1. Initiation splits X₂ into two radicals.
Light or heat breaks the halogen bond homolytically, making two chain-carrying radicals; this step is slow and happens rarely.
2. Propagation step 1 abstracts a hydrogen.
A chlorine radical pulls an H off the alkane, producing HCl and a carbon radical that carries the chain forward.
3. Propagation step 2 regenerates the carrier.
The carbon radical grabs a chlorine from Cl₂, forming the R–Cl product and releasing a fresh chlorine radical to repeat the cycle.
4. Termination combines radicals and ends the chain.
When any two radicals meet they pair up — removing carriers so the chain stops, sometimes making a side product.
5. Overall: the two propagation steps add up to the reaction.
Summed, the cycle converts alkane plus Cl₂ into the chloroalkane plus HCl — one initiation feeds many product-making cycles.
Ethane + Cl₂, under light, gives chloroethane + HCl.
Summary
Initiation makes radicals · propagation step 1 abstracts H · propagation step 2 regenerates the carrier · termination pairs radicals to stop the chain · one flash of light yields many products.
Quiz yourself
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Initiation — light or heat homolytically splits X₂ into two X• radicals. It is slow and accounts for very few events.
Each propagation cycle consumes reactants and regenerates a radical carrier, so the same chain repeats thousands of times before it terminates.
(1) Cl• + CH₃CH₃ → •CH₂CH₃ + HCl. (2) •CH₂CH₃ + Cl₂ → CH₃CH₂Cl + Cl•.
Propagation regenerates a carrier and keeps the chain going; termination combines two radicals, destroying carriers and ending the chain.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.