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

Conjugation Chemistry

Organic Chemistry

16. Conjugated Systems

Conjugation Chemistry

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3:40 minutes

Problem 72c

Textbook Question

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

Verified step by step guidance

Step 1: Analyze the structures of compounds A and B. Compound A is acetophenone, which contains a phenyl group directly attached to a carbonyl group. Compound B is propiophenone, which has a phenyl group attached to a carbonyl group via a methylene (-CH2-) bridge.

Step 2: Understand the principle of UV spectroscopy. UV spectroscopy measures the absorption of ultraviolet light by a compound, which is influenced by the presence of conjugated systems. Conjugation increases the wavelength of maximum absorption (λmax).

Step 3: Compare the conjugation in the two compounds. In acetophenone (A), the phenyl group is directly conjugated with the carbonyl group, leading to a higher degree of conjugation. In propiophenone (B), the methylene bridge interrupts conjugation between the phenyl group and the carbonyl group, resulting in less conjugation.

Step 4: Predict the UV absorption behavior. Due to the greater conjugation in acetophenone (A), it will have a higher λmax compared to propiophenone (B). Propiophenone (B) will absorb UV light at a shorter wavelength because of the reduced conjugation.

Step 5: Use UV spectroscopy to distinguish between the two compounds. Measure the λmax for each compound. Acetophenone (A) will show a λmax at a longer wavelength, while propiophenone (B) will show a λmax at a shorter wavelength. This difference in λmax can be used to identify and distinguish 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-Vis Spectroscopy

UV-Vis spectroscopy is an analytical technique that measures the absorbance of ultraviolet or visible light by a substance. It is particularly useful for identifying compounds with conjugated systems, as these systems absorb light at specific wavelengths due to electronic transitions. The resulting spectrum provides information about the energy levels of electrons in the molecule, which can be used to distinguish between different compounds.

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 electronic transitions, resulting in absorption of light at longer wavelengths (lower energy). The extent of conjugation in a molecule affects its UV-Vis spectrum, making it a key factor in distinguishing between compounds with different conjugated systems.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the context of UV spectroscopy, different functional groups can influence the absorption properties of a compound. For example, carbonyl groups (C=O) and aromatic rings can significantly affect the UV spectrum, allowing for differentiation between compounds based on their functional group composition.

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Textbook Question

Identify the following dienes as being in the s-cis or s-trans conformation. If they are in the s-trans conformation, draw them in the s-cis conformation. [It may not always be possible.]

(e)

Textbook Question

At pH = 7, one of the ions shown here is purple and the other is blue. Which is which?

Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(a)

(b)

(c)

Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(d)

(e)

(f)

Textbook Question

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

Textbook Question

One milligram of a compound of molecular weight 160 is dissolved in 10 mL of ethanol, and the solution is poured into a 1-cm UV cell. The UV spectrum is taken, and there is an absorption at λ_max = 247 nm. The maximum absorbance at 247 nm is 0.50. Calculate the value of e for this absorption.

Textbook Question

The UV spectrum of 1-phenylprop-2-en-1-ol shows an intense absorption at 220 nm and a weaker absorption at 258 nm. When this compound is treated with dilute sulfuric acid, it rearranges to an isomer with an intense absorption at 250 nm and a weaker absorption at 290 nm. Suggest a structure for the isomeric product and propose a mechanism for its formation.

Textbook Question

A 4.0 × 10^-5 M solution of a compound in hexane shows an absorbance of 0.40 at 252 nm in a cell with a 1 cm light path. What is the molar absorptivity of the compound in hexane at 252 nm?

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

Key Concepts

UV-Vis Spectroscopy

Conjugation

Functional Groups

Watch next

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