Textbook Question
Choose the bond in each pair that you expect to vibrate at the higher wavenumber.
(b) C―H vs. C―O
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Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 18a
Mullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 18aChapter 13, Problem 18a
Would you expect an acetylenic C―H to absorb at a higher or lower wavenumber than the C―H in ethene?
Verified step by step guidance1
Understand the concept of wavenumber in IR spectroscopy: Wavenumber is inversely proportional to wavelength and is directly related to the energy of the bond vibration. Higher wavenumbers correspond to higher energy vibrations.
Identify the types of C―H bonds in question: An acetylenic C―H bond is found in alkynes, where the carbon is sp-hybridized. In ethene, the C―H bond is found in alkenes, where the carbon is sp2-hybridized.
Consider the hybridization of the carbon atoms: sp-hybridized carbons (as in alkynes) have more s-character (50%) compared to sp2-hybridized carbons (33% s-character) in alkenes. More s-character results in a stronger, shorter bond.
Relate bond strength to wavenumber: Stronger bonds typically absorb at higher wavenumbers because they require more energy to vibrate.
Conclude based on bond strength and hybridization: Since the acetylenic C―H bond is stronger due to higher s-character, it will absorb at a higher wavenumber compared to the C―H bond in ethene.
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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 functional groups in organic molecules by measuring the absorption of infrared light, which causes molecular vibrations. Different bonds absorb at characteristic wavenumbers, allowing for the identification of specific types of bonds, such as C―H, C=C, and C≡C, based on their unique vibrational frequencies.
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Bond Strength and Wavenumber
The wavenumber at which a bond absorbs infrared light is related to the bond strength and the masses of the atoms involved. Stronger bonds, such as triple bonds, typically absorb at higher wavenumbers compared to double or single bonds. This is because stronger bonds require more energy to vibrate, resulting in higher frequency and wavenumber absorption.
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Acetylenic vs. Ethylenic C―H Bonds
Acetylenic C―H bonds, found in alkynes, are associated with sp-hybridized carbon atoms, while ethylenic C―H bonds, found in alkenes, involve sp2-hybridized carbons. The sp-hybridized C―H bond is stronger and shorter than the sp2-hybridized C―H bond, leading to a higher wavenumber absorption for acetylenic C―H in infrared spectroscopy compared to ethylenic C―H.
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Related Practice
Textbook Question
The IR spectrum for anisole contains two C―O stretching bands in the fingerprint region. Match the band to the bond that gives rise to it. Why are these bands so intense?
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Textbook Question
Match each of the four IR spectra to one of the given compounds. [One of the compounds does not match a spectrum.]
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Textbook Question
Calculate the reduced mass for the following bonds.
(a) C―H
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Textbook Question
For each pair, choose the molecule that you expect to have the highest wavenumber for its C=O stretch.
(d)
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Textbook Question
Calculate the reduced mass for the following bonds.
(c) C―Cl
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