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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6:23 minutes

Problem 51

Textbook Question

The ¹H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at δ6.10, δ4.33, and δ7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pK_a.

Verified step by step guidance

Understand the relationship between chemical shift and electron density: In 1H NMR spectroscopy, the chemical shift is influenced by the electron density around the hydrogen atoms. A higher electron density results in shielding, leading to a lower chemical shift (downfield), while a lower electron density results in deshielding, leading to a higher chemical shift (upfield).

Analyze the compounds: Nitromethane (CH3NO2), dinitromethane (CH2(NO2)2), and trinitromethane (CH(NO2)3) have progressively more electron-withdrawing nitro (-NO2) groups attached to the central carbon. Nitro groups are highly electronegative and withdraw electron density from the hydrogen atoms, causing deshielding and increasing the chemical shift.

Match the chemical shifts: The compound with the highest number of nitro groups (trinitromethane) will have the most deshielded hydrogens and the highest chemical shift (δ 7.52). The compound with the fewest nitro groups (nitromethane) will have the least deshielded hydrogens and the lowest chemical shift (δ 6.10). Dinitromethane, with an intermediate number of nitro groups, will have a chemical shift between the two (δ 4.33).

Correlate chemical shift with pKa: The pKa of a compound is influenced by the electron-withdrawing ability of substituents. More nitro groups increase the acidity of the compound (lower pKa) by stabilizing the conjugate base through resonance and inductive effects. Thus, trinitromethane has the lowest pKa, followed by dinitromethane, and then nitromethane. This trend aligns with the chemical shift values, as more acidic compounds tend to have more deshielded hydrogens.

Summarize the matching: Based on the analysis, nitromethane corresponds to δ 6.10, dinitromethane corresponds to δ 4.33, and trinitromethane corresponds to δ 7.52. The chemical shift values reflect the increasing deshielding effect of the nitro groups and correlate with the decreasing pKa values of the compounds.

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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 is a key parameter in nuclear magnetic resonance (NMR) spectroscopy that indicates the resonance frequency of a nucleus relative to a standard reference. It is measured in parts per million (ppm) and reflects the electronic environment surrounding the nucleus. Different functional groups and substituents can cause variations in chemical shifts, allowing chemists to deduce structural information about the compound.

pKa and Acidity

pKa is a measure of the acidity of a compound, representing the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a stronger acid, meaning it more readily donates protons (H+). The relationship between pKa and chemical shift arises because electron-withdrawing groups, such as nitro groups, can stabilize the negative charge of the conjugate base, influencing the chemical environment and thus the NMR chemical shifts.

Electron-Withdrawing Effects

Electron-withdrawing groups (EWGs) are substituents that pull electron density away from the rest of the molecule, often through inductive or resonance effects. In the context of nitromethane and its derivatives, the presence of nitro groups significantly affects the electron density around the hydrogen atoms, leading to shifts in their NMR signals. This alteration in electron density correlates with changes in acidity, as stronger EWGs typically increase acidity by stabilizing the conjugate base.

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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)

Textbook Question

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

(b)

Textbook Question

Compound A with molecular formula C₆H₁₀ has two peaks in its ¹H NMR spectrum, both of which are singlets (with ratio 9 : 1). Compound A reacts with an acidic aqueous solution containing mercuric sulfate to form compound B, which gives a positive iodoform test (Problem 58) and has an 1H NMR spectrum that shows two singlets (with ratio 3 : 1). Identify A and B.

Textbook Question

Which set of underlined hydrogens has its ¹H NMR signal at a higher frequency?

a. CH₃CH₂CH₃ or CH₃OCH₂CH₃

b. CH₃CH=CH₂ or CH₃OCH=CH₂

Textbook Question

How would you expect the IR and ¹H NMR spectra for propanamide and N,N-diethylpropanamide to differ?

Textbook Question

The chemical shifts of the C-2 hydrogen in the spectra of pyrrole, pyridine, and pyrrolidine are 2.82 ppm, 6.42 ppm, and 8.50 ppm. Match each heterocycle with its chemical shift.

Textbook Question

[18]-Annulene shows two signals in its ¹H NMR spectrum: one at 9.25 ppm and the other to the right of the TMS signal at –2.88 ppm. What hydrogens are responsible for each of the signals? (Hint: Look at the direction of the induced magnetic field outside and inside the benzene ring in Figure 14.6.)

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The 1H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at δ6.10, δ4.33, and δ7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pKa.

Key Concepts

Chemical Shift in NMR Spectroscopy

pKa and Acidity

Electron-Withdrawing Effects

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The ¹H NMR chemical shifts of nitromethane, dinitromethane, and trinitromethane are at δ6.10, δ4.33, and δ7.52. Match each chemical shift with the compound. Explain how chemical shift correlates with pK_a.