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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4:14 minutes

Problem 47

Textbook Question

a. Which is the most stable: 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?
b. Which compound has the largest heat of hydrogenation?
c. Which compound has the smallest heat of hydrogenation?

Verified step by step guidance

Step 1: Understand the stability of alkenes. Alkene stability is influenced by the degree of substitution of the double bond. More substituted alkenes are generally more stable due to hyperconjugation and inductive effects. Tetrasubstituted alkenes are the most stable, followed by trisubstituted, disubstituted, and monosubstituted alkenes.

Step 2: Analyze the substitution pattern of each compound. For 3,4-dimethyl-2-hexene, the double bond is between carbons 2 and 3, and it is trisubstituted. For 2,3-dimethyl-2-hexene, the double bond is between carbons 2 and 3, and it is tetrasubstituted. For 4,5-dimethyl-2-hexene, the double bond is between carbons 2 and 3, and it is disubstituted.

Step 3: Compare the stability of the compounds. Based on the substitution pattern, 2,3-dimethyl-2-hexene (tetrasubstituted) is the most stable, followed by 3,4-dimethyl-2-hexene (trisubstituted), and then 4,5-dimethyl-2-hexene (disubstituted).

Step 4: Understand the relationship between heat of hydrogenation and stability. The heat of hydrogenation is inversely related to alkene stability. More stable alkenes release less heat upon hydrogenation, while less stable alkenes release more heat.

Step 5: Determine the compounds with the largest and smallest heat of hydrogenation. The least stable compound, 4,5-dimethyl-2-hexene, will have the largest heat of hydrogenation. The most stable compound, 2,3-dimethyl-2-hexene, will have the smallest heat of hydrogenation.

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Key Concepts

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

Stability of Alkenes

The stability of alkenes is influenced by the degree of substitution at the double bond. More substituted alkenes, such as tetrasubstituted or trisubstituted, are generally more stable due to hyperconjugation and the inductive effect of alkyl groups. In this context, comparing the substitution patterns of 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, and 4,5-dimethyl-2-hexene will help determine which is the most stable.

Heat of Hydrogenation

The heat of hydrogenation is a measure of the energy released when an alkene is converted to an alkane through the addition of hydrogen. A higher heat of hydrogenation indicates a less stable alkene, as more energy is required to break the double bond. Thus, analyzing the heats of hydrogenation for the given compounds will reveal which has the largest and smallest values, correlating with their stability.

Zaitsev's Rule

Zaitsev's Rule states that in elimination reactions, the more substituted alkene is typically the major product. This principle can be applied to predict the stability of the alkenes formed from the compounds in the question. Understanding this rule aids in determining which compound will have a lower heat of hydrogenation due to its greater stability, as more substituted alkenes are favored.

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Related Practice

Textbook Question

Rank each group of compounds in order of increasing heat of hydrogenation.

(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

Textbook Question

Consider the Cope rearrangement, a reaction we describe in Chapter 20.

(a) Using the knowledge we have gained here in Chapter 9, suggest a one-step, concerted mechanism that explains the formation of B from A.

(b) Which side of the reaction would you expect to be favored? Justify your answer.

Textbook Question

Rank the following alkenes in order of stability (1 = most stable; 5 = least stable). Explain your order.

Textbook Question

The same alkane is obtained from the catalytic hydrogenation of both alkene A and alkene B. The heat of hydrogenation of alkene A is 29.8 kcal/mol, and the heat of hydrogenation of alkene B is 31.4 kcal/mol. Which alkene is more stable?

Textbook Question

Of the compounds you named in Problem 45:

a. Which is the most stable?

Textbook Question

Of the compounds you named in Problem 45:

b. Which is the least stable?

Textbook Question

Which species in each pair is more stable?

Textbook Question

Use the data in Table 7-2 to predict the energy difference between 2,3-dimethylbut-1-ene and 2,3-dimethylbut-2-ene. Which of these double-bond isomers is more stable?

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a. Which is the most stable: 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?b. Which compound has the largest heat of hydrogenation?c. Which compound has the smallest heat of hydrogenation?

Key Concepts

Stability of Alkenes

Heat of Hydrogenation

Zaitsev's Rule

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a. Which is the most stable: 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?
b. Which compound has the largest heat of hydrogenation?
c. Which compound has the smallest heat of hydrogenation?