Write a mechanism for the light-initiated reaction of cyclohexane with chlorine to give chlorocyclohexane. Label the initiation and propagation steps.

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Step 1: The reaction begins with the initiation step, where light energy (hv) causes homolytic cleavage of the chlorine molecule (Cl₂). This produces two chlorine radicals (Cl•). The equation for this step is: Cl₂ → 2Cl•.

Step 2: In the first propagation step, one chlorine radical abstracts a hydrogen atom from cyclohexane, forming a cyclohexyl radical and HCl. The equation for this step is: C₆H₁₂ + Cl• → C₆H₁₁• + HCl.

Step 3: In the second propagation step, the cyclohexyl radical reacts with another chlorine molecule (Cl₂), forming chlorocyclohexane and regenerating a chlorine radical. The equation for this step is: C₆H₁₁• + Cl₂ → C₆H₁₁Cl + Cl•.

Step 4: The chain reaction continues as the regenerated chlorine radical can initiate further propagation steps, maintaining the radical chain mechanism.

Step 5: Termination steps occur when two radicals combine to form a stable molecule, effectively ending the chain reaction. Examples include: Cl• + Cl• → Cl₂ or C₆H₁₁• + Cl• → C₆H₁₁Cl.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Free Radical Halogenation

Free radical halogenation is a reaction mechanism where alkanes react with halogens (like Cl2) in the presence of light or heat to form alkyl halides. This process involves the formation of free radicals, which are highly reactive species with unpaired electrons. The reaction proceeds through three main steps: initiation, propagation, and termination.

Initiation Step

The initiation step in free radical halogenation involves the generation of free radicals from the halogen molecules. In this case, the chlorine molecules (Cl2) are dissociated by light (hv) to form two chlorine radicals (Cl·). This step is crucial as it sets the stage for the subsequent propagation steps where the radicals will react with cyclohexane.

Propagation Steps

Propagation steps are the series of reactions where the free radicals react with the substrate, leading to the formation of products while generating new radicals. In the reaction of cyclohexane with chlorine, a chlorine radical abstracts a hydrogen atom from cyclohexane, forming chlorocyclohexane and another radical. This new radical can then react with another Cl2 molecule, continuing the chain reaction until termination occurs.

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Textbook Question

Draw the important resonance forms of the following free radicals.

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Textbook Question

For each alkane,

3. which monobrominated derivatives could you form in good yield by free-radical bromination?

c. 2-methylpentane