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 45b

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

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Identify the functional group in the compound. In 3-methylhexan-2-one, the functional group is a ketone (C=O) located at the second carbon.

Understand the McLafferty rearrangement mechanism. This involves a six-membered cyclic transition state where a hydrogen atom from the γ-carbon (the third carbon away from the carbonyl carbon) is transferred to the oxygen of the carbonyl group, leading to cleavage of the bond between the α-carbon (adjacent to the carbonyl) and the β-carbon.

Locate the γ-hydrogen in the molecule. In 3-methylhexan-2-one, the γ-carbon is the fourth carbon in the chain, and it has hydrogens available for the rearrangement.

Determine the fragments formed. The McLafferty rearrangement results in two fragments: (1) a neutral alkene formed from the γ-carbon and beyond, and (2) a charged fragment containing the carbonyl group and the α-carbon.

Calculate the mass of the charged fragment. Add the atomic masses of the atoms in the charged fragment (including the transferred hydrogen) to predict its mass. Use the molecular formula of the fragment to ensure accuracy.

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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 specific rearrangement to form charged fragments. This typically involves the transfer of a hydrogen atom and the cleavage of a bond, resulting in the formation of a double bond and a new charged species. It is particularly common in compounds with a carbonyl group, such as ketones and aldehydes.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. In this process, molecules are ionized, and the resulting ions are separated based on their mass. The data obtained allows for the identification of molecular structures and the prediction of fragmentation patterns, such as those seen in the McLafferty rearrangement.

Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart during mass spectrometry. These patterns are influenced by the molecular structure and functional groups present. Understanding these patterns is crucial for predicting the masses of the resulting fragments, as they provide insights into the stability and reactivity of the ions formed during the ionization process.

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The following compounds undergo McLafferty rearrangement in the mass spectrometer. Predict the masses of the resulting charged fragments. (b) 3-methylhexan-2-one

Key Concepts

McLafferty Rearrangement

Mass Spectrometry

Fragmentation Patterns

Watch next

The following compounds undergo McLafferty rearrangement in the mass spectrometer. Predict the masses of the resulting charged fragments.
(b) 3-methylhexan-2-one