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Ch.4 - The Study of Chemical Reactions
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.4 - The Study of Chemical ReactionsProblem 3b
Chapter 4, Problem 3b

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.
b.

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Step 1: Analyze the first step of the proposed mechanism, CH₄ + hv → •CH₃ + H•. This step suggests that methane undergoes homolytic cleavage upon exposure to light to form a methyl radical (•CH₃) and a hydrogen radical (H•). However, experimental evidence shows that methane does not absorb light in the visible or UV range, meaning this step cannot occur as described. Instead, the initiation step in free-radical chlorination involves the homolytic cleavage of Cl₂ into two chlorine radicals (Cl•) upon exposure to light.
Step 2: Examine the second step, •CH₃ + Cl₂ → CH₃Cl + Cl•. This step implies that the methyl radical reacts directly with chlorine gas to form chloromethane (CH₃Cl) and a chlorine radical. While this reaction could theoretically occur, experimental evidence shows that the propagation steps in free-radical chlorination involve a chlorine radical (Cl•) reacting with methane (CH₄) to form HCl and a methyl radical (•CH₃), followed by the methyl radical reacting with Cl₂ to form CH₃Cl and regenerate Cl•.
Step 3: Evaluate the third step, Cl• + H• → HCl. This step suggests that a chlorine radical reacts with a hydrogen radical to form HCl. While this reaction is chemically plausible, experimental evidence indicates that hydrogen radicals (H•) are not formed in significant quantities during the free-radical chlorination of methane. Instead, the formation of HCl occurs when a chlorine radical abstracts a hydrogen atom directly from methane (CH₄).
Step 4: Consider the overall mechanism and compare it to the experimentally supported mechanism. The proposed mechanism fails to account for the initiation step involving Cl₂ and the propagation steps that regenerate radicals, which are essential for sustaining the chain reaction in free-radical chlorination. The experimental mechanism involves initiation (Cl₂ → 2Cl•), propagation (Cl• + CH₄ → HCl + •CH₃, •CH₃ + Cl₂ → CH₃Cl + Cl•), and termination steps.
Step 5: Summarize the experimental evidence that disproves the proposed mechanism. Methane does not absorb light to initiate the reaction, hydrogen radicals are not formed in significant quantities, and the propagation steps in the proposed mechanism do not align with the experimentally observed chain reaction process. The correct mechanism involves the generation and regeneration of chlorine radicals to sustain the reaction.

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Key Concepts

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

Free Radical Mechanism

Free radical mechanisms involve the formation and reaction of unpaired electrons, which are highly reactive species. In the context of chlorination, the process typically includes initiation, propagation, and termination steps. Understanding how radicals are generated and how they react with other molecules is crucial for analyzing proposed mechanisms.
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Chlorination of Methane

The chlorination of methane is a classic example of a free radical substitution reaction where chlorine replaces hydrogen atoms in methane. This process is initiated by the homolytic cleavage of Cl-Cl bonds under UV light, leading to the formation of chlorine radicals. Analyzing the specific steps and products formed during this reaction helps in evaluating the validity of proposed mechanisms.
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Experimental Evidence

Experimental evidence in organic chemistry often includes observations from reaction conditions, product distributions, and kinetic studies. For the chlorination of methane, evidence such as the lack of certain products or unexpected reaction rates can disprove proposed mechanisms. Understanding how to interpret this evidence is essential for validating or refuting mechanistic pathways.
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Related Practice
Textbook Question
The reaction of tert-butyl chloride with methanol (CH3)3C—Cl Tert-butylchloride + CH3—OH methanol —> (CH3)C—OCH3 methyltert-butylether + HCl is found to follow the rate equation rate= Kt[(CH3)3C—Cl] a. What is the kinetic order with respect to tert-butyl chloride?
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Textbook Question
Use bond-dissociation enthalpies [TABLE 4-2], p. 167) to calculate values of ΔH° for the following reactions. a. CH3—CH3 + I2 —> CH3CH2I + HI
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Textbook Question
The bromination of methane proceeds through the following steps: 1. Br2 + 2 Br• ΔH° (per mole)/+190 kJ (45 kcal) Ea (per mole)/ 190 kJ (45 kcal) 2. CH4 + Br• —> CH3+ HBr +73 kJ (17 kcal) 79 kJ (19 kcal) 3. • CH3 + Br2 —> CH3Br + Br -112 kJ (-27 kcal) 4 kJ (1 kcal) d. Compute the overall value of ΔH° for the bromination
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Textbook Question

Explain why free-radical halogenation usually gives mixtures of products.

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Textbook Question

Each of the following proposed mechanisms for the free-radical chlorination of methane is wrong. Explain how the experimental evidence disproves each mechanism.

a.

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Textbook Question

How could an industrial plant control the proportions of methane and chlorine to favor production of CCl4? To favor production of CH3Cl?

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