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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3:26 minutes

Problem 13dBruice - 8th Edition

Textbook Question

Rank each set of compounds in order of decreasing lmax: b.

Verified step by step guidance

Identify the conjugated systems in each compound. Compounds A, C, and D have aromatic rings, while B does not.

Recognize that the presence of an aromatic ring generally increases the \( \lambda_{max} \) due to extended conjugation.

Consider the effect of substituents on the aromatic ring. Electron-donating groups (EDGs) like the dimethylamino group in A and C can increase \( \lambda_{max} \).

Note that in compound D, the nitrogen is positively charged, which can affect the electron-donating ability of the dimethylamino group, potentially decreasing \( \lambda_{max} \) compared to A and C.

Rank the compounds based on the extent of conjugation and the effect of substituents: A > C > D > B.

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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 electrons. This delocalization lowers the energy of the molecule and can lead to increased stability and altered electronic properties, such as shifts in absorption wavelengths (lmax) in UV-Vis spectroscopy.

lmax (Lambda max)

lmax is the wavelength at which a compound exhibits maximum absorbance in UV-Vis spectroscopy. It is influenced by the extent of conjugation in a molecule; more extensive conjugation typically results in a longer lmax due to lower energy transitions between molecular orbitals.

Electron-Withdrawing and Electron-Donating Groups

Electron-withdrawing groups (EWGs) can decrease electron density in a conjugated system, often leading to a blue shift in lmax, while electron-donating groups (EDGs) can increase electron density, resulting in a red shift. The presence and position of these groups in the molecular structure significantly affect the electronic transitions and, consequently, the lmax.