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

Enthalpy

Organic Chemistry

6. Thermodynamics and Kinetics

Enthalpy

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4:42 minutes

Problem 56

Textbook Question

Calculate ∆H° for the following alkene addition reaction, one we discuss further in Chapter 7. Predict the sign of ∆S° . (The BDE for C―C π bond is approximately 65 kcal/mol.)

Verified step by step guidance

Identify the bonds broken and formed during the reaction. In an alkene addition reaction, the π bond of the alkene is broken, and two new σ bonds are formed (e.g., C-H or C-X bonds, depending on the reaction).

Use the bond dissociation energies (BDEs) to calculate the enthalpy change (∆H°). The formula is: ∆H° = Σ(BDE of bonds broken) - Σ(BDE of bonds formed). For this reaction, the BDE of the C=C π bond is approximately 65 kcal/mol, and you will need the BDEs of the bonds formed (e.g., C-H or C-X).

Predict the sign of ∆S° (entropy change). In an alkene addition reaction, two reactants combine to form a single product, which decreases the number of molecules and thus decreases entropy. Therefore, ∆S° is expected to be negative.

Substitute the known BDE values into the ∆H° formula to calculate the enthalpy change. Ensure that you account for all bonds broken and formed in the reaction.

Summarize the thermodynamic predictions: ∆H° will likely be negative (exothermic) if the bonds formed are stronger than the bonds broken, and ∆S° will be negative due to the decrease in molecular randomness.

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Key Concepts

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

Enthalpy Change (∆H°)

Enthalpy change (∆H°) refers to the heat content change of a system at constant pressure during a chemical reaction. In the context of alkene addition reactions, it is crucial to calculate the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products. A negative ∆H° indicates an exothermic reaction, while a positive value indicates an endothermic reaction.

Bond Dissociation Energy (BDE)

Bond dissociation energy (BDE) is the energy required to break a specific bond in a molecule, resulting in the formation of free radicals. In this question, the BDE for the C―C π bond is given as approximately 65 kcal/mol, which is essential for calculating the enthalpy change of the reaction. Understanding BDE helps predict how much energy is needed to initiate the reaction and how it influences the overall reaction energetics.

Entropy Change (∆S°)

Entropy change (∆S°) measures the disorder or randomness in a system. In chemical reactions, a positive ∆S° indicates an increase in disorder, while a negative ∆S° suggests a decrease. For the alkene addition reaction, predicting the sign of ∆S° involves considering the number of reactant and product molecules; typically, if fewer gas molecules are produced, ∆S° is negative, reflecting a decrease in disorder.

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Related Practice

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

c. (CH₃)₃C—OH + HCl → (CH₃)₃C—Cl + H₂O

Textbook Question

Use bond-dissociation enthalpies (Table 4-2, p. 167) to calculate values of ΔH° for the following reactions.

d. CH₃CH₂CH₃ + H₂ → CH₃CH₃ + CH₄

Textbook Question

If the following reaction is favorable, what can we say about the sign of ∆H°? Explain your answer.

Textbook Question

Give the approximate bond-dissociation energy for each indicated bond.

Textbook Question

Reaction (c), on the other hand, is favored (∆G° < 0). Identify the bonds formed and broken and explain this result in light of (a) and (b).

(c)

Textbook Question

Calculate ∆H° for the following equilibrium processes.

(b)

Textbook Question

The combustion of alkanes is exothermic (∆H° < 0) . Would you expect the combustion of butane or cyclobutane to be more exothermic?

Textbook Question

Reactions (a) and (b) are disfavored overall (∆G° > 0), yet they are favored based on ∆H°. Identify the bonds formed and broken for (a) and (b).

(b)

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