Radical substitution — chain mechanism
Radical chlorination of methane
How the Radical chlorination of methane works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Cl₂, CH₄. Conditions: UV light (hν) or heat; gas phase. Bonds formed: C–Cl σ; H–Cl; bonds broken: Cl–Cl σ; C–H σ. Stereochemistry: Planar •CH₃ radical (no stereochemistry). Rate and selectivity: Chain reaction; propagation ≫ initiation; polyhalogenation unless CH₄ in excess.
At a glance
Step 1: Initiation: homolysis
Initiation: light or heat homolytically cleaves Cl–Cl. Each thin, single-barbed 'fishhook' arrow moves just one electron (not a pair) — the signature of radical mechanisms — giving two chlorine radicals.
Step 2: Propagation 1: H abstraction
Propagation 1: a chlorine radical abstracts a hydrogen atom from methane. One fishhook breaks the C–H bond, the other forms the new H–Cl bond, leaving a methyl radical.
Step 3: Propagation 2: Cl₂ attacked
Propagation 2: the methyl radical attacks a fresh Cl₂ molecule, forming chloromethane and regenerating a chlorine radical — which goes on to repeat step 1. This chain can run thousands of times per initiation event.
Abbreviations and terms
- Δ
- Heat. Reaction is run at elevated temperature.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Radical chlorination of methane | Barton-McCombie deoxygenation | Kharasch addition (peroxide-mediated HBr addition) | |
|---|---|---|---|
| Reagents | Cl₂, CH₄ | 1) NaH, CS₂, CH₃I 2) Bu₃SnH, AIBN (or (TMS)₃SiH) | HBr, ROOR (peroxide initiator) or light, alkene |
| Conditions | UV light (hν) or heat; gas phase | Toluene, 80–110 °C, radical chain | Heat or hν, non-polar solvent |
| Stereochemistry | Planar •CH₃ radical (no stereochemistry) | Stereocentre lost (planar radical) | Anti-Markovnikov; radical chain |
| Rate / selectivity | Chain reaction; propagation ≫ initiation; polyhalogenation unless CH₄ in excess | Driven by strong C=O/Sn–S bonds; chain propagation | Chain propagation; H-abstraction from HBr is fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Radical chlorination of methane?
Cl₂, CH₄. Conditions: UV light (hν) or heat; gas phase.
Which bonds are formed and which are broken?
Formed: C–Cl σ; H–Cl. Broken: Cl–Cl σ; C–H σ.
What is the stereochemical outcome?
Planar •CH₃ radical (no stereochemistry)
What controls the rate or the selectivity?
Chain reaction; propagation ≫ initiation; polyhalogenation unless CH₄ in excess
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make