Textbook Question
For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them:
i.
j.
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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 44
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 44Chapter 14, Problem 44
Rank the following compounds in order of increasing lmax:
Verified step by step guidance1
Step 1: Understand the concept of λmax (maximum wavelength of absorption). λmax is influenced by the extent of conjugation in a molecule. Greater conjugation leads to a higher λmax because the energy gap between the HOMO and LUMO decreases.
Step 2: Analyze the structures provided. Compound A has one double bond, Compound B has two conjugated double bonds, Compound C has three conjugated double bonds, and Compound D has two conjugated double bonds but lacks aromaticity.
Step 3: Rank the compounds based on the extent of conjugation. Compound C has the most conjugation (three double bonds), followed by Compounds B and D (two conjugated double bonds), and finally Compound A (one double bond).
Step 4: Consider additional factors such as aromaticity. Compound B has aromaticity due to the benzene ring, which stabilizes the conjugated system and increases λmax compared to Compound D.
Step 5: Final ranking in order of increasing λmax: Compound A < Compound D < Compound B < Compound C.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Conjugation
Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for delocalization of electrons. This delocalization lowers the energy of the molecule and increases the wavelength of light absorbed, leading to higher λmax values. The extent of conjugation directly influences the color and reactivity of organic compounds.
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Conjugated states
λmax (Lambda Max)
λmax is the wavelength at which a compound exhibits maximum absorbance in UV-Vis spectroscopy. It is influenced by the electronic structure of the molecule, particularly the presence of conjugated systems. Compounds with more extensive conjugation typically have higher λmax values due to lower energy gaps between their ground and excited states.
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Electron-Withdrawing and Electron-Donating Groups
Electron-withdrawing groups (EWGs) decrease electron density in a molecule, often lowering λmax, while electron-donating groups (EDGs) increase electron density, typically raising λmax. The presence and position of these groups in relation to the conjugated system can significantly affect the electronic transitions and, consequently, the absorption properties of the compounds.
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Related Practice
Textbook Question
At pH = 7, one of the ions shown here is purple and the other is blue. Which is which?
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Textbook Question
In the mass spectrum of the following compounds, which is the tallest—the peak at m/z = 57 or the peak at m/z = 71?
a. 3-methylpentane
b. 2-methylpentane
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Textbook Question
a. How could you use IR spectroscopy to determine whether the following reaction had occurred?
b. After purifying the product, how could you determine whether all the NH2NH2 had been removed?
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Textbook Question
What would be the difference in the colors of the compounds at pH = 3?
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Textbook Question
For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them:
c. CH3CH2CH2OH and CH3CH2OCH3
d.
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