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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4:58 minutes

Problem 17d

Textbook Question

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

Verified step by step guidance

Identify the compound provided in the problem. If the compound is not explicitly stated, assume it is a generic organic molecule. The goal is to determine the principal fragments formed during mass spectrometry analysis.

Recall that in mass spectrometry, the molecule is ionized (usually by electron impact), leading to the formation of a molecular ion (M⁺). This ion can then fragment into smaller ions and neutral species. Use curved arrows to represent the movement of electrons during bond cleavage.

Analyze the structure of the compound to identify the most likely bonds to break. Bonds adjacent to functional groups, or those that lead to the formation of stable carbocations (e.g., tertiary carbocations, allylic carbocations, or benzyl carbocations), are more likely to break.

Draw the molecular ion (M⁺) and use curved arrows to show the electron movement that leads to the formation of the first principal fragment. For example, if a bond breaks to form a carbocation and a neutral radical, indicate this with curved arrows starting from the bond and pointing toward the atoms involved.

Repeat the process for other possible fragmentations, focusing on those that produce stable ions. Label each fragment with its corresponding m/z value (mass-to-charge ratio) based on the molecular weights of the fragments. Ensure that the fragments are consistent with the rules of mass spectrometry.

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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 chemists to identify fragments produced during ionization.

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart during ionization in mass spectrometry. These patterns are influenced by the molecular structure and stability of the resulting fragments. Understanding common fragmentation pathways helps predict the principal fragments that will appear in the mass spectrum of a compound.

Curved Arrows in Mechanisms

Curved arrows are used in organic chemistry to illustrate the movement of electrons during chemical reactions, particularly in mechanisms. They indicate the direction of electron flow, helping to visualize how bonds are formed or broken. In the context of mass spectrometry, curved arrows can be used to depict the fragmentation process, showing how a molecule dissociates into smaller ions.

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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.

(b)

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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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

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

Key Concepts

Mass Spectrometry

Fragmentation Patterns

Curved Arrows in Mechanisms

Watch next

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