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

15. Analytical Techniques:IR, NMR, Mass Spect

Infrared Spectroscopy Table

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Infrared Spectroscopy Table

Previous problemNext problem

3:45 minutes

Problem 26

Textbook Question

Which shows an O–H stretch at a larger wavenumber: ethanol dissolved in carbon disulfide or an undiluted sample of ethanol?

Verified step by step guidance

Understand the concept of hydrogen bonding: The O-H stretch in an IR spectrum is influenced by hydrogen bonding. Stronger hydrogen bonding lowers the wavenumber of the O-H stretch due to the weakening of the O-H bond.

Analyze the environment of ethanol in carbon disulfide: When ethanol is dissolved in carbon disulfide (CS₂), a non-polar solvent, hydrogen bonding between ethanol molecules is disrupted because CS₂ does not participate in hydrogen bonding.

Analyze the environment of undiluted ethanol: In an undiluted sample of ethanol, ethanol molecules are in close proximity and can form strong intermolecular hydrogen bonds with each other.

Compare the effects on the O-H stretch: In the case of ethanol dissolved in CS₂, the O-H bond is less involved in hydrogen bonding, so the O-H stretch appears at a higher wavenumber. In undiluted ethanol, the strong hydrogen bonding shifts the O-H stretch to a lower wavenumber.

Conclude: Ethanol dissolved in carbon disulfide shows an O-H stretch at a larger wavenumber compared to undiluted ethanol due to the absence of significant hydrogen bonding in the dissolved state.

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Key Concepts

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

Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify molecular structures by measuring the absorption of infrared light, which causes molecular vibrations. Different functional groups absorb light at characteristic wavenumbers, allowing for the identification of specific bonds, such as O-H stretches in alcohols.

Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen, interacts with another electronegative atom. In ethanol, hydrogen bonding can affect the O-H stretch frequency; stronger hydrogen bonding typically results in a lower wavenumber due to increased bond strength and reduced vibrational frequency.

Solvent Effects

The choice of solvent can significantly influence the vibrational frequencies observed in infrared spectroscopy. In non-polar solvents like carbon disulfide, ethanol's hydrogen bonding interactions are diminished, potentially leading to a higher wavenumber for the O-H stretch compared to undiluted ethanol, where strong intermolecular hydrogen bonds are present.

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Related Practice

Textbook Question

The acid-catalyzed hydrolysis of an ester results in the formation of an equal amount of carboxylic acid and alcohol.

(b) Once separated, how could you distinguish between the carboxylic acid and alcohol using IR spectroscopy?

Textbook Question

The absence of what band in the IR spectrum of the product in Figure 18.59 would be consistent with full conversion of the carboxylic acid to an acid chloride?

Textbook Question

The IR spectrum of trans-oct-2-enoic acid is shown. Point out the spectral characteristics that allow you to tell that this is a carboxylic acid, and show which features lead you to conclude that the acid is unsaturated and conjugated.

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Textbook Question

Following a LiAlH₄ reduction, an IR spectrum suggested that the product was a mixture of the starting amide and the desired amine.

(b) Once separated, how could you distinguish between the amide and the amine using IR spectroscopy?

Open Question

Answer each of the following questions based on the images below.

a) Which compounds show an intense peak ~ 1700 cm^-1?

b) Which compound shows an intense, broad peak at ~ 3400 cm^-1?

c) Which compound has a peak at ~1700 cm^-1, but no peaks at 2700 cm^-1?

d) Which compound has no signal beyond the fingerprint region?

Multiple Choice

Identify which carbonyl group will exhibit a signal at a lower wavenumber