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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4:23 minutes

Problem 12c,d

Textbook Question

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

Verified step by step guidance

Step 1: Understand the concept of chemical shift in NMR spectroscopy. Chemical shift is influenced by the electronic environment around a proton. Electronegative atoms, pi bonds, and other factors can deshield protons, causing them to resonate at higher frequencies (downfield).

Step 2: Analyze the structure in image i. The red proton is adjacent to a chlorine atom, which is highly electronegative. This electronegativity deshields the red proton, increasing its chemical shift. The blue protons are further away from the chlorine atom and are less affected by its electronegativity, resulting in a lower chemical shift.

Step 3: Analyze the structure in image ii. The red proton is adjacent to a bromine atom, which is less electronegative than chlorine but still deshields the proton to some extent. Additionally, the red proton is closer to the carbonyl group (C=O), which is highly deshielding due to its electron-withdrawing nature. The blue protons are adjacent to a hydroxyl group (-OH), which also has an electron-withdrawing effect but is less deshielding compared to the carbonyl group.

Step 4: Compare the chemical shifts of the red and blue protons in both structures. In image i, the red proton has a higher chemical shift due to the strong deshielding effect of the chlorine atom. In image ii, the red proton has a higher chemical shift due to the combined deshielding effects of the bromine atom and the carbonyl group.

Step 5: Conclude that in both cases (c and d), the red proton has the greater chemical shift compared to the blue protons due to its proximity to electronegative atoms and electron-withdrawing groups.

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Key Concepts

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

Chemical Shift in NMR Spectroscopy

Chemical shift refers to the resonance frequency of a nucleus relative to a standard in a magnetic field, typically measured in parts per million (ppm). In proton NMR, the chemical shift is influenced by the electronic environment surrounding the protons, with electronegative atoms or groups causing downfield shifts (higher ppm) due to deshielding effects.

Deshielding and Shielding Effects

Shielding occurs when surrounding electrons reduce the magnetic field experienced by a nucleus, resulting in a lower chemical shift. Conversely, deshielding happens when electronegative atoms withdraw electron density from protons, increasing their chemical shift. Protons near electronegative atoms, such as halogens or oxygen, will typically resonate at higher frequencies.

Comparative Analysis of Proton Environments

In NMR analysis, comparing the environments of protons in different molecular structures is crucial for determining their chemical shifts. Protons in similar environments will have similar shifts, while those influenced by different electronegative groups or steric effects will show significant differences. This comparative analysis helps identify which protons resonate at higher frequencies.

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Related Practice

Textbook Question

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

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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

The methyl hydrogens in propane appear at a chemical shift of 0.9 ppm, whereas the methyl hydrogens of propene appear around 2.5 ppm. Explain.

Textbook Question

Alkene hydrogens usually appear at similar chemical shifts between 5 and 6 ppm. The alkene hydrogens in the structure shown are very different. Rationalize the difference in chemical shifts between Hₐ and H₆ in the structure shown.

Textbook Question

For the following molecules, give the chemical shift for each indicated hydrogen.

(d)

Textbook Question

For the following molecules, give the chemical shift for each indicated hydrogen.

(e)

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

Key Concepts

Chemical Shift in NMR Spectroscopy

Deshielding and Shielding Effects

Comparative Analysis of Proton Environments

Watch next

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