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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6:15 minutes

Problem 41

Textbook Question

In the mass spectrum of the following compounds, which is the tallest—the peak at m/z = 57 or the peak at m/z = 71?
a. 3-methylpentane
b. 2-methylpentane

Verified step by step guidance

Step 1: Understand the problem. The question is asking about the relative intensities of peaks at m/z = 57 and m/z = 71 in the mass spectra of two isomers: 3-methylpentane and 2-methylpentane. The tallest peak corresponds to the most stable fragment ion formed during fragmentation in the mass spectrometer.

Step 2: Recall the concept of fragmentation in mass spectrometry. When a molecule is ionized, it can break into smaller fragments. The stability of these fragments determines the intensity of the corresponding peaks in the mass spectrum. More stable fragments produce taller peaks.

Step 3: Analyze the fragmentation of 3-methylpentane. For m/z = 57, the fragment corresponds to a tert-butyl cation (C₄H₉⁺), which is highly stable due to hyperconjugation and inductive effects. For m/z = 71, the fragment corresponds to a primary carbocation (C₅H₁₁⁺), which is less stable.

Step 4: Analyze the fragmentation of 2-methylpentane. For m/z = 57, the fragment corresponds to a secondary carbocation (C₄H₉⁺), which is moderately stable. For m/z = 71, the fragment corresponds to a secondary carbocation (C₅H₁₁⁺), which is also moderately stable.

Step 5: Compare the stability of the fragments. In 3-methylpentane, the peak at m/z = 57 is taller because the tert-butyl cation is more stable than the primary carbocation at m/z = 71. In 2-methylpentane, the peaks at m/z = 57 and m/z = 71 are closer in intensity because both correspond to secondary carbocations with similar stability. Thus, the tallest peak depends on the compound being analyzed.

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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 helps identify the composition of a sample by generating a mass spectrum, which displays peaks corresponding to different ions. The height of each peak indicates the relative abundance of the corresponding ion, allowing for the comparison of different compounds based on their fragmentation patterns.

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart into smaller ions during mass spectrometry. Different compounds will produce characteristic fragments based on their structure, which can be predicted using knowledge of organic chemistry. Understanding these patterns is crucial for interpreting mass spectra and determining which peaks correspond to which compounds.

Isomeric Compounds

Isomeric compounds, such as 3-methylpentane and 2-methylpentane, have the same molecular formula but different structural arrangements. This structural difference can lead to variations in their fragmentation patterns and, consequently, their mass spectra. Recognizing how isomers can affect peak heights in a mass spectrum is essential for accurately determining which compound produces the tallest peak at a given m/z value.

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