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Which is the mechanism for this conversion? A - AQA - A-Level Chemistry - Question 23 - 2020 - Paper 3
Question 23
Which is the mechanism for this conversion? A. Addition-elimination B. Electrophilic substitution C. Free-radical substitution D. Nucleophilic substitution
Worked Solution & Example Answer:Which is the mechanism for this conversion? A - AQA - A-Level Chemistry - Question 23 - 2020 - Paper 3
Step 1
C. Free-radical substitution
Answer
The conversion from toluene (CH₃ attached to a benzene ring) to benzyl chloride (CH₂Cl attached to a benzene ring) suggests that a free-radical substitution mechanism is involved.
In free-radical substitution, the benzyl carbon undergoes a reaction where a chlorine radical replaces a hydrogen atom, typically facilitated by a chlorine gas (Cl₂) in the presence of UV light or heat. This process can be summarized in the following steps:
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Initiation:
The Cl₂ molecules dissociate into two chlorine radicals (Cl•) upon exposure to heat or light.
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Propagation:
The chlorine radicals attack the benzyl group, replacing a hydrogen atom and forming a benzyl radical (C₆H₅CH₂•).
The benzyl radical can then react with another chlorine molecule to form benzyl chloride and regenerate another chlorine radical, continuing the cycle.
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Termination:
Two radicals can combine to terminate the reaction.
Overall, this identifies free-radical substitution as the correct mechanism for the conversion.