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 5
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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Verified step by step guidance1
Examine the infrared spectra for each compound and identify the major peaks above 1600 cm-1. Infrared spectroscopy is used to identify functional groups based on characteristic absorption frequencies.
Look for a peak in the range of 1650–1700 cm-1. This range typically corresponds to the C=O stretching vibration of a conjugated ketone or an amide. Conjugation lowers the frequency compared to a non-conjugated ketone.
Check for a peak in the range of 1735–1750 cm-1. This range is characteristic of the C=O stretching vibration in esters. Esters generally have higher frequencies compared to ketones due to the electron-withdrawing effect of the oxygen atom.
Identify peaks in the range of 2100–2260 cm-1. This range is indicative of a nitrile (C≡N) or an alkyne (C≡C) functional group. Nitriles typically absorb closer to 2200 cm-1, while alkynes absorb closer to 2100 cm-1.
Correlate the identified peaks with the functional groups present in each compound. Assign the functional group(s) based on the absorption frequencies and ensure that the major peaks above 1600 cm-1 are consistent with the expected vibrations for the functional groups.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared Spectroscopy
Infrared spectroscopy is an analytical technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different functional groups absorb light at characteristic wavelengths, allowing for the identification of specific bonds and structures. Peaks in the IR spectrum correspond to vibrational transitions of molecular bonds, with major peaks typically appearing above 1600 cm-1 indicating the presence of certain functional groups.
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Functional Groups
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Common functional groups include ketones, esters, amides, nitriles, and alkynes, each with distinct structural features and reactivity. Understanding these groups is essential for predicting the behavior of organic compounds and interpreting their IR spectra.
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Peak Assignments in IR Spectra
In IR spectroscopy, peak assignments refer to the identification of specific peaks in the spectrum that correspond to particular functional groups. Peaks above 1600 cm-1 are typically associated with double bonds and other functional groups, such as carbonyls (C=O) and alkynes (C≡C). Assigning these peaks accurately is crucial for determining the functional groups present in a compound and understanding its chemical properties.
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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
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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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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