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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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 31
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 31Chapter 14, Problem 31
Which of the following compounds has a vibration that is infrared inactive?
1-butyne, 2-butyne, H2, H2O, Cl2, ethene
Verified step by step guidance1
Step 1: Understand the concept of infrared (IR) activity. A vibration is IR active if it results in a change in the dipole moment of the molecule. Molecules or bonds that do not exhibit a change in dipole moment during vibration are IR inactive.
Step 2: Analyze the molecular structure of 1-butyne. It is a terminal alkyne with the structure CH≡C-CH2-CH3. The C≡C bond in 1-butyne is polar, and its stretching vibration will cause a change in dipole moment, making it IR active.
Step 3: Analyze the molecular structure of 2-butyne. It is an internal alkyne with the structure CH3-C≡C-CH3. The C≡C bond in 2-butyne is symmetric, and its stretching vibration does not cause a change in dipole moment, making it IR inactive.
Step 4: Analyze the molecular structure of H2. It is a diatomic molecule with a nonpolar covalent bond. Since there is no dipole moment in H2, its vibrations do not cause a change in dipole moment, making it IR inactive.
Step 5: Compare the compounds. Based on the analysis, 2-butyne and H2 are IR inactive, while 1-butyne is IR active. The key factor is whether the vibration causes a change in dipole moment.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared Activity
Infrared activity refers to the ability of a molecule to absorb infrared radiation, which is determined by changes in the dipole moment during molecular vibrations. For a compound to be infrared active, it must have a permanent dipole moment or undergo a change in dipole moment during vibration. Molecules that are symmetrical or nonpolar often exhibit infrared inactivity.
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Molecular Symmetry
Molecular symmetry plays a crucial role in determining infrared activity. Symmetrical molecules, such as H2, do not have a permanent dipole moment and do not exhibit vibrations that change the dipole moment, making them infrared inactive. Understanding the symmetry of a molecule helps predict its vibrational modes and their infrared activity.
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Types of Hydrocarbons
Hydrocarbons can be classified into alkanes, alkenes, and alkynes, each with distinct structural features. 1-butyne and 2-butyne are alkynes, which contain triple bonds and can exhibit different vibrational characteristics. Recognizing the type of hydrocarbon and its structure is essential for analyzing its infrared activity and understanding how molecular vibrations relate to functional groups.
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Related Practice
Textbook Question
A 4.0 × 10-5 M solution of a compound in hexane shows an absorbance of 0.40 at 252 nm in a cell with a 1 cm light path. What is the molar absorptivity of the compound in hexane at 252 nm?
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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
Identify the compound that gives the mass spectrum and infrared spectrum shown here.
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Textbook Question
Predict the λmax of the following compound:
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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.
e.
f.
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