36 exam-style questions on Electron sharing reactions (radicals) — topic R3.3 of Reactivity 3: What are the mechanisms of chemical change? in the IB Chemistry syllabus — with answers and explanations. Samples below; the full set is free with an account.
State the term for the step in a radical chain mechanism in which two radicals combine to form a molecule with no unpaired electrons.
Answer: Termination
Termination removes radicals from the system by combining two of them, giving a species with all electrons paired; this reduces the total number of radicals.
In the chlorination of methane, which equation represents the initiation step?
Answer: Cl₂ → 2Cl•
Initiation is the homolytic fission of the Cl–Cl bond by UV light to form two chlorine radicals; the C–H bond is much stronger and does not break first.
Identify the type of bond fission that produces radicals.
Answer: Homolytic fission
In homolytic fission each atom takes one electron from the shared pair, producing two radicals; heterolytic fission gives ions because one atom takes both electrons.
The monochlorination of methane requires ultraviolet light. State the role of the UV light.
Answer: It provides energy to break the Cl–Cl bond homolytically
UV photons supply the energy for homolytic fission of Cl₂ into two radicals in the initiation step; it does not ionise the chlorine.
Determine the two propagation steps for the reaction of methane with chlorine.
Answer: CH₄ + Cl• → CH₃• + HCl ; CH₃• + Cl₂ → CH₃Cl + Cl•
In propagation a radical reacts to form a product plus a new radical, so the chain continues; each step regenerates a radical (Cl• then CH₃•).
Suggest why the radical substitution of methane with chlorine produces a mixture of products such as CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄.
Answer: Chlorine radicals can further substitute the hydrogens remaining on already-chlorinated products
As long as radicals and C–H bonds are present, further substitution occurs, so chloromethane is chlorinated again, giving a mixture of poly-substituted products.
That was 6 of 36 Electron sharing reactions (radicals) questions.
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36 Electron sharing reactions (radicals) questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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