Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 19m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 18m
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 Acids3h 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

HOMO LUMO

Organic Chemistry

16. Conjugated Systems

HOMO LUMO

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2:38 minutes

Problem 13cBruice - 8th Edition

Textbook Question

Rank the following compounds in order of increasing lmax:

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Identify the conjugated systems in each compound. Conjugation occurs when there are alternating single and double bonds, allowing for delocalization of π-electrons.

Determine the extent of conjugation in each compound. The more extensive the conjugation, the longer the wavelength (λmax) of absorption.

Compare the number of conjugated double bonds in each compound. More conjugated double bonds typically result in a higher λmax.

Rank the compounds based on the extent of conjugation. The compound with the least conjugation will have the lowest λmax, and the one with the most conjugation will have the highest λmax.

Consider any additional factors such as the presence of aromatic rings or other structural features that might influence the λmax.

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Key Concepts

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

Conjugation

Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for delocalization of π electrons. This delocalization lowers the energy of the electronic system and increases the wavelength of light absorbed, leading to higher λmax (lmax) values. The more extensive the conjugation, the longer the wavelength of light absorbed.

λmax (lmax)

λmax (lmax) is the wavelength at which a compound exhibits maximum absorbance in UV-Vis spectroscopy. It is influenced by the electronic structure of the molecule, particularly the presence of conjugated systems. Compounds with greater conjugation typically have higher λmax values due to the stabilization of excited states.

Substituent Effects

Substituents on a conjugated system can significantly affect its electronic properties and, consequently, its λmax. Electron-donating groups can increase electron density in the π system, enhancing conjugation and raising λmax, while electron-withdrawing groups can decrease electron density, lowering λmax. Understanding these effects is crucial for predicting the order of λmax in different compounds.