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 48

Textbook Question

Would you expect the following molecules to absorb in the UV–visible region of the electromagnetic spectrum?
(a)
(b)
(c)
(d)
(e)

Verified step by step guidance

Understand that UV-visible absorption is related to the presence of conjugated systems in a molecule. Conjugated systems involve alternating single and multiple bonds, which allow for delocalization of π-electrons.

Examine each molecule to identify the presence of conjugated systems. Look for alternating single and double bonds, or aromatic rings, which are common indicators of conjugation.

Consider the length of the conjugated system. Generally, the longer the conjugated system, the lower the energy required for electronic transitions, and the more likely the molecule will absorb in the UV-visible region.

Evaluate the presence of functional groups that can affect UV-visible absorption. Groups such as carbonyls, nitro groups, or amines can shift absorption wavelengths due to their electronic effects.

For each molecule, determine if the structure includes features that typically result in UV-visible absorption, such as extended conjugation or specific functional groups. If these features are present, the molecule is likely to absorb in the UV-visible region.

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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 single and multiple bonds, allowing for delocalization of electrons. This delocalization lowers the energy gap between the ground and excited states, enabling absorption in the UV-visible region. Molecules with extended conjugation typically absorb at longer wavelengths, making them more likely to be visible in the UV-visible spectrum.

Chromophores

Chromophores are parts of a molecule responsible for its color, typically due to the presence of conjugated pi-bond systems. These groups absorb specific wavelengths of light, leading to electronic transitions. The presence of chromophores in a molecule is a key factor in determining whether it will absorb in the UV-visible region, as they are directly involved in the absorption process.

Electronic Transitions

Electronic transitions occur when electrons in a molecule absorb energy and move from a lower energy level (ground state) to a higher energy level (excited state). In the context of UV-visible spectroscopy, these transitions often involve pi to pi* or n to pi* transitions. The likelihood and wavelength of absorption depend on the energy difference between these states, which is influenced by the molecular structure and presence of conjugated systems.

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