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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4:05 minutes

Problem 37

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.

Verified step by step guidance

The chemical shift in NMR spectroscopy is influenced by the electronic environment around a nucleus. In this case, we are comparing the OH proton of a carboxylic acid and the OH proton of an alcohol.

In a carboxylic acid, the OH group is directly attached to a carbonyl group (C=O). The carbonyl group is highly electronegative due to the oxygen atom, which withdraws electron density from the OH group through both inductive and resonance effects.

The inductive effect occurs because the electronegative oxygen atom in the carbonyl group pulls electron density away from the OH proton, deshielding it. This deshielding causes the OH proton to resonate at a higher frequency (downfield) in the NMR spectrum.

Additionally, the resonance effect in a carboxylic acid allows the lone pair of electrons on the OH oxygen to delocalize into the carbonyl group. This further enhances the deshielding of the OH proton, contributing to its higher chemical shift.

In contrast, in an alcohol, the OH group is not adjacent to a carbonyl group. The absence of strong electron-withdrawing effects means the OH proton in an alcohol is less deshielded and resonates at a lower frequency (upfield) compared to the OH proton in a carboxylic acid.

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Key Concepts

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

Chemical Shift

Chemical shift refers to the resonant frequency of a nucleus relative to a standard in a magnetic field, typically measured in parts per million (ppm). It provides insight into the electronic environment surrounding the nucleus, influenced by factors such as electronegativity and hybridization. In NMR spectroscopy, different functional groups exhibit distinct chemical shifts, allowing for the identification of molecular structures.

Hydrogen Bonding

Hydrogen bonding is a type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom. In carboxylic acids, the presence of both a hydroxyl (-OH) and a carbonyl (C=O) group allows for strong intramolecular hydrogen bonding, which stabilizes the structure and affects the chemical environment of the OH proton, leading to a higher chemical shift compared to alcohols.

Electronegativity and Inductive Effect

Electronegativity is the tendency of an atom to attract electrons towards itself, influencing the distribution of electron density in a molecule. In carboxylic acids, the electronegative oxygen atoms in the carbonyl group exert an inductive effect, pulling electron density away from the hydroxyl proton. This results in a deshielding effect, causing the OH proton in carboxylic acids to resonate at a higher frequency than that in alcohols, where such strong electron-withdrawing effects are absent.

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Related Practice

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

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

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)?

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.

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