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

6. Thermodynamics and Kinetics

Entropy

Organic Chemistry

6. Thermodynamics and Kinetics

Entropy

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14:31 minutes

Problem 43b

Textbook Question

Through the course of this chapter, we have discussed only alkane chlorination and bromination, yet there are two other halogens we have not discussed.
(b) Is radical iodination a favorable reaction? Do you expect it to be selective? Show your calculations.

Verified step by step guidance

Step 1: Begin by understanding the concept of radical halogenation. Radical halogenation involves the substitution of a hydrogen atom in an alkane with a halogen atom through a radical mechanism. The favorability of the reaction depends on the bond dissociation energies (BDEs) of the involved bonds.

Step 2: Analyze the bond dissociation energies (BDEs) for the relevant bonds. For radical iodination, the key bonds to consider are the C-H bond in the alkane and the I-I bond in molecular iodine. Compare these values to determine if the reaction is thermodynamically favorable. Use the equation ΔH = BDE(C-H) - BDE(I-I) to calculate the enthalpy change.

Step 3: Consider the activation energy and reaction kinetics. Radical iodination typically has a high activation energy due to the weak I-I bond and the low reactivity of iodine radicals. This makes the reaction slow and less favorable compared to chlorination or bromination.

Step 4: Evaluate the selectivity of the reaction. Radical halogenation reactions are selective based on the stability of the intermediate radicals formed. Iodine radicals are less reactive, which could lead to lower selectivity compared to bromination, but the reaction is generally not favorable enough to proceed efficiently.

Step 5: Summarize the findings. Radical iodination is not a favorable reaction due to its unfavorable thermodynamics (positive ΔH) and slow kinetics. Additionally, the selectivity is not expected to be significant because the reaction does not proceed efficiently under typical conditions.

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Key Concepts

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

Radical Halogenation

Radical halogenation is a reaction mechanism where alkanes react with halogens (like Cl2 or Br2) to form alkyl halides through a radical chain process. This involves the generation of halogen radicals, which abstract hydrogen atoms from alkanes, leading to the formation of new carbon-halogen bonds. Understanding this mechanism is crucial for analyzing the reactivity and selectivity of different halogens in substitution reactions.

Selectivity in Halogenation

Selectivity in halogenation refers to the preference of a halogen to react with certain hydrogen atoms over others in an alkane. Bromination is generally more selective than chlorination due to the stability of the bromine radical, which favors the formation of more stable products. In the case of iodination, the selectivity is lower, and understanding this concept helps predict the outcomes of radical iodination reactions.

Thermodynamics and Kinetics of Iodination

The thermodynamics and kinetics of iodination involve evaluating the energy changes and reaction rates associated with the formation of iodine radicals. Iodine is less reactive than chlorine and bromine, making radical iodination less favorable and slower. Calculating the activation energy and comparing the bond dissociation energies of the halogens can provide insights into the feasibility and selectivity of radical iodination.

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Related Practice

Textbook Question

a. For which reaction in each set will ∆S° be more significant?

b. For which reaction will ∆S° be positive?

1. A ⇌ B or A + B ⇌ C

2. A ⇌ B + C or A + B ⇌ C + D

Textbook Question

a. Which of the following reactions has the larger ∆S° value?

b. Is the ∆S° value positive or negative?

Textbook Question

At what temperature does the entropy change of a process not contribute to the favorability of a process?

Textbook Question

The hydrogenation of alkenes is a reaction we study in Chapter 9.

(b) Is this reaction favored or disfavored in terms of entropy?

Textbook Question

The dehydrogenation of butane to trans-but-2-ene has ΔH° = +116 kJ/mol (+27.6 kcal/mol) and ΔS° = +117J/kelvin-mol (+28.0 cal/kelvin-mol). a. Compute the value of ΔG° for dehydrogenation at room temperature (25 °C or 298 °K). Is dehydrogenation favored or disfavored?HINT: When you are doing synthesis problems, avoid using these high-temperature industrial methods. They require specialized equipment, and they produce variable mixtures of products.

Textbook Question

The dehydrogenation of butane to trans-but-2-ene has ΔH° = +116 kJ/mol (+27.6 kcal/mol) and ΔS° = +117J/kelvin-mol (+28.0 cal/kelvin-mol).

a. Compute the value of ΔG° for dehydrogenation at room temperature (25 °C or 298 °K). Is dehydrogenation favored or disfavored?

HINT: When you are doing synthesis problems, avoid using these high-temperature industrial methods. They require specialized equipment, and they produce variable mixtures of products.

Textbook Question

Estimate the value of entropy (∆S > 0 or ∆S < 0) for the elimination step shown.

Textbook Question

Would you expect ∆S to be greater than, less than, or equal to zero in the following reactions?

(b)

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Through the course of this chapter, we have discussed only alkane chlorination and bromination, yet there are two other halogens we have not discussed. (b) Is radical iodination a favorable reaction? Do you expect it to be selective? Show your calculations.

Key Concepts

Radical Halogenation

Selectivity in Halogenation

Thermodynamics and Kinetics of Iodination

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Through the course of this chapter, we have discussed only alkane chlorination and bromination, yet there are two other halogens we have not discussed.
(b) Is radical iodination a favorable reaction? Do you expect it to be selective? Show your calculations.