Textbook Question
Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption bands:
b.
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Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy
Bruice8th EditionOrganic ChemistryISBN: 9780135213711
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Table of contents
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 1)31
- Ch. 1 - Remembering General Chemistry: Electronic Structure and Bonding (Part 2)65
- Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry84
- Ch. 3 - An Introduction to Organic Compounds:Nomenclature, Physical Properties, and Structure183
- Ch. 4 - Isomers: The Arrangement of Atoms in Space121
- Ch. 5 - Alkenes: Structure, Nomenclature, and an Introduction to Reactivity • Thermodynamics and Kinetics83
- Ch. 6 - The Reactions of Alkenes • The Stereochemistry of Addition Reactions175
- Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis119
- Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene148
- Ch. 9 - Substitution and Elimination Reactions of Alkyl Halides212
- Ch. 10 - Reactions of Alcohols, Ethers, Epoxides, Amines, and Sulfur-Containing Compounds131
- Ch. 11 - Organometallic Compounds60
- Ch. 12 - Radicals90
- Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy62
- Ch. 14 - NMR Spectroscopy95
- Ch. 15 - Reactions of Carboxylic Acids and Carboxylic Acid Derivatives123
- Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives141
- Ch. 17 - Reactions at the Alpha-Carbon101
- Ch. 18 - Reactions of Benzene and Substituted Benzenes144
- Ch. 19 - More About Amines • Reactions of Heterocyclic Compounds49
- Ch. 20 - The Organic Chemistry of Carbohydrates80
- Ch. 21 - Amino Acids, Peptides, and Proteins57
- Ch. 22 - Catalysis in Organic Reactions and in Enzymatic Reactions35
- Ch. 23 - The Organic Chemistry of the Coenzymes—Compounds Derived from Vitamins1
- Ch. 28 - Pericyclic Reactions58
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Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 28b
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 28bChapter 14, Problem 28b
A nitrogen-containing compound shows no absorption band at ~3400 cm-1 and no absorption bands between ~1700 cm-1 and ~1600 cm-1 or between 2260 cm-1 and 2220 cm-1. What class of compound is it?
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Analyze the absence of an absorption band at ~3400 cm⁻¹: This region typically corresponds to N-H stretching vibrations. The lack of absorption here suggests that the compound does not contain an amine (primary or secondary) or an amide functional group, as these would exhibit N-H stretching bands.
Examine the absence of absorption bands between ~1700 cm⁻¹ and ~1600 cm⁻¹: This region is characteristic of C=O stretching vibrations. The absence of absorption here indicates that the compound does not contain a carbonyl group, ruling out amides, ketones, aldehydes, esters, and carboxylic acids.
Consider the absence of absorption bands between ~2260 cm⁻¹ and ~2220 cm⁻¹: This region is associated with C≡N stretching vibrations. The lack of absorption here suggests that the compound does not contain a nitrile functional group.
Based on the absence of these characteristic absorption bands, conclude that the compound is likely a tertiary amine. Tertiary amines do not have N-H bonds, so they do not exhibit absorption at ~3400 cm⁻¹, and they lack the functional groups that would produce absorption in the other specified regions.
Verify the conclusion by considering the molecular structure of tertiary amines: Tertiary amines contain a nitrogen atom bonded to three alkyl or aryl groups, and their IR spectra typically lack the N-H and C=O stretching bands observed in other nitrogen-containing compounds.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared Spectroscopy
Infrared (IR) spectroscopy is a technique used to identify functional groups in organic compounds based on their absorption of infrared light. Different functional groups absorb IR radiation at characteristic wavelengths, allowing chemists to deduce the presence or absence of specific bonds and structures in a molecule.
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Absorption Bands
Absorption bands in IR spectroscopy correspond to specific vibrational transitions of molecular bonds. For example, a strong absorption band around 3400 cm-1 typically indicates the presence of O-H or N-H bonds, while bands between 1700 cm-1 and 1600 cm-1 are associated with carbonyl (C=O) groups. The absence of these bands provides critical information about the compound's structure.
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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. In the context of nitrogen-containing compounds, the presence or absence of certain functional groups, such as amines, amides, or nitriles, can be inferred from the IR absorption patterns, guiding the identification of the compound's class.
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Related Practice
Textbook Question
How can IR spectroscopy be used to distinguish between the following compounds?
e. cyclohexene and cyclohexane
f. a primary amine and a tertiary amine
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Textbook Question
For each of the following pairs of compounds, name one absorption band that can be used to distinguish between them.
c.
d.
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Textbook Question
Which shows an O–H stretch at a larger wavenumber: ethanol dissolved in carbon disulfide or an undiluted sample of ethanol?
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Textbook Question
How can IR spectroscopy be used to distinguish between the following compounds?
a. a ketone and an aldehyde
b. a cyclic ketone and an open-chain ketone
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Textbook Question
Why is the C–O absorption band of 1-hexanol at a smaller wavenumber (1060 cm–1) than the C–O absorption band of pentanoic acid (1220 cm–1)?
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