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

Carbon NMR

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Carbon NMR

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5:58 minutes

Problem 54

Textbook Question

How many signals would you expect to see in the 13C NMR of the following compounds? In each case, show which carbon atoms are equivalent in the ¹³C NMR.

Verified step by step guidance

Step 1: Analyze the symmetry of each structure to determine which carbon atoms are equivalent. Equivalent carbons will produce the same signal in the 13C NMR spectrum.

Step 2: For structure (i), cyclohexane, all six carbon atoms are equivalent due to the high symmetry of the molecule. This results in one signal in the 13C NMR spectrum.

Step 3: For structure (ii), thiacyclohexane, the sulfur atom breaks the symmetry of the ring. The carbons adjacent to the sulfur are equivalent to each other, and the remaining carbons are equivalent in pairs. This results in three distinct signals in the 13C NMR spectrum.

Step 4: For structure (iii), cyclohexanone, the carbonyl group breaks the symmetry of the ring. The carbonyl carbon is unique, and the carbons adjacent to the carbonyl are equivalent to each other. The remaining carbons are equivalent in pairs. This results in four distinct signals in the 13C NMR spectrum.

Step 5: For structure (iv), cyclohexane-1,4-dione, the two carbonyl groups further reduce the symmetry. The carbonyl carbons are equivalent to each other, and the carbons adjacent to each carbonyl are equivalent in pairs. The remaining carbons are also equivalent in pairs. This results in three distinct signals in the 13C NMR spectrum.

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Key Concepts

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

Carbon NMR Spectroscopy

Carbon-13 NMR (Nuclear Magnetic Resonance) spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic environment of carbon atoms. Each unique carbon environment in a molecule produces a distinct signal in the NMR spectrum, allowing chemists to infer the number of different carbon types present.

Chemical Equivalence

In NMR spectroscopy, chemically equivalent atoms are those that are in identical environments and thus produce the same signal. For carbon atoms, this means they are bonded to the same atoms and have the same electronic environment. Identifying equivalent carbons is crucial for predicting the number of signals in the NMR spectrum.

Functional Groups and Their Effects

Functional groups, such as carbonyls and hydroxyls, significantly influence the chemical environment of adjacent carbon atoms, affecting their equivalence. For example, the presence of a carbonyl group can deshield nearby carbons, leading to shifts in their NMR signals. Understanding how functional groups interact with carbon atoms is essential for accurate NMR analysis.

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Related Practice

Textbook Question

The standard ¹³C NMR spectrum of phenyl propanoate is shown here. Predict the appearance of the DEPT-90 and DEPT-135 spectra.

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Textbook Question

Different types of protons and carbons in alkanes tend to absorb at similar chemical shifts, making structure determination difficult. Explain how the ¹³C NMR spectrum, including the DEPT technique, would allow you to distinguish among the following four isomers.

Multiple Choice

How many signals would be expected in the ¹³C NMR spectrum for the following molecule?

Multiple Choice

An alkene/aromatic signal will often appear ~ 160–170 ppm in a ¹³C NMR spectrum. Why?

Textbook Question

(a) Show which carbon atoms correspond with which peaks in the 13C NMR spectrum of butan-2-one (Figure 13-45).

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Textbook Question

Hexamethylbenzene undergoes free-radical chlorination to give one monochlorinated product (C₁₂H₁₇Cl) and four dichlorinated products (C₁₂H₁₆Cl₂). These products are easily separated by GC-MS, but the dichlorinated products are difficult to distinguish by their mass spectra. Draw the monochlorinated product and the four dichlorinated products, and explain how ¹³C NMR would easily distinguish among these compounds.

Textbook Question

(a) Draw all six isomers of formula C₄H₈ (including stereoisomers).

(b) For each structure, show how many types of H would appear in the proton NMR spectrum.

(d) If an unknown compound of formula C₄H₈ shows two types of H and three types of C, can you determine its structure from this information?

Textbook Question

The three isomers of dimethylbenzene are commonly named ortho-xylene, meta-xylene, and para-xylene. These three isomers are difficult to distinguish using proton NMR, but they are instantly identifiable using ¹³C NMR.

(a) Describe how carbon NMR distinguishes these three isomers.

(b) Explain why they are difficult to distinguish using proton NMR.

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How many signals would you expect to see in the 13C NMR of the following compounds? In each case, show which carbon atoms are equivalent in the 13C NMR.

Key Concepts

Carbon NMR Spectroscopy

Chemical Equivalence

Functional Groups and Their Effects

Watch next

How many signals would you expect to see in the 13C NMR of the following compounds? In each case, show which carbon atoms are equivalent in the ¹³C NMR.