Textbook Question
Which of the bonds shown in red are expected to have IR-active stretching frequencies?
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Ch. 12 - Infrared Spectroscopy and Mass Spectrometry
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 12, Problem 7
Identify which of these four mass spectra indicate the presence of sulfur, chlorine, bromine, iodine, or nitrogen. Suggest a molecular formula for each.
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Verified step by step guidance1
Examine the mass spectrum provided. Identify the molecular ion peak (M+) which represents the molecular weight of the compound. This is typically the highest m/z value with significant intensity.
Look for isotopic patterns that are characteristic of specific elements. For example, chlorine and bromine exhibit distinct isotopic patterns due to their natural abundance (e.g., Cl-35 and Cl-37, Br-79 and Br-81).
Determine if the spectrum shows evidence of sulfur. Sulfur has a minor isotope (S-34) that can create a small peak at +2 m/z relative to the main molecular ion peak.
Check for nitrogen by applying the nitrogen rule. If the molecular ion peak has an odd m/z value, the compound likely contains an odd number of nitrogen atoms.
Suggest a molecular formula based on the molecular ion peak and any isotopic patterns observed. Use the molecular weight and elemental clues to propose a plausible formula.
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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 provides information about the molecular weight and structure of compounds by generating a mass spectrum, which displays the abundance of detected ions as a function of their mass-to-charge ratio (m/z). This technique is crucial for identifying the presence of specific elements in a sample based on their characteristic mass peaks.
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01:25
How to Read a Mass Spectrum
Isotopic Patterns
Isotopic patterns refer to the distribution of isotopes of an element in a mass spectrum. Elements like chlorine, bromine, and iodine have distinct isotopic signatures that can be identified by their characteristic peak ratios. For example, chlorine typically shows a 3:1 ratio of peaks corresponding to its isotopes (35Cl and 37Cl), while bromine shows a 1:1 ratio for its isotopes (79Br and 81Br), aiding in the identification of these elements in a compound.
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Molecular Formula Determination
Determining a molecular formula involves analyzing the mass spectrum to deduce the elemental composition of a compound. By identifying the m/z values of the significant peaks and their corresponding isotopes, one can infer the presence of specific elements such as sulfur, chlorine, bromine, iodine, or nitrogen. This process often includes calculating the ratio of the detected ions to propose a plausible molecular formula that matches the observed data.
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Related Practice
Textbook Question
Ethers are not easily differentiated by their infrared spectra, but they tend to form predictable fragments in the mass spectrum. The following compounds give similar but distinctive mass spectra.
Both compounds give prominent peaks at m/z 116, 73, 57, and 43. But one compound gives a distinctive strong peak at 87, and the other compound gives a strong peak at 101. Determine which compound gives the peak at 87 and which one gives the peak at 101. Propose fragmentations to account for the ions at m/z 116, 101, 87, and 73.
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Textbook Question
Show the fragmentations that give rise to the peaks at m/z 43, 57, and 85 in the mass spectrum of 2,4-dimethylpentane (Figure 12-17).
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Textbook Question
The infrared spectra for three compounds are provided. Each compound has one or more of the following functional groups: conjugated ketone, ester, amide, nitrile, and alkyne. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm–1.
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Textbook Question
Account for the peaks at m/z 87, 111, and 126 in the mass spectrum of 2,6-dimethylheptan-4-ol.
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Textbook Question
Spectra are given for three compounds. Each compound has one or more of the following functional groups: alcohol, amine, ketone, aldehyde, and carboxylic acid. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm–1.
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