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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2:18 minutes

Problem 13bBruice - 8th Edition

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

Identify the structural differences between compounds A and B. Compound A has three conjugated double bonds, while compound B has two conjugated double bonds and one isolated double bond.

Understand that UV spectroscopy is used to analyze the extent of conjugation in a molecule. More conjugated systems absorb at longer wavelengths (lower energy) compared to less conjugated systems.

Predict that compound A, with more conjugation, will have a UV absorption at a longer wavelength compared to compound B.

Consider the effect of the additional conjugation in compound A, which will result in a bathochromic shift (red shift) in its UV spectrum compared to compound B.

Use the UV spectra to distinguish between the two compounds by comparing the wavelength of maximum absorption (λmax) for each compound. Compound A should have a higher λmax than compound B.

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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 unique absorption patterns.

Conjugation and Absorption

Conjugation refers to the presence of alternating double and single bonds in a molecule, which allows for delocalization of π electrons. This delocalization lowers the energy gap between the ground and excited states, resulting in absorption of longer wavelengths (lower energy) light. Compounds with different degrees of conjugation will exhibit distinct UV absorption spectra, making it a key factor in distinguishing between them.

Functional Groups and UV Absorption

Different functional groups in organic compounds can significantly influence their UV absorption characteristics. For example, carbonyl groups (C=O) and aromatic rings typically absorb UV light at specific wavelengths due to their electronic transitions. Identifying the presence and type of functional groups in compounds A and B can help predict their UV absorption behavior, aiding in their differentiation through spectroscopy.