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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6:21 minutes

Problem 15a

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)

Verified step by step guidance

Step 1: Analyze the stability of the isomers based on conjugation. Conjugation occurs when π-electrons can delocalize over adjacent π-systems or lone pairs. The more conjugated an isomer is, the more stable it tends to be.

Step 2: Examine the position of the double bonds relative to the benzene ring. Double bonds directly conjugated with the benzene ring (i.e., in resonance with the aromatic system) will increase stability.

Step 3: Consider steric hindrance. Isomers with bulky groups or double bonds positioned in a way that causes steric clashes will be less stable.

Step 4: Evaluate hyperconjugation effects. Hyperconjugation occurs when alkyl groups adjacent to double bonds or aromatic rings donate electron density, stabilizing the molecule.

Step 5: Rank the isomers based on the above factors. The most stable isomer will have the highest degree of conjugation and minimal steric hindrance, while the least stable isomer will lack conjugation and/or have significant steric hindrance.

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Key Concepts

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

Isomer Stability

Isomer stability refers to the relative energy levels of different isomers of a compound. Generally, more stable isomers have lower energy due to factors such as steric hindrance, resonance stabilization, and the overall molecular conformation. Understanding which structural features contribute to stability is crucial for predicting the most and least stable isomers.

Steric Hindrance

Steric hindrance occurs when atoms within a molecule are forced too close together, leading to increased repulsion and higher energy states. In isomers, bulky groups can create steric strain, making certain conformations less stable. Evaluating the spatial arrangement of substituents helps determine which isomer is more stable.

Resonance Structures

Resonance structures are different ways of drawing a molecule that represent the same compound, highlighting the delocalization of electrons. Isomers that can delocalize electrons through resonance tend to be more stable due to the distribution of charge and energy. Identifying resonance contributors is essential for assessing the stability of isomers.

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Related Practice

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.

(c)

Textbook Question

In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene could not have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbons. This is known as Bredt's rule. Explain why there cannot be a double bond at this position.

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

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

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.

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

c. 2-methylbut-1-ene or 2-methylbut-2-ene

d. cis-4-methylpent-2-ene or 2-methylpent-2-ene

Textbook Question

Which species in each pair is more stable?

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