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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3:49 minutes

Problem 13a,b

Textbook Question

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

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 or groups deshield the proton, causing a higher chemical shift (downfield signal).

Step 2: Analyze the first pair of protons in part (i). The proton attached to the carbon bonded to chlorine (Cl) is more deshielded compared to the proton attached to the carbon bonded to iodine (I). Chlorine is more electronegative than iodine, pulling electron density away from the proton more effectively.

Step 3: Compare the second pair of protons in part (ii). The proton attached to the carbon bonded to bromine (Br) is less deshielded compared to the proton in the aldehyde group. The aldehyde proton is highly deshielded due to the electron-withdrawing effect of the carbonyl group (C=O).

Step 4: Summarize the findings. In part (i), the proton near chlorine has the greater chemical shift. In part (ii), the aldehyde proton has the greater chemical shift due to the strong deshielding effect of the carbonyl group.

Step 5: Conclude that the chemical shift is determined by the electronegativity and electron-withdrawing effects of nearby atoms or functional groups. This principle can be applied to predict chemical shifts in other molecules.

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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 hydrogen atoms. Protons near electronegative atoms or in deshielded environments resonate at higher frequencies, resulting in greater chemical shifts.

Electronegativity and Shielding Effects

Electronegativity is the tendency of an atom to attract electrons towards itself. In NMR, protons attached to carbon atoms adjacent to electronegative atoms (like Cl, Br, or O) experience deshielding, which increases their chemical shift. This means that protons in such environments will resonate at higher frequencies compared to those in more shielded environments, such as those attached to carbon atoms in saturated hydrocarbons.

Integration and Multiplicity in NMR

Integration in NMR spectroscopy refers to the area under the peaks in the spectrum, which correlates to the number of protons contributing to that signal. Multiplicity indicates the splitting of NMR signals due to neighboring protons, following the n+1 rule, where n is the number of adjacent protons. Understanding these concepts helps in interpreting the NMR spectrum and identifying the environment of specific protons.

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Related Practice

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

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)

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