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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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 37a
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 37aChapter 14, Problem 37a
Rank each set of compounds in order of decreasing λmax:
a. 
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 longer λmax because the energy gap between the HOMO and LUMO decreases.
Step 2: Analyze the conjugation in each compound. Compound A has two benzene rings connected directly, providing significant conjugation. Compound B has a benzene ring conjugated with an alkene group, which is less conjugated than A. Compound C has a single benzene ring with no extended conjugation. Compound D has three conjugated systems (two benzene rings and an alkene), making it the most conjugated.
Step 3: Rank the compounds based on their conjugation. Compound D has the most conjugation, followed by A, then B, and finally C.
Step 4: Explain why conjugation affects λmax. Conjugation allows delocalization of π-electrons, reducing the energy required for electronic transitions and increasing the wavelength of absorption.
Step 5: Final ranking in order of decreasing λmax: D > A > B > 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 between double bonds and lone pairs, allowing for delocalization of electrons. This delocalization lowers the energy of the molecule and can lead to increased stability and extended π-systems, which typically results in a higher λmax (lmax) in UV-Vis spectroscopy.
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03:27
Conjugated states
Aromaticity
Aromatic compounds are cyclic, planar structures with a ring of resonance that follow Hückel's rule (4n + 2 π electrons). Aromaticity contributes to the stability of the compound and can influence the absorption of light, often resulting in a higher λmax due to the increased electron delocalization within the aromatic system.
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Substituent Effects
The presence of different substituents on a benzene ring can significantly affect the electronic properties of the compound. Electron-donating groups (EDGs) can increase the electron density and enhance conjugation, leading to a higher λmax, while electron-withdrawing groups (EWGs) can decrease electron density and lower λmax. Understanding these effects is crucial for ranking the compounds based on their lmax.
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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
Identify the compound that gives the mass spectrum and infrared spectrum shown here.
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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
Predict the λmax of the following compound:
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Textbook Question
Rank each set of compounds in order of decreasing λmax:
b.
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