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Organic Chemistry · An Overview of Organic Reactions

Radical Reactions

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

The polar reactions of the previous topics move electrons in pairs; radical reactions move them one at a time. A radical is any species with an unpaired electron — written with a dot, like Cl ·  or CH3 · . Radicals form when a bond breaks homolytically, each fragment keeping one bonding electron, usually created by heat, light, or an initiator. Radical reactions typically run as chain reactions with three phases — , , — and the classic example is chlorination of methane, CH4 + Cl2 → CH3Cl + HCl. Because radicals are neutral and extremely reactive, their chemistry differs sharply from ionic chemistry: fishhook arrows, no charge guidance, and selectivity set by bond strengths.

Why this matters

Radical chemistry is everywhere. It is how saturated hydrocarbons are functionalized industrially, how polyethylene and other polymers are made, and how combustion releases energy. In biology, radicals are both essential and dangerous: enzymes deliberately generate them, while stray radicals damage lipids, DNA, and proteins — which is why antioxidants such as vitamin E intercept them. Atmospheric chemistry depends on radical chains too: chlorine radicals from CFCs destroy stratospheric ozone, while HO ·  cleans pollutants from the air. Understanding chain mechanisms explains all of this, plus exam staples like predicting the major monohalogenation product of an alkane.

The college version

Core Concepts

What a radical is

A radical is a neutral (or charged) species with at least one unpaired electron. Carbon radicals are sp2 hybridized with the unpaired electron in a p orbital; they are electron-poor but carry no charge, so they are neither nucleophiles nor electrophiles in the ionic sense. Radicals form by : the bond breaks so each fragment takes one electron, as in Cl2 → 2 Cl ·  under ultraviolet light. Because an unpaired electron wants a partner, radicals are extremely reactive; most exist for fractions of a second.

The three phases of a chain reaction

A chain reaction has an initiation step, one or more propagation steps, and termination steps. Initiation creates radicals from stable molecules: light or heat breaks the weak Cl–Cl bond, giving two chlorine atoms. Propagation is the working cycle — each step consumes a radical and produces another, repeating many times per initiating event. Termination removes radicals by combining two into a stable molecule, ending the chain.

The chlorination of methane, step by step

Initiation: Cl2 light⟶ 2 Cl · . Propagation step 1: a chlorine atom abstracts a hydrogen from methane, Cl · + CH4 → HCl + CH3 · , forming a methyl radical. Propagation step 2: the methyl radical reacts with chlorine, CH3 · + Cl2 → CH3Cl + Cl · , producing chloromethane and regenerating the chlorine atom. Termination: any two radicals combine, e.g., Cl · + Cl ·  → Cl2 or CH3 · + Cl ·  → CH3Cl. Propagation regenerates a radical; termination destroys them — that is the engine of the chain.

Radical stability and reactivity

The ease of abstracting a hydrogen depends on C–H bond strength, which correlates with radical stability: weaker bonds give more stable radicals. Stability order: tertiary > secondary > primary > methyl, matching bond dissociation energies. Chlorine atoms are so reactive that they abstract hydrogens almost indiscriminately, giving mixtures; bromine atoms are more selective and strongly prefer the weakest (most substituted) C–H bond. The more reactive the radical, the less selective it is.

Fishhook arrows and radical notation

Radical mechanisms are drawn with fishhook (half-headed) arrows, each representing one electron; a homolytic cleavage is two fishhooks leaving the bond in opposite directions, one per fragment. Because radicals are neutral, no formal charges appear — the mechanism is tracked by electron count. Full-headed (two-electron) arrows signal polar chemistry; fishhooks signal radicals.

How It Works / Step-by-Step Process

  1. Identify the weak bond that will break homolytically (usually the halogen–halogen bond, X2, or a peroxide O–O bond).
  2. Write the initiation step: energy (light or heat) splits it into two radicals.
  3. Write propagation step 1: the radical abstracts a hydrogen (or adds to a bond), forming a new stable molecule and a new radical.
  4. Write propagation step 2: the new radical reacts with a reagent molecule, forming the product and regenerating a radical.
  5. Write possible termination steps: pair every combination of radicals into stable products.
  6. To predict the major product of halogenation, compare C–H bond strengths: the weakest (most substituted) C–H is abstracted preferentially — strongly so for bromine, weakly for chlorine.

Common Confusions

Do not confuseWithDifference
Homolytic cleavageHeterolytic cleavageHomolytic splits electrons evenly (radicals); heterolytic gives both electrons to one atom (ions).
Fishhook arrowFull-headed arrowFishhook = one electron (radical step); full head = two electrons (polar step).
InitiationPropagationInitiation creates radicals from stable molecules; propagation cycles a radical through product-forming steps.
Radical chargeIonic chargeRadicals are usually neutral; they react because of an unpaired electron, not because of charge.
"Chlorination is selective like bromination"Chlorination is unselectiveCl ·  attacks all C–H bonds (statistical mixtures); Br ·  strongly prefers 3° > 2° > 1°.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

A radical is like a kid holding a single mitten in winter — that lonely hand wants another mitten so badly it will grab anything nearby. Radicals form when a bond breaks and each side keeps one electron, and they instantly try to pair up with other electrons. In a chain reaction, one radical makes a new radical, which makes another, like a line of dominoes — until two radicals bump into each other and both calm down.

