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

HOMO LUMO

Organic Chemistry

16. Conjugated Systems

HOMO LUMO

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Problem 21e

Textbook Question

Which of the following molecules would you expect to absorb in the visible region of the electromagnetic spectrum?
(e)

Verified step by step guidance

Identify the structural features of the molecule that can lead to absorption in the visible region. Typically, conjugated systems with alternating single and double bonds are responsible for such absorption.

Examine the given structure for conjugation. The molecule contains a conjugated system involving a carbonyl group (C=O) and a double bond (C=C).

Consider the extent of conjugation. The presence of a conjugated system can lower the energy gap between the HOMO and LUMO, allowing absorption of longer wavelengths, potentially in the visible region.

Evaluate the length of the conjugated system. Longer conjugated systems tend to absorb at longer wavelengths. In this molecule, the conjugation is limited to a few bonds.

Conclude whether the conjugation is sufficient for visible light absorption. Generally, more extensive conjugation is required for absorption in the visible region, so assess if the current conjugation is adequate.

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Key Concepts

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

Conjugation

Conjugation in organic chemistry refers to the overlap of p-orbitals across adjacent single and multiple bonds, allowing for delocalization of electrons. This delocalization can lower the energy gap between molecular orbitals, enabling absorption of longer wavelengths of light, such as those in the visible spectrum. The molecule in the image shows a conjugated system, which is crucial for its potential to absorb visible light.

Chromophores

Chromophores are parts of molecules responsible for their color, typically due to the presence of conjugated pi-bond systems. These structures absorb specific wavelengths of light, resulting in the emission or reflection of others, which we perceive as color. In the molecule shown, the conjugated system acts as a chromophore, potentially absorbing visible light.

Electromagnetic Spectrum

The electromagnetic spectrum encompasses all wavelengths of electromagnetic radiation, from gamma rays to radio waves. The visible region, which ranges from approximately 400 to 700 nm, is where human eyes can detect light. Molecules that absorb in this region often have extended conjugation, as seen in the provided structure, which can absorb visible light due to its conjugated system.