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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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 45i,j
Bruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 45i,jChapter 14, Problem 45i,j
For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them:
i. 
j. 
Verified step by step guidance1
Step 1: Analyze the first pair of compounds (I). The first compound contains a double bond (alkene), while the second compound contains a triple bond (alkyne). In IR spectroscopy, alkenes typically show a C=C stretching vibration around 1600-1680 cm⁻¹, whereas alkynes exhibit a C≡C stretching vibration around 2100-2260 cm⁻¹. These distinct absorption bands can be used to differentiate between the two compounds.
Step 2: For the second pair of compounds (II), the first compound is an aldehyde, and the second compound is a ketone. Aldehydes exhibit a characteristic C-H stretching