Textbook Question
Besides the molecular mass, how do you know that the fragment that gives rise to the peak at m/z 111 in the mass spectrum for 1-bromo-4-chlorobenzene contains only chlorine?
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Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 34
Mullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 34Chapter 13, Problem 34
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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Consider the concept of the molecular ion (M) in mass spectrometry, which is the ion formed by the removal of an electron from a molecule, resulting in a radical cation. This ion is significant because it represents the molecular weight of the compound.
Understand that the stability of the molecular ion is crucial for its presence in the mass spectrum. More stable ions are more likely to be detected.
Analyze the bond strengths in benzyl fluoride and benzyl iodide. The C-F bond in benzyl fluoride is much stronger than the C-I bond in benzyl iodide. This difference in bond strength affects the stability of the molecular ion.
Recognize that the weaker C-I bond in benzyl iodide makes the molecular ion less stable and more prone to fragmentation, leading to its reduced presence in the mass spectrum.
Conclude that the stronger C-F bond in benzyl fluoride contributes to a more stable molecular ion, which is why it is more prominent in the mass spectrum compared to benzyl iodide.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Molecular Ion Formation
In mass spectrometry, the molecular ion (M) is formed when a molecule loses an electron, resulting in a positively charged ion. This ion represents the entire molecule and is crucial for determining the molecular weight. The stability of the molecular ion can vary depending on the structure and the elements present in the molecule.
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Bond Strength and Stability
The bond strength between carbon and halogens (like fluorine and iodine) affects the stability of the molecular ion. Carbon-fluorine bonds are stronger than carbon-iodine bonds, making the molecular ion of benzyl fluoride more stable and detectable in mass spectrometry, whereas the weaker carbon-iodine bond in benzyl iodide leads to rapid fragmentation.
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Fragmentation Patterns
Fragmentation in mass spectrometry occurs when the molecular ion breaks into smaller ions. The tendency to fragment depends on the stability of the molecular ion and the bonds within the molecule. Benzyl iodide's molecular ion is less stable due to the weak C-I bond, leading to significant fragmentation and a reduced presence of the molecular ion peak in its mass spectrum.
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Related Practice
Textbook Question
Suggest three ways that the following tertiary amine can fragment by α-cleavage. At which m/z would you expect to see peaks corresponding to these fragments?
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Textbook Question
The molecule that gave the mass spectrum shown here contains a halogen. Which halogen is present?
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Textbook Question
In the mass spectrum of hexan-3-ol, identify the fragments arising from two α-cleavage fragmentations and one dehydration fragmentation.
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Textbook Question
Match each of the four IR spectra to one of the given compounds. [One of the compounds does not match a spectrum.]
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Textbook Question
The presence of what atom is indicated by the following mass spectrum?
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