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

9. Alkenes and Alkynes

Alkene Stability

Organic Chemistry

9. Alkenes and Alkynes

Alkene Stability

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4:37 minutes

Problem 48

Textbook Question

A double bond in a six-membered ring is usually more stable in an endocyclic position than in an exocyclic position. Hydrogenation data on two pairs of compounds follow. One pair suggests that the energy difference between endocyclic and exocyclic double bonds is about 9 kJ/mol. The other pair suggests an energy difference of about 5 kJ/mol. Which number do you trust as being more representative of the actual energy difference? Explain your answer.

Verified step by step guidance

Step 1: Analyze the heats of hydrogenation data provided for the two pairs of compounds. The first pair shows an energy difference of 5 kJ/mol (116 kJ/mol for exocyclic and 111 kJ/mol for endocyclic). The second pair shows an energy difference of 9 kJ/mol (110 kJ/mol for exocyclic and 105 kJ/mol for endocyclic).

Step 2: Understand the concept of stability in double bonds. Endocyclic double bonds are generally more stable than exocyclic double bonds due to better conjugation and less steric strain. This stability is reflected in lower heats of hydrogenation for endocyclic double bonds.

Step 3: Evaluate the reliability of the data. The heats of hydrogenation for the second pair (difference of 9 kJ/mol) are closer in magnitude and consistent with the expected trend of endocyclic double bonds being more stable. The first pair (difference of 5 kJ/mol) shows a smaller difference, which might be influenced by additional factors such as substituent effects or experimental error.

Step 4: Consider the structural differences between the compounds in each pair. The second pair has simpler structures with fewer substituents, which reduces the likelihood of confounding factors affecting the heats of hydrogenation. This makes the second pair's data more representative of the intrinsic stability difference between endocyclic and exocyclic double bonds.

Step 5: Conclude that the energy difference of 9 kJ/mol is more representative of the actual stability difference between endocyclic and exocyclic double bonds, as it aligns better with theoretical expectations and is supported by cleaner experimental data.

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Key Concepts

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

Endocyclic vs. Exocyclic Double Bonds

Endocyclic double bonds are located within a ring structure, while exocyclic double bonds are outside the ring. The stability of these bonds can differ significantly due to steric and electronic factors. Generally, endocyclic double bonds are more stable because they benefit from the ring's structural constraints, which can lead to less steric hindrance and more favorable orbital overlap.

Heats of Hydrogenation

Heats of hydrogenation refer to the energy released when a double bond is converted to a single bond through the addition of hydrogen. This value is indicative of the stability of the double bond; lower heats of hydrogenation suggest greater stability. By comparing the heats of hydrogenation for different compounds, one can infer the relative stabilities of their double bonds, aiding in the assessment of endocyclic versus exocyclic configurations.

Energy Difference Interpretation

The energy difference between endocyclic and exocyclic double bonds can be assessed through experimental data, such as heats of hydrogenation. In this case, two pairs of compounds suggest different energy differences (9 kJ/mol and 5 kJ/mol). The more representative value may depend on factors like the specific compounds studied and their structural features, which can influence the stability of the double bonds and the accuracy of the heat measurements.

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Related Practice

Textbook Question

Rank the following compounds from most stable to least stable:

trans-3-hexene, cis-3-hexene, cis-2,5-dimethyl-3-hexene, (Z)-3,4-dimethyl-3-hexene

Textbook Question

The energy difference between cis- and trans-but-2-ene is about 4 kJ/mol; however, the trans isomer of 4,4-dimethylpent-2-ene is nearly 16 kJ/mol more stable than the cis isomer. Explain this large difference.

Textbook Question

For each set of isomers, choose the isomer that you expect to be most stable and the isomer you expect to be least stable.

(a)

Textbook Question

Explain why each of the following alkenes is stable or unstable.

(a) 1,2-dimethylcyclopentene

(b) trans-1,2-dimethylcyclopentene

(d) trans-1,2-dimethylcyclodecene

Textbook Question

Using Table 7-2 as a guide, predict which member of each pair is more stable, as well as by about how many kJ/mol or kcal/mol.

a. cis,cis-hexa-2,4-diene or trans,trans-hexa-2,4-diene

b. 2-methylbut-1-ene or 3-methylbut-1-ene