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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3:28 minutes

Problem 3

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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Verified step by step guidance

Understand the McLafferty rearrangement: This is a fragmentation process observed in mass spectrometry, where a molecule undergoes a specific cleavage involving a γ-hydrogen (a hydrogen atom three carbons away from a carbonyl group) and the carbonyl group. This results in the formation of a neutral alkene and a charged fragment.

Analyze the structure of butan-2-one: The molecular structure of butan-2-one is CH₃-CO-CH₂-CH₃. Identify the γ-hydrogens relative to the carbonyl group (C=O).

Determine the presence of γ-hydrogens: In butan-2-one, the γ-hydrogens would be located on the terminal methyl groups (CH₃). However, for a McLafferty rearrangement to occur, there must be a γ-hydrogen that can participate in a six-membered cyclic transition state.

Evaluate the feasibility of the six-membered transition state: In butan-2-one, the γ-hydrogens are not positioned in a way that allows the formation of a six-membered cyclic transition state. This is because the molecule lacks the necessary spatial arrangement for the rearrangement to occur.

Conclude why no McLafferty products are observed: Since the structural requirements for the McLafferty rearrangement are not met in butan-2-one, no products from this rearrangement are observed in its mass spectrum.

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Key Concepts

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

McLafferty Rearrangement

The McLafferty rearrangement is a fragmentation process observed in mass spectrometry, where a molecule undergoes a rearrangement to form a more stable product. This typically involves the transfer of a hydrogen atom and the formation of a double bond, resulting in the cleavage of a carbon-carbon bond. It is most commonly seen in compounds with a gamma-hydrogen, which is crucial for the rearrangement to occur.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It provides information about the molecular weight and structure of compounds by ionizing chemical species and sorting the resulting ions based on their mass. The resulting spectrum displays peaks corresponding to different ions, allowing for the identification of molecular fragments and the analysis of complex mixtures.

Structural Features of Butan-2-one

Butan-2-one, a ketone, has a specific structure that lacks a gamma-hydrogen atom necessary for the McLafferty rearrangement to occur. The absence of this hydrogen means that the molecule cannot undergo the rearrangement process, leading to the absence of expected products in the mass spectrum. Understanding the structural features of organic compounds is essential for predicting their behavior in mass spectrometry.

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Related Practice

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.

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

Identify the compound that gives the mass spectrum and infrared spectrum shown here.

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Textbook Question

Identify the peaks in the mass spectrum of octan-4-one that correspond to (b) the McLafferty rearrangement.

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Textbook Question

2-Methylnonan-4-one can undergo two different McLafferty rearrangements. Draw the products of each of them.

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

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

Key Concepts

McLafferty Rearrangement

Mass Spectrometry

Structural Features of Butan-2-one

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