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

Mass Spect:Fragmentation

Organic Chemistry

15. Analytical Techniques:IR, NMR, Mass Spect

Mass Spect:Fragmentation

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Problem 34

Textbook Question

The molecular ion (M) is significant in the mass spectrum of benzyl fluoride, but it is barely present in the mass spectrum of benzyl iodide. Why might this be the case?
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Verified step by step guidance

Consider the concept of the molecular ion (M) in mass spectrometry, which is the ion formed by the removal of an electron from a molecule, resulting in a radical cation. This ion is significant because it represents the molecular weight of the compound.

Understand that the stability of the molecular ion is crucial for its presence in the mass spectrum. More stable ions are more likely to be detected.

Analyze the bond strengths in benzyl fluoride and benzyl iodide. The C-F bond in benzyl fluoride is much stronger than the C-I bond in benzyl iodide. This difference in bond strength affects the stability of the molecular ion.

Recognize that the weaker C-I bond in benzyl iodide makes the molecular ion less stable and more prone to fragmentation, leading to its reduced presence in the mass spectrum.

Conclude that the stronger C-F bond in benzyl fluoride contributes to a more stable molecular ion, which is why it is more prominent in the mass spectrum compared to benzyl iodide.

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Key Concepts

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

Molecular Ion Formation

In mass spectrometry, the molecular ion (M) is formed when a molecule loses an electron, resulting in a positively charged ion. This ion represents the entire molecule and is crucial for determining the molecular weight. The stability of the molecular ion can vary depending on the structure and the elements present in the molecule.

Bond Strength and Stability

The bond strength between carbon and halogens (like fluorine and iodine) affects the stability of the molecular ion. Carbon-fluorine bonds are stronger than carbon-iodine bonds, making the molecular ion of benzyl fluoride more stable and detectable in mass spectrometry, whereas the weaker carbon-iodine bond in benzyl iodide leads to rapid fragmentation.

Fragmentation Patterns

Fragmentation in mass spectrometry occurs when the molecular ion breaks into smaller ions. The tendency to fragment depends on the stability of the molecular ion and the bonds within the molecule. Benzyl iodide's molecular ion is less stable due to the weak C-I bond, leading to significant fragmentation and a reduced presence of the molecular ion peak in its mass spectrum.

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Related Practice

Textbook Question

For each of the molecules shown, predict the structure of at least one major fragment in the mass spectrum.

(d)

Textbook Question

For each of the molecules shown, predict the structure of at least one major fragment in the mass spectrum.

(e)

Textbook Question

Identify the peaks in the mass spectrum of octan-4-one that correspond to (a) α-cleavage.

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Textbook Question

For each of the molecules shown, predict the structure of at least one major fragment in the mass spectrum.

(b)

Textbook Question

The mass spectrum of tert-butylamine follows shows an intense base peak at m/z 58, and very little else. Use a diagram to show the cleavage that accounts for the base peak. Suggest why no molecular ion is visible in this spectrum.

Textbook Question

The mass spectrum for a compound with molecular weight of 102 is shown below. Its IR spectrum has a broad, strong absorption at 3600 cm^–1 and a medium absorption at 1360 cm^–1.

a. Identify the compound.

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Textbook Question

Using curved arrows, show the principal fragments you would expect to see in the mass spectrum of each of the following compounds:

Textbook Question

The following compounds undergo McLafferty rearrangement in the mass spectrometer. Predict the masses of the resulting charged fragments.

(b) 3-methylhexan-2-one

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The molecular ion (M) is significant in the mass spectrum of benzyl fluoride, but it is barely present in the mass spectrum of benzyl iodide. Why might this be the case?<IMAGE>

Key Concepts

Molecular Ion Formation

Bond Strength and Stability

Fragmentation Patterns

Watch next

The molecular ion (M) is significant in the mass spectrum of benzyl fluoride, but it is barely present in the mass spectrum of benzyl iodide. Why might this be the case?
<IMAGE>