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

16. Conjugated Systems

Stability of Conjugated Intermediates

Organic Chemistry

16. Conjugated Systems

Stability of Conjugated Intermediates

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3:41 minutes

Problem 69

Textbook Question

Which compound would you expect to have the greater heat of hydrogenation: 1,2-pentadiene or 1,4-pentadiene?

Verified step by step guidance

Understand the concept of heat of hydrogenation: It is the amount of heat released when one mole of an unsaturated compound (containing double or triple bonds) is hydrogenated to form a saturated compound. The greater the instability of the starting compound, the higher the heat of hydrogenation.

Analyze the structure of 1,2-pentadiene: This compound contains two double bonds that are conjugated (adjacent to each other). Conjugation provides some stabilization due to delocalization of π-electrons.

Analyze the structure of 1,4-pentadiene: This compound contains two double bonds that are isolated (separated by at least one single bond). Isolated double bonds do not benefit from conjugation and are generally less stable than conjugated double bonds.

Compare the stability of the two compounds: Since 1,2-pentadiene has conjugated double bonds, it is more stable than 1,4-pentadiene, which has isolated double bonds. Greater stability corresponds to a lower heat of hydrogenation.

Conclude: The compound with the greater heat of hydrogenation will be 1,4-pentadiene because it is less stable than 1,2-pentadiene due to the lack of conjugation.

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Key Concepts

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

Heat of Hydrogenation

The heat of hydrogenation refers to the amount of heat released when an alkene reacts with hydrogen to form an alkane. This value is indicative of the stability of the alkene; less stable alkenes release more heat upon hydrogenation. Therefore, comparing the heats of hydrogenation of different alkenes can provide insights into their relative stabilities.

Alkene Stability

Alkene stability is influenced by the degree of substitution of the double bond. More substituted alkenes (those with more alkyl groups attached to the double bond) are generally more stable due to hyperconjugation and the inductive effect. In the case of 1,2-pentadiene and 1,4-pentadiene, the position of the double bonds affects their substitution and, consequently, their stability.

Conjugation

Conjugation occurs when double bonds are separated by a single bond, allowing for the delocalization of π electrons across multiple atoms. This delocalization stabilizes the molecule, making conjugated systems like 1,4-pentadiene more stable than non-conjugated systems. Understanding the role of conjugation is crucial for predicting the heat of hydrogenation and the relative stabilities of the compounds in question.

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