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

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Infrared Spectroscopy

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4:05 minutes

Problem 21a(3,4)

Textbook Question

a. Which occurs at a larger wavenumber:
3. a C–N stretch or a C=N stretch?
4. a C=O stretch or a C–O stretch?

Verified step by step guidance

Understand the concept of bond stretching frequencies in IR spectroscopy: The wavenumber of a bond stretch in IR spectroscopy depends on the bond strength and the masses of the atoms involved. Stronger bonds and lighter atoms result in higher wavenumbers.

For part 3: Compare the bond strengths of C-N and C=N. A double bond (C=N) is stronger than a single bond (C-N), so the C=N stretch will occur at a larger wavenumber than the C-N stretch.

For part 4: Compare the bond strengths of C=O and C-O. A double bond (C=O) is stronger than a single bond (C-O), so the C=O stretch will occur at a larger wavenumber than the C-O stretch.

Relate the bond strength to the vibrational frequency: Stronger bonds vibrate at higher frequencies, which correspond to larger wavenumbers in the IR spectrum.

Summarize the findings: The C=N stretch occurs at a larger wavenumber than the C-N stretch, and the C=O stretch occurs at a larger wavenumber than the C-O stretch.

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Key Concepts

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

Wavenumber and Infrared Spectroscopy

Wavenumber is a measure of the frequency of a wave, typically expressed in cm⁻¹, and is commonly used in infrared (IR) spectroscopy to identify molecular vibrations. In IR spectroscopy, different types of chemical bonds absorb infrared light at characteristic wavenumbers, allowing for the identification of functional groups in a molecule.

Bond Strength and Stretching Frequencies

The strength of a chemical bond influences the frequency at which it vibrates. Stronger bonds, such as double bonds (C=N, C=O), generally stretch at higher wavenumbers compared to weaker bonds, like single bonds (C-N, C-O). This is due to the greater energy required to stretch stronger bonds, resulting in higher vibrational frequencies.

Functional Group Identification

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Identifying functional groups through their IR absorption peaks allows chemists to deduce the structure and reactivity of organic compounds, making it essential for understanding molecular behavior.

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Related Practice

Textbook Question

Which of the following compounds has a vibration that is infrared inactive?

1-butyne, 2-butyne, H₂, H₂O, Cl₂, ethene

Textbook Question

Assuming that the force constant is approximately the same for C–C, C–N, and C–O bonds, predict the relative positions of their stretching vibrations in an IR spectrum.

Textbook Question

Why is the C–O absorption band of 1-hexanol at a smaller wavenumber (1060 cm^–1) than the C–O absorption band of pentanoic acid (1220 cm^–1)?

Textbook Question

Which occurs at a larger wavenumber:

a. the C–O stretch of phenol or the C–O stretch of cyclohexanol?

b. the C=O stretch of a ketone or the C=O stretch of an amide?

c. the C–N stretch of cyclohexylamine or the C–N stretch of aniline?

Textbook Question

Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption bands:

Textbook Question

Calculate the reduced mass for the following bonds.

(a) C―H