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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2:12 minutes

Problem 36

Textbook Question

Predict the λ_max of the following compound:

Verified step by step guidance

Step 1: Understand the concept of λmax (maximum wavelength of absorption). λmax is influenced by conjugation in the molecule. The more conjugated double bonds present, the higher the λmax value due to the stabilization of the π-electron system.

Step 2: Analyze the given compounds. The first compound, but-3-en-2-one, has one conjugated double bond and a λmax of 219 nm. The second compound, (E)-hexa-3,5-dien-2-one, has two conjugated double bonds and a λmax of 249 nm. The third compound, cyclopent-2-enone, has one conjugated double bond but is part of a cyclic structure.

Step 3: Consider the effect of the cyclic structure in cyclopent-2-enone. Cyclic conjugated systems can have slightly different λmax values compared to acyclic systems due to ring strain and orbital overlap. Cyclopent-2-enone is expected to have a λmax slightly higher than but-3-en-2-one due to the stabilization provided by the cyclic structure.

Step 4: Use the base λmax value for an α,β-unsaturated ketone (approximately 215 nm) and add any increments for structural features. For example, cyclic conjugation may add a small increment to the λmax value.

Step 5: Predict the λmax for cyclopent-2-enone based on the trends observed in the other compounds and the structural features of the molecule. Compare it to the λmax values of but-3-en-2-one and (E)-hexa-3,5-dien-2-one to make an educated estimate.

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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 between double bonds and lone pairs, allowing for delocalization of electrons. This delocalization lowers the energy of the molecule and can lead to increased stability and shifts in the absorption spectrum, affecting the λmax value. Compounds with extended conjugation typically absorb light at longer wavelengths.

λmax (Lambda Max)

λmax is the wavelength at which a compound exhibits maximum absorbance in UV-Vis spectroscopy. It is influenced by the electronic structure of the molecule, particularly the presence of conjugated systems. Understanding λmax is crucial for predicting how different structural features, such as double bonds and functional groups, affect a compound's light absorption properties.

Structural Effects on Absorption

The structure of a molecule significantly impacts its electronic transitions and, consequently, its λmax. Factors such as the number of conjugated double bonds, steric hindrance, and the presence of electron-withdrawing or donating groups can alter the energy levels of the molecular orbitals. Analyzing these structural features helps predict the λmax for compounds like cyclopent-2-en-one based on known values of similar structures.

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