Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

11. Radical Reactions

Free Radical Halogenation

Organic Chemistry

11. Radical Reactions

Free Radical Halogenation

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7:31 minutes

Problem 9c,d

Textbook Question

b. Calculate ΔH° for each step in this reaction.
c. Calculate the overall value of ΔH° for this reaction.

Verified step by step guidance

Step 1: Understand the reaction mechanism. Free-radical chlorination involves three main steps: initiation, propagation, and termination. In the initiation step, the chlorine molecule (Cl₂) absorbs energy from light (hv) and undergoes homolytic cleavage to form two chlorine radicals (Cl•).

Step 2: Write the propagation steps. In the first propagation step, a chlorine radical (Cl•) reacts with ethane (CH₃—CH₃) to abstract a hydrogen atom, forming a methyl radical (CH₃—CH₂•) and HCl. In the second propagation step, the methyl radical reacts with another chlorine molecule (Cl₂) to form the product (CH₃—CH₂Cl) and regenerate a chlorine radical (Cl•).

Step 3: Analyze the termination steps. Termination occurs when two radicals combine to form a stable molecule, such as Cl• + Cl• → Cl₂ or CH₃—CH₂• + Cl• → CH₃—CH₂Cl. These steps reduce the number of free radicals in the system.

Step 4: To calculate ΔH° for each step, use bond dissociation energies (BDEs). For example, calculate the energy change for breaking the Cl—Cl bond in the initiation step, the energy change for breaking the C—H bond in ethane during propagation, and the energy change for forming new bonds (C—Cl and H—Cl). Use the formula ΔH° = Σ(BDE bonds broken) - Σ(BDE bonds formed).

Step 5: Calculate the overall ΔH° for the reaction by summing the ΔH° values for all steps. This will give the net energy change for the reaction. Ensure that all bond dissociation energy values are correctly referenced from a reliable source, such as a chemistry textbook or database.

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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 chlorination of ethane, the process typically includes initiation (formation of radicals), propagation (reaction of radicals with ethane), and termination (recombination of radicals). Understanding this mechanism is crucial for proposing a detailed reaction pathway.

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. It is essential for calculating the energy changes associated with each step of the reaction mechanism. By determining the ΔH° for individual steps, one can assess the overall energy profile of the reaction.

Bond Dissociation Energy

Bond dissociation energy refers to the energy required to break a specific bond in a molecule, resulting in the formation of free radicals. This concept is vital for calculating ΔH° in the chlorination reaction, as it allows for the quantification of energy changes when bonds are broken and formed during the reaction steps.

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Related Practice

Textbook Question

Using cyclohexane as one of your starting materials, show how you would synthesize the following compounds.

(a)

Textbook Question

When exactly 1 mole of methane is mixed with exactly 1 mole of chlorine and light is shone on the mixture, a chlorination reaction occurs. The products are found to contain substantial amounts of di-, tri-, and tetrachloromethane, as well as unreacted methane.

b. How would you run this reaction to get a good conversion of methane to CH₃Cl? Of methane to CCl₄?

Textbook Question

Write the propagation steps leading to the formation of dichloromethane (CH₂Cl₂) from chloromethane.

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.

Textbook Question

For each alkane,

1. draw all the possible monochlorinated derivatives.

a. Cyclopentane

b. Methylcyclopentane

Textbook Question

For each alkane,

2. determine whether free-radical chlorination would be a good way to make any of these monochlorinated derivatives. Will the reaction give mostly one major product?

a. Cyclopentane

b. Methylcyclopentane

Textbook Question

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

Textbook Question

How many dibrominated products could each of the compounds form if stereoisomers are included?

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