Radical Mechanisms

1. Homolytic Cleavage and Fishhook Arrows

Homolysis breaks a bond symmetrically so each atom keeps one electron: A–B ⇀ A• + B•. It forms radicals and needs an energy input — heat (Δ), light (hν), or a radical initiator (peroxide). The opposite, heterolysis, gives both electrons to one atom, forming ions (A–B ⟶ A⁺ + :B⁻) — the polar world.

Fishhook arrows are single-headed and show movement of one electron at a time. Conventions used here: ⇀ = fishhook (single-electron, radical); ⟶ = double-headed curved arrow (electron pair, polar). Homolysis is drawn with two fishhooks, one from each atom, each carrying one electron outward. Arrows move electrons, never atoms. Telling the two arrow types apart is the fastest way to classify a mechanism as radical vs polar.

2. Initiation, Propagation, Termination

• Initiation — radicals are created, almost always by homolysis of a weak bond (Cl₂ ⇀ 2 Cl•; RO–OR ⇀ 2 RO•). Endothermic, and the rate-limiting "spark."
• Propagation — radicals are transferred, not created or destroyed. Each step consumes one radical and produces one, so the net radical count is conserved while a reactant becomes product. These steps repeat — the "chain."
• Termination — radicals are destroyed by coupling (Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl) or disproportionation (one radical abstracts H from another).

3. Chain Carriers, Chain Length, Bookkeeping

Chain carriers are the radicals that recycle through propagation (Cl• and R• in chlorination). A molecule that appears only as a stable product is not a carrier. Chain length is the average number of propagation cycles per initiation event (often thousands) — long chains are efficient.

Bookkeeping checks each step: (1) atom balance — same atoms both sides; (2) charge balance — radicals are neutral, so net charge is zero unless ions are drawn; (3) unpaired-electron balance — in propagation, one radical in ⇒ one radical out (fishhooks come in pairs; every bond made/broken accounts for two electrons). A step that consumes a radical but produces none (or two) is not propagation — it is initiation or termination.

How it works

1. A weak bond (X–X or RO–OR) homolyzes under heat/light, forming two radicals (initiation).
2. A radical abstracts an atom (usually H) from a molecule, making a new radical; that radical then abstracts from X₂, regenerating the original radical (propagation).
3. The two propagation steps repeat, converting reactant to product while conserving radicals.
4. Occasionally two radicals collide and terminate the chain; rate and products follow radical stability (topic 36) and step energies.

Common confusions

Memory aids

"I Pass Terms" — Initiation, Propagation, Termination. Within propagation, remember "one in, one out." For arrows: "fishhooks are for fish (single electrons), double-headers are for pairs (polar)." Homolysis rhymes with "solo-lysis" — each atom goes solo with one electron.

Quick review

Topic Recap

Radical reactions run as chains with three stages: initiation (homolysis creates radicals), propagation (radicals conserved while products form), and termination (radicals combine). Fishhook arrows track single-electron movement and mark radical mechanisms, distinct from double-headed polar arrows. Chain carriers recycle through propagation, and chain length measures efficiency. Bookkeeping — atom, charge, and unpaired-electron balance — plus step energies decide whether a radical pathway is viable and what it produces.

Knowledge Check

1. What three stages make up a radical chain, and what happens in each?
2. How many electrons does a fishhook arrow represent, and when is it used instead of a double-headed arrow?
3. In Cl• + CH₄ → HCl + •CH₃, what is conserved, and which species are chain carriers?
4. Write one termination step for methane chlorination and explain why it ends the chain.
5. Why must propagation steps be exothermic (or nearly so) for an efficient chain?

Answers and Rationales

1. Initiation (radicals created by homolysis), propagation (radical + molecule → new radical + new molecule, conserving radicals), termination (two radicals combine/disproportionate). Rationale: these three roles describe a chain's full life cycle.
2. A fishhook represents one electron and is used for radical steps; a double-headed arrow represents an electron pair and is used for polar steps. Rationale: the arrow type matches the electrons moved.
3. The number of radicals is conserved (one Cl• consumed, one •CH₃ produced); the chain carriers are Cl• and •CH₃. Rationale: propagation transfers but never creates/destroys radicals.
4. E.g., •CH₃ + Cl• → CH₃Cl (or Cl• + Cl• → Cl₂). It ends the chain because two radicals are consumed and none produced. Rationale: no radical left means propagation stops.
5. Endothermic steps have high barriers and are slow, so the chain dies early; exothermic steps are fast and sustain many cycles. Rationale: step energetics gate chain efficiency.