IB Chemistry HL R3.3 Electron sharing reactions questions

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Question 1

Multiple choiceHard1 mark

The alkane shown reacts with Cl2\mathrm{Cl_2} under UV light. Which sequence shows initiation followed by the two propagation steps in which one H atom at C2 is replaced by Cl? A dot denotes one unpaired electron.

C1C2C3H3CCH2CH3
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Answer: C. UV light causes homolytic fission of Cl2\mathrm{Cl_2} producing two chlorine radicals. A chlorine radical abstracts H from C2, forming HCl\mathrm{HCl} and CH3CH⋅CH3\mathrm{CH_3CH\boldsymbol{\cdot}CH_3} This radical reacts with Cl2\mathrm{Cl_2} to form CH3CHClCH3\mathrm{CH_3CHClCH_3} and regenerate Cl⋅\mathrm{Cl\boldsymbol{\cdot}} sustaining the chain. A: abstracts H at a terminal carbon, so the product is 1-chloropropane. B: uses radical–radical combination, a termination step that consumes the radicals rather than regenerating one. D: shows heterolytic fission, producing ions rather than the radicals needed to initiate the chain. Remember: a propagation step consumes one radical and produces another.

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