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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2:18 minutes

Problem 72a

Textbook Question

How can you use UV spectroscopy to distinguish between the compounds in each of the following pairs?
a.

Verified step by step guidance

Step 1: Analyze the structures of compounds A and B. Compound A contains conjugated double bonds (a system of alternating single and double bonds), while compound B has isolated double bonds (double bonds separated by single bonds without conjugation). This structural difference is key to their UV spectroscopy behavior.

Step 2: Understand the principle of UV spectroscopy. UV spectroscopy measures the absorption of ultraviolet light by molecules, which is influenced by the presence of conjugated systems. Conjugated systems lower the energy gap between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), resulting in absorption at longer wavelengths.

Step 3: Predict the UV absorption behavior. Compound A, with its conjugated double bonds, will absorb UV light at a longer wavelength (lower energy) compared to compound B, which has isolated double bonds. This is because conjugation increases the delocalization of π-electrons, stabilizing the molecule and shifting the absorption maximum (λmax) to a longer wavelength.

Step 4: Compare the λmax values. In a UV spectrum, compound A will show a λmax at a longer wavelength due to its conjugated system, while compound B will have a λmax at a shorter wavelength because its double bonds are not conjugated.

Step 5: Use UV spectroscopy data to distinguish the compounds. By recording the UV spectra of both compounds, you can identify compound A by its longer λmax and compound B by its shorter λmax. This difference in absorption wavelengths provides a clear method to differentiate between the two compounds.

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Key Concepts

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

UV Spectroscopy Principles

UV spectroscopy is a technique that measures the absorption of ultraviolet light by molecules. When a molecule absorbs UV light, electrons are excited from a ground state to a higher energy state. The specific wavelengths of light absorbed can provide information about the electronic structure of the molecule, allowing for differentiation between compounds based on their functional groups and conjugation.

Conjugation and Electron Delocalization

Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for electron delocalization. This delocalization lowers the energy gap between the ground and excited states, resulting in absorption of longer wavelengths (lower energy) of UV light. Compounds with different degrees of conjugation will absorb UV light at different wavelengths, making it a key factor in distinguishing between them.

Functional Groups and Their UV Absorption

Different functional groups absorb UV light at characteristic wavelengths due to their unique electronic structures. For example, alkenes and aromatic compounds typically show distinct absorption patterns. By analyzing the UV absorption spectrum, one can identify specific functional groups present in the compounds, aiding in their differentiation based on the observed peaks in the spectrum.

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