Ch.4 - The Study of Chemical Reactions

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.4 - The Study of Chemical Reactions

Problem 9a

Chapter 4, Problem 9a

a. Propose a mechanism for the free-radical chlorination of ethane,

Verified step by step guidance

Step 1: Initiation - The process begins with the homolytic cleavage of the Cl-Cl bond under the influence of UV light (hv). This generates two chlorine radicals (Cl•). The reaction can be represented as:

{Cl}_{2} \to {Cl}_{•} + {Cl}_{•}

Step 2: Propagation - In the first propagation step, a chlorine radical abstracts a hydrogen atom from ethane (CH3—CH3), forming a methyl radical (CH3—CH2•) and HCl. The reaction can be represented as:

{Cl}_{•} + {CH}_{3} — {CH}_{3} \to {CH}_{3} — {CH}_{2} • + HCl

Step 3: Propagation - In the second propagation step, the methyl radical reacts with another Cl2 molecule, forming the chlorinated product (CH3—CH2Cl) and regenerating a chlorine radical. The reaction can be represented as:

{CH}_{3} — {CH}_{2} • + {Cl}_{2} \to {CH}_{3} — {CH}_{2} Cl + {Cl}_{•}

Step 4: Termination - The reaction can terminate when two radicals combine to form a stable molecule. For example, two chlorine radicals can combine to regenerate Cl2, or a methyl radical can combine with a chlorine radical to form CH3—CH2Cl. These termination steps reduce the number of radicals in the system.

Step 5: Summary - The overall reaction involves the conversion of ethane (CH3—CH3) and chlorine (Cl2) into chloroethane (CH3—CH2Cl) and HCl, with the mechanism proceeding through initiation, propagation, and termination steps. The free-radical mechanism is driven by the generation and interaction of reactive radical species.

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Key Concepts

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

Free Radical Mechanism

The free radical mechanism involves a series of steps where free radicals are generated and react with other molecules. In the case of chlorination, the process typically includes initiation, propagation, and termination steps. Initiation generates free radicals from chlorine molecules, propagation involves the reaction of these radicals with ethane to form chlorinated products, and termination occurs when two radicals combine to form a stable molecule.

Chlorination of Alkanes

Chlorination of alkanes is a substitution reaction where hydrogen atoms in an alkane are replaced by chlorine atoms. This reaction is facilitated by the presence of light (hv) or heat, which provides the energy needed to break the Cl-Cl bond, generating chlorine radicals. The resulting chlorinated products can vary based on the number of hydrogen atoms replaced and the reaction conditions.

Selectivity in Radical Reactions

Selectivity in radical reactions refers to the preference of radicals to abstract certain hydrogen atoms over others, leading to different products. In the case of ethane, the formation of primary versus secondary radicals can influence the distribution of chlorinated products. The stability of the resulting radical intermediates plays a crucial role in determining which hydrogen atoms are more likely to be replaced during the chlorination process.

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Related Practice

Textbook Question

The reaction of tert-butyl chloride with methanol

is found to follow the rate equation

rate = k_r[(CH₃)₃C—Cl]

a. What is the kinetic order with respect to tert-butyl chloride?

3202

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Textbook Question

b. Calculate ΔH° for each step in this reaction.

c. Calculate the overall value of ΔH° for this reaction.

1088

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Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(a)

1224

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Textbook Question

a. Using bond-dissociation enthalpies from Table 4-2. (page 167), calculate the heat of reaction for each step in the free-radical bromination of methane

b. Calculate the overall heat of the reaction.

1818

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Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(c)

1257

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Textbook Question

For each reaction, estimate whether ΔS° for the reaction is positive, negative, or impossible to predict.

(b)

1189

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