Textbook Question
The IR and mass spectra for three different compounds are shown below. Identify each compound.
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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 72a
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 72aChapter 14, Problem 72a
How can you use UV spectroscopy to distinguish between the compounds in each of the following pairs?
a. 
Verified step by step guidance1
Step 1: Analyze the structures of compounds A and B. Compound A contains conjugated double bonds (a system of alternating single and double bonds), while compound B has isolated double bonds (double bonds separated by single bonds without conjugation). This structural difference is key to their UV spectroscopy behavior.
Step 2: Understand the principle of UV spectroscopy. UV spectroscopy measures the absorption of ultraviolet light by molecules, which is influenced by the presence of conjugated systems. Conjugated systems lower the energy gap between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), resulting in absorption at longer wavelengths.
Step 3: Predict the UV absorption behavior. Compound A, with its conjugated double bonds, will absorb UV light at a longer wavelength (lower energy) compared to compound B, which has isolated double bonds. This is because conjugation increases the delocalization of π-electrons, stabilizing the molecule and shifting the absorption maximum (λmax) to a longer wavelength.
Step 4: Compare the λmax values. In a UV spectrum, compound A will show a λmax at a longer wavelength due to its conjugated system, while compound B will have a λmax at a shorter wavelength because its double bonds are not conjugated.
Step 5: Use UV spectroscopy data to distinguish the compounds. By recording the UV spectra of both compounds, you can identify compound A by its longer λmax and compound B by its shorter λmax. This difference in absorption wavelengths provides a clear method to differentiate between the two compounds.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
UV Spectroscopy Principles
UV spectroscopy is a technique that measures the absorption of ultraviolet light by molecules. When a molecule absorbs UV light, electrons are excited from a ground state to a higher energy state. The specific wavelengths of light absorbed can provide information about the electronic structure of the molecule, allowing for differentiation between compounds based on their functional groups and conjugation.
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The UV-Vis Spectroscopy Concept 1
Conjugation and Electron Delocalization
Conjugation refers to the overlap of p-orbitals across adjacent double bonds or lone pairs, allowing for electron delocalization. This delocalization lowers the energy gap between the ground and excited states, resulting in absorption of longer wavelengths (lower energy) of UV light. Compounds with different degrees of conjugation will absorb UV light at different wavelengths, making it a key factor in distinguishing between them.
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Functional Groups and Their UV Absorption
Different functional groups absorb UV light at characteristic wavelengths due to their unique electronic structures. For example, alkenes and aromatic compounds typically show distinct absorption patterns. By analyzing the UV absorption spectrum, one can identify specific functional groups present in the compounds, aiding in their differentiation based on the observed peaks in the spectrum.
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Related Practice
Textbook Question
How can you use UV spectroscopy to distinguish between the compounds in each of the following pairs?
b.
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Textbook Question
Each of the IR spectra shown below is the spectrum of one of the following compounds. Identify the compound that produced each spectrum.
a. <IMAGE>
Textbook Question
Each of the IR spectra shown below is the spectrum of one of the following compounds. Identify the compound that produced each spectrum.
b. <IMAGE>
Textbook Question
Phenolphthalein is an acid–base indicator. In solutions of pH < 8.5, it is colorless; in solutions of pH > 8.5, it is deep red-purple. Account for the change in color.
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Textbook Question
How can you use UV spectroscopy to distinguish between the compounds in each of the following pairs?
c.
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