually looks like
No HL extension. Reactivity 3.3 is entirely SL and shared — it is the shortest sub-topic in the course (2 hours) and introduces radical chemistry qualitatively, with equations but no calculations. The HL depth of reaction mechanisms comes in R3.4 (SN1/SN2, electrophilic addition and substitution).
Linking questions
Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.
- Official IB
How is it possible for a radical to be an atom, a molecule, a cation or an anion? Consider examples of each type.
- Official IB
Why do chlorofluorocarbons (CFCs) in the atmosphere break down to release chlorine radicals but typically not fluorine radicals?
- Official IB
What is the reverse process of homolytic fission?
- Official IB
Chlorine radicals released from CFCs are able to break down ozone, O₃, but not oxygen, O₂, in the stratosphere. What does this suggest about the relative strengths of bonds in the two allotropes?
- Official IB
Why are alkanes described as kinetically stable but thermodynamically unstable?
- Official IB
What is the difference between the bond-breaking that forms a radical and the bond-breaking that occurs in nucleophilic substitution reactions?
- Official IB
Why is bromine water decolourized in the dark by alkenes but not by alkanes?
Bridge: GCSE → IB HL
Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.