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 22
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 22Chapter 14, Problem 22
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?
Verified step by step guidance1
Step 1: Understand the factors affecting wavenumber in IR spectroscopy. Wavenumber is inversely proportional to bond length and directly proportional to bond strength. Additionally, resonance, hybridization, and hydrogen bonding can influence the wavenumber of a functional group.
Step 2: For part (a), compare the C-O stretch of phenol and cyclohexanol. Phenol has a conjugated aromatic ring, which can delocalize electrons and reduce the bond strength of the C-O bond, leading to a lower wavenumber. Cyclohexanol lacks this conjugation, so its C-O bond is stronger and occurs at a higher wavenumber.
Step 3: For part (b), compare the C=O stretch of a ketone and an amide. The amide group has resonance between the lone pair on nitrogen and the carbonyl group, which reduces the double-bond character of the C=O bond, lowering its wavenumber. Ketones lack this resonance effect, so their C=O stretch occurs at a higher wavenumber.
Step 4: For part (c), compare the C-N stretch of cyclohexylamine and aniline. Aniline has a conjugated aromatic ring, which allows resonance between the lone pair on nitrogen and the ring, reducing the bond strength of the C-N bond and lowering its wavenumber. Cyclohexylamine lacks this conjugation, so its C-N stretch occurs at a higher wavenumber.
Step 5: Summarize the trends: (a) Cyclohexanol has a larger wavenumber than phenol for the C-O stretch. (b) Ketones have a larger wavenumber than amides for the C=O stretch. (c) Cyclohexylamine has a larger wavenumber than aniline for the C-N stretch.
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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 by measuring the absorption of infrared light. Different bonds absorb IR radiation at characteristic wavenumbers, allowing chemists to deduce the presence of specific functional groups based on their absorption peaks.
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Wavenumber and Bond Strength
Wavenumber, measured in cm⁻¹, is inversely related to the wavelength of light and is commonly used in IR spectroscopy. Generally, stronger bonds and more electronegative atoms lead to higher wavenumbers. For example, C=O stretches typically occur at higher wavenumbers than C-O stretches due to the greater bond strength of the carbonyl group.
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Substituent Effects on Stretching Frequencies
The presence of different substituents on a molecule can influence the stretching frequencies of bonds. Electron-donating or withdrawing groups can alter the electron density around a bond, affecting its strength and, consequently, the wavenumber at which it absorbs IR radiation. For instance, the presence of an amine group can lower the wavenumber of a C-N stretch compared to a simple alkyl amine.
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Related Practice
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
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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Textbook Question
The reaction of (Z)-2-pentene with water and a trace of H2SO4 forms two products. Identify the products from their mass spectra.
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