Worked example

Example 1: Predicting the major monobromination product

Which product predominates when 2-methylpropane, (CH3)3CH, is treated with bromine and light?

Compare the C–H bonds: 2-methylpropane has nine equivalent primary C–H bonds (about 423 kJ/mol each) and one tertiary C–H bond (about 404 kJ/mol). Bromine radicals are selective, so they abstract the weakest hydrogen — the tertiary one:

(CH3)3CH + Br2 light⟶ (CH3)3CBr + HBr

The major product is 2-bromo-2-methylpropane (tert-butyl bromide). With chlorine, the product would be a statistical mixture dominated by 1-chloro-2-methylpropane, because chlorine radicals attack the nine primary hydrogens almost indiscriminately — a vivid demonstration of the reactivity–selectivity principle.

Example 2: Energy bookkeeping for the propagation steps

Estimate the enthalpy change of each propagation step of methane chlorination using bond dissociation energies: C–H (methane) ≈ 438 kJ/mol, H–Cl ≈ 431 kJ/mol, Cl–Cl ≈ 242 kJ/mol, C–Cl ≈ 350 kJ/mol. The formula is ΔH°= (bonds broken) − (bonds formed).

Propagation step 1:

ΔH°= D(C–H) - D(H–Cl) = 438 - 431 = +7 kJ/mol

This step is slightly endothermic but still fast, because the entropy of breaking one molecule into two species favors it and the chlorine atom is extremely reactive. Propagation step 2:

ΔH°= D(Cl–Cl) - D(C–Cl) = 242 - 350 = -108 kJ/mol

The second step is strongly exothermic. The two steps together give an overall ΔH° ≈ -101 kJ/mol for the reaction CH4 + Cl2 → CH3Cl + HCl, consistent with chlorination being a favorable, chain-propagating process. Units check: every term is kJ per mole of bonds, so the kJ/mol units cancel properly.

Example 3: Recognizing the phases in a real chain

For C2H6 + Cl2 → C2H5Cl + HCl: initiation Cl2 → 2 Cl · ; propagation 1 Cl · + C2H6 → HCl + C2H5 · ; propagation 2 C2H5 · + Cl2 → C2H5Cl + Cl · ; termination C2H5 · + Cl ·  → C2H5Cl, 2 C2H5 ·  → C4H10, 2 Cl ·  → Cl2. The butane side product signals a chain ended.

Key takeaways

  • A radical has an unpaired electron, shown with a dot (Cl · , CH3 · ); it forms by homolytic cleavage.
  • Chain reactions: initiation (radicals formed), propagation (radical consumed, radical produced), termination (radicals combine).
  • Methane chlorination: Cl2 light⟶ 2 Cl · ; Cl · + CH4 → HCl + CH3 · ; CH3 · + Cl2 → CH3Cl + Cl · .
  • Radical stability: 3° > 2° > 1° > methyl; weaker C–H bond → more stable radical → easier abstraction.
  • Cl ·  is reactive but unselective (mixtures); Br ·  is selective, preferring 3° > 2° > 1° C–H.
  • Radical mechanisms use fishhook (one-electron) arrows; polar mechanisms use full-headed (two-electron) arrows.
  • Real-world radicals: polymers, combustion, biological antioxidants, and ozone destruction by chlorine from CFCs.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. What is the difference between homolytic and heterolytic bond cleavage, and which one produces radicals?

    Show answer

    Homolytic cleavage gives each fragment one electron (forming radicals); heterolytic cleavage gives both electrons to one atom (forming ions). Radicals come from homolysis.

  2. Name the three phases of a radical chain reaction and what each phase does.

    Show answer

    Initiation (radicals created from stable molecules), propagation (radical consumed and a new one produced, forming product), termination (two radicals combine into stable molecules, ending the chain).

  3. Write the two propagation steps for the chlorination of methane.

    Show answer

    Cl · + CH4 → HCl + CH3 ·  and CH3 · + Cl2 → CH3Cl + Cl · .

  4. Why does bromination of 2-methylpropane give 2-bromo-2-methylpropane as the major product?

    Show answer

    Because bromine radicals are selective and abstract the weakest C–H bond — the tertiary one (≈ 404 kJ/mol) — giving the tertiary radical and then the tertiary bromide; the primary C–H bonds are stronger (≈ 423 kJ/mol).

  5. Which arrow notation (fishhook or full-headed) is used in radical mechanisms, and why?

    Show answer

    Fishhook (half-headed) arrows, because each represents a single electron moving, matching the one-electron steps of homolysis and radical reactions.

  6. In methane chlorination, which propagation step is exothermic and which is slightly endothermic?

    Show answer

    Propagation step 2 (CH3 · + Cl2 → CH3Cl + Cl · ) is exothermic (−108 kJ/mol); propagation step 1 (Cl · + CH4 → HCl + CH3 · ) is slightly endothermic (+7 kJ/mol).

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

radical (free radical)
A species with an unpaired electron, e.g., Cl ·  or CH3 · .
homolytic cleavage
Bond breaking in which each fragment keeps one electron.
initiation
The step that creates radicals from stable molecules.
propagation
A step that consumes one radical and produces another.
termination
A step in which two radicals combine into stable molecules.
fishhook arrow
A half-headed arrow representing one-electron movement.

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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