Free-radical substitution
Free-radical substitution · Cl2 or Br2 · UV light
Reagents: Cl2 or Br2 · Conditions: UV light
Step 1 of 3: Build the initiation step.
UV light provides the energy for homolytic fission of the Br–Br bond, giving two bromine radicals (dots required).
Br2 → 2Br• under UV.
Step 2 of 3: Build BOTH propagation steps.
Br• abstracts a hydrogen from ethane, giving HBr and an ethyl radical.
The ethyl radical attacks Br2, forming the product and regenerating Br• — the chain continues.
The two propagation steps together convert ethane + Br2 into bromoethane + HBr, regenerating Br• each cycle.
Step 3 of 3: Build ONE valid termination step.
Termination = any two radicals combining. All three versions score; the butane dimer is the one examiners love to see.
Two radicals combine and the chain stops. Overall: CH3CH3 + Br2 → CH3CH2Br + HBr.
Product: bromoethane
Watched it? Now draw it yourself — that's what the exam asks.
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