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

H NMR Table

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

H NMR Table

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7:50 minutes

Problem 24

Textbook Question

Explain the relative chemical shifts of the benzene ring protons in Figure 14.18.
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Identify the structure in the image: The molecule is a substituted benzene ring with an -OH group attached to one of the carbons. The protons on the benzene ring are labeled as 'a', 'b', and 'c'.

Understand the concept of chemical shifts: In NMR spectroscopy, chemical shifts are influenced by the electronic environment around the protons. Electron-withdrawing or electron-donating groups attached to the benzene ring affect the deshielding or shielding of nearby protons.

Analyze the effect of the -OH group: The -OH group is an electron-donating group through resonance. It increases electron density on the benzene ring, particularly at the ortho and para positions relative to the -OH group. This affects the chemical shifts of the protons labeled 'a', 'b', and 'c'.

Explain the relative shifts: Protons 'a' are ortho to the -OH group and experience increased electron density due to resonance, making them more shielded and causing their chemical shift to be upfield (lower ppm). Protons 'b' are meta to the -OH group and are less affected by the electron-donating effect, resulting in a chemical shift that is slightly downfield compared to 'a'. Proton 'c' is para to the -OH group and experiences moderate shielding due to resonance, leading to a chemical shift that is intermediate between 'a' and 'b'.

Summarize the reasoning: The relative chemical shifts of the benzene ring protons are determined by their positions relative to the electron-donating -OH group. Protons 'a' are the most shielded (upfield), followed by 'c', and then 'b', which is the least shielded (downfield).

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Key Concepts

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

Chemical Shifts in NMR Spectroscopy

Chemical shifts in NMR spectroscopy refer to the resonance frequency of a nucleus relative to a standard in a magnetic field. They are influenced by the electronic environment surrounding the nucleus, which can be affected by factors such as electronegativity, hybridization, and molecular symmetry. In aromatic compounds like benzene, protons experience different shifts due to the delocalized π-electrons, leading to distinct signals for each type of hydrogen.

Aromaticity and Its Effects

Aromaticity is a property of cyclic compounds that have a planar structure, complete delocalization of π-electrons, and follow Huckel's rule (4n + 2 π-electrons). This delocalization stabilizes the molecule and affects the chemical shifts of protons in the benzene ring. The protons on the benzene ring will resonate at different frequencies based on their positions relative to substituents and the overall electron density in the ring.

Substituent Effects on Proton Shifts

The presence of substituents on a benzene ring can significantly influence the chemical shifts of the protons. Electron-withdrawing groups (EWGs) can deshield protons, causing them to resonate at lower fields (higher ppm), while electron-donating groups (EDGs) can shield protons, resulting in higher field shifts (lower ppm). Understanding these effects is crucial for interpreting NMR spectra and predicting the relative positions of signals for different protons in substituted benzene derivatives.

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Related Practice

Textbook Question

In a 300-MHz spectrometer, the protons in iodomethane absorb at a position 650 Hz downfield from TMS.

(a) What is the chemical shift of these protons?

(b) What is the chemical shift of the iodomethane protons in a 60-MHz spectrometer?

Textbook Question

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on.

Textbook Question

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on.

Textbook Question

Label each set of chemically equivalent protons, using a for the set that will be at the lowest frequency in the ¹H NMR spectrum, b for the next lowest, and so on. Indicate the multiplicity of each signal.

Textbook Question

Without referring to Table 14.1, label the proton or set of protons in each compound that gives the signal at the lowest frequency a, at the next lowest b, and so on.

c. ClCH₂CH₂CH₂Cl

Textbook Question

Explain why the chemical shift of the OH proton of a carboxylic acid is at a higher frequency than the chemical shift of an OH proton of an alcohol.

Textbook Question

Which underlined proton (or sets of protons) has the greater chemical shift (that is, the higher frequency signal)?

Textbook Question

Which underlined proton (or sets of protons) has the greater chemical shift (that is, the higher frequency signal)?

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Explain the relative chemical shifts of the benzene ring protons in Figure 14.18.<IMAGE>

Key Concepts

Chemical Shifts in NMR Spectroscopy

Aromaticity and Its Effects

Substituent Effects on Proton Shifts

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Explain the relative chemical shifts of the benzene ring protons in Figure 14.18.
<IMAGE>