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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3:48 minutes

Problem 58b

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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Verified step by step guidance

Analyze the molecular weight of the compound (102) and the IR spectrum data. The broad, strong absorption at 3600 cm⁻¹ suggests the presence of an -OH group (alcohol or phenol). The medium absorption at 1360 cm⁻¹ indicates possible C-H bending vibrations, often associated with methyl groups.

Identify the base structure of the compound. Given the molecular weight and functional group information, hypothesize a structure that includes an alcohol group (-OH) and possibly a methyl group.

Determine the fragment responsible for the m/z = 84 peak. Subtract the mass of the fragment lost from the molecular ion (102 - 84 = 18). A mass of 18 corresponds to the loss of a water molecule (H₂O), which is common in alcohol-containing compounds.

Propose a mechanism for the formation of the m/z = 84 peak. The loss of H₂O can occur via a dehydration reaction. This involves the -OH group and a hydrogen atom from a neighboring carbon atom, forming a double bond in the process. Use curved arrows to show the movement of electrons: (1) the lone pair on the oxygen of the -OH group abstracts a proton, (2) the bond between the oxygen and carbon breaks, and (3) a double bond forms between adjacent carbons.

Verify the stability of the resulting fragment (m/z = 84). The fragment should be a stable carbocation or neutral molecule, depending on the specific structure of the compound. Ensure that the proposed mechanism aligns with the observed mass spectrum and IR data.

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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 molecular weight of compounds and their fragmentation patterns. In this context, the peak at m/z = 84 indicates a specific fragment of the original molecule, which can provide insights into its structure and functional groups.

Infrared (IR) Spectroscopy

Infrared spectroscopy is a technique that measures the absorption of infrared light by a compound, providing information about its molecular vibrations and functional groups. The broad absorption at 3600 cm–1 suggests the presence of an -OH group, while the medium absorption at 1360 cm–1 may indicate a C-H bending vibration, which can help deduce the compound's structure.

Fragmentation Mechanism

Fragmentation mechanisms describe how a molecule breaks apart during mass spectrometry, leading to the formation of smaller ions. Understanding these mechanisms is crucial for interpreting mass spectra, as they reveal how specific fragments, like the one at m/z = 84, are generated from the parent compound, often involving the cleavage of certain bonds or the loss of functional groups.

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