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 24
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 24Chapter 14, Problem 24
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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Understand that the C-O absorption band in IR spectroscopy corresponds to the stretching vibration of the carbon-oxygen bond. The position of this band depends on the bond strength and the surrounding chemical environment.
Recognize that 1-hexanol contains a hydroxyl (-OH) group attached to a carbon atom, forming an alcohol. In contrast, pentanoic acid contains a carboxylic acid functional group (-COOH), where the C-O bond is part of a carboxyl group.
Explain that in pentanoic acid, the C-O bond is part of a resonance structure. The resonance between the carbonyl group (C=O) and the hydroxyl group (-OH) in the carboxylic acid delocalizes electron density, giving the C-O bond partial double-bond character. This increases the bond strength and shifts the absorption band to a higher wavenumber (1220 cm⁻¹).
In 1-hexanol, the C-O bond is a single bond without resonance effects. The lack of resonance and the single-bond character make the bond weaker, resulting in a lower wavenumber for the absorption band (1060 cm⁻¹).
Conclude that the difference in wavenumber arises from the resonance stabilization in pentanoic acid, which strengthens the C-O bond, compared to the single-bond character of the C-O bond in 1-hexanol.
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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 a technique used to identify molecular structures by measuring the absorption of infrared light at different wavelengths. Different functional groups absorb infrared light at characteristic wavenumbers, allowing chemists to deduce the presence of specific bonds in a molecule. The position of these absorption bands can provide insights into molecular interactions and the environment surrounding the functional groups.
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Functional Groups and Their Influence on Wavenumbers
Functional groups, such as alcohols and carboxylic acids, have distinct vibrational modes that influence their infrared absorption wavenumbers. The presence of electronegative atoms, hydrogen bonding, and molecular structure can shift these bands. For instance, the C-O bond in alcohols typically appears at lower wavenumbers compared to carboxylic acids due to differences in bonding and molecular interactions.
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02:36Identifying Functional Groups
Hydrogen Bonding
Hydrogen bonding is a type of intermolecular force that occurs when a hydrogen atom covalently bonded to an electronegative atom interacts with another electronegative atom. In the case of pentanoic acid, the presence of a carboxylic group allows for strong hydrogen bonding, which can stabilize the molecule and affect the vibrational frequency of the C-O bond, resulting in a higher wavenumber compared to 1-hexanol, which lacks such extensive hydrogen bonding.
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Related Practice
Textbook Question
Which occurs at a larger wavenumber:
a. the C–O stretch of phenol or the C–O stretch of cyclohexanol?
b. the C=O stretch of a ketone or the C=O stretch of an amide?
c. the C–N stretch of cyclohexylamine or the C–N stretch of aniline?
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Textbook Question
a. Which occurs at a larger wavenumber:
3. a C–N stretch or a C=N stretch?
4. a C=O stretch or a C–O stretch?
Textbook Question
a. Which occurs at a larger wavenumber:
1. a C=C stretch or a C=C stretch?
2. a C–H stretch or a C–H bend?
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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
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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