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 13a

Textbook Question

The mass spectra of 1-methoxybutane, 2-methoxybutane, and 2-methoxy-2-methylpropane are shown below. Match each compound with its spectrum.
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Analyze the molecular structure of each compound (1-methoxybutane, 2-methoxybutane, and 2-methoxy-2-methylpropane) to identify key features that could influence fragmentation patterns in mass spectrometry. For example, consider the position of the methoxy group (-OCH₃) and the branching of the carbon chain.

Understand the general principles of mass spectrometry: molecules are ionized, and the resulting ions fragment in predictable ways. Look for characteristic peaks in the spectra, such as the molecular ion peak (M⁺), base peak (most intense peak), and other significant fragment peaks.

For 1-methoxybutane, predict the fragmentation pattern. The methoxy group is attached to the first carbon, so cleavage near this group may produce a fragment with a mass corresponding to CH₃O⁺ (31 m/z) and another fragment corresponding to the remaining alkyl chain.

For 2-methoxybutane, the methoxy group is attached to the second carbon. This structure may lead to fragmentation that produces a peak for CH₃O⁺ (31 m/z) and other fragments resulting from cleavage at the adjacent carbons, such as a peak for a secondary carbocation.

For 2-methoxy-2-methylpropane, the highly branched structure and the methoxy group on a tertiary carbon will influence fragmentation. Expect a strong molecular ion peak (M⁺) and significant peaks for tertiary carbocations or other stable fragments. Match these predicted patterns to the spectra to identify each compound.

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Key Concepts

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

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It involves ionizing chemical species and sorting the ions based on their mass. The resulting mass spectrum provides information about the molecular weight and structure of the compounds, allowing for the identification of different substances based on their fragmentation patterns.

Isomerism

Isomerism refers to the phenomenon where compounds have the same molecular formula but different structural arrangements or spatial orientations. In the case of 1-methoxybutane, 2-methoxybutane, and 2-methoxy-2-methylpropane, understanding their structural differences is crucial for interpreting their mass spectra, as these variations can lead to distinct fragmentation patterns.

Fragmentation Patterns

Fragmentation patterns are the specific ways in which a molecule breaks apart into smaller ions during mass spectrometry. Each compound has a unique fragmentation pattern based on its structure, which can be analyzed to deduce the original molecule. Recognizing these patterns is essential for matching the compounds to their respective mass spectra in the given question.

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a. What fragment is responsible for this peak?

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

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The reaction of (Z)-2-pentene with water and a trace of H₂SO₄ forms two products. Identify the products from their mass spectra.

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A mass spectrum shows significant peaks at m/z = 87, 115, 140, and 143. Which of the following compounds is responsible for that mass 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.

b. Show the mechanism for formation of the peak at m/z = 84.

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Propose a fragmentation to account for each numbered peak in the mass spectrum of n-butyl isopropyl ether.

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