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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6:40 minutes

Problem 29

Textbook Question

The ultimate test of fluency in MS and IR is whether you can determine a moderately complex structure from just the MS and the IR, with no additional information. The IR and MS of a compound are shown below. Use everything you know about IR and MS, plus reasoning and intuition, to determine a likely structure. Then show how your proposed structure is consistent with these spectra.
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Verified step by step guidance

Examine the mass spectrum provided. Identify the molecular ion peak (M+) which represents the molecular weight of the compound. In this case, the peak at m/z = 107 is likely the molecular ion peak.

Look for significant fragment peaks in the mass spectrum. For example, peaks at m/z = 77 and 79 suggest the presence of a benzene ring or aromatic structure due to common fragmentation patterns.

Analyze the IR spectrum (not shown here). Identify key functional group absorptions, such as C=O stretching (around 1700 cm⁻¹), O-H stretching (broad peak around 3200-3600 cm⁻¹), or C-H stretching (around 2800-3000 cm⁻¹). These will help narrow down the functional groups present in the compound.

Combine the information from the mass spectrum and IR spectrum. For example, if the IR spectrum shows a strong C=O stretch, the compound may contain a ketone or aldehyde. If an O-H stretch is present, it may be an alcohol or carboxylic acid.

Propose a structure consistent with the molecular weight (107), fragmentation patterns (aromatic ring), and functional groups identified in the IR spectrum. Ensure the structure accounts for all observed peaks and spectral data.

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Key Concepts

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

Mass Spectrometry (MS)

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It provides information about the molecular weight of compounds and their structural features by generating a mass spectrum, which displays the relative abundance of ions at different m/z (mass-to-charge) values. Key peaks in the spectrum correspond to the molecular ion and fragment ions, which can help deduce the structure of the compound.

Infrared Spectroscopy (IR)

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 resulting IR spectrum displays peaks corresponding to specific bond vibrations, allowing chemists to identify functional groups present in the molecule. This information is crucial for confirming the presence of certain structures and guiding the interpretation of the mass spectrum.

Structural Elucidation

Structural elucidation involves determining the molecular structure of a compound based on analytical data from techniques like MS and IR. By analyzing the mass spectrum for molecular weight and fragmentation patterns, alongside the IR spectrum for functional group identification, chemists can propose a likely structure. This process requires integrating data from both techniques to ensure consistency and accuracy in the proposed molecular structure.

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Related Practice

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

Textbook Question

An acid-catalyzed reaction was carried out using methyl cellosolve (2-methoxyethanol) as the solvent. When the 2-methoxyethanol was redistilled, a higher-boiling fraction (bp 162°C) was also recovered. The mass spectrum of this fraction showed the molecular weight to be 134. The IR and NMR spectra are shown here. Determine the structure of this compound, and propose a mechanism for its formation.

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Textbook Question

The following spectra for A and B correspond to two structural isomers. The NMR singlet at δ1.16 in spectrum A disappears when the sample is shaken with D₂O. The singlet at δ0.6 ppm in the spectrum of B disappears on shaking with D₂O. Propose structures for these isomers, and show how your structures correspond to the spectra. Show what cleavage is responsible for the base peak at m/z 44 in the mass spectrum of A and the prominent peak at m/z 58 in the mass spectrum of B.

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Textbook Question

An unknown compound gives a mass spectrum with a weak molecular ion at m/z 113 and a prominent ion at m/z 68. Its NMR and IR spectra are shown here. Determine the structure, and show how it is consistent with the observed absorptions. Propose a favorable fragmentation to explain the prominent MS peak at m/z 68.

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Textbook Question

For each NMR spectrum, propose a structure consistent with the spectrum and the additional information provided.

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a. Elemental analysis shows the molecular formula to be C₈H₇OCl. The IR spectrum shows a moderate absorption at 1602 cm^–1 and a strong absorption at 1690 cm^–1.

b. The mass spectrum shows a double molecular ion of ratio 1:1 at m/z 184 and 186.

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Textbook Question

An unknown compound gives the following mass, IR, and NMR spectra. Propose a structure, and show how it is consistent with the spectra. Show the fragmentations that give the prominent peaks at m/z 127 and 155 in the mass spectrum.

Textbook Question

A student added 3-phenylpropanoic acid (PhCH₂CH₂COOH) to a molten salt consisting of a 1:1 mixture of NaCl and AlCl₃ maintained at 170 °C. After 5 minutes, he poured the molten mixture into water and extracted it into dichloromethane. Evaporation of the dichloromethane gave a 96% yield of the product whose spectra follow. The mass spectrum of the product shows a molecular ion at m/z 132. What is the product?

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Textbook Question

Why were no products from the McLafferty rearrangement observed in the spectrum of butan-2-one (Figure 18-3)?

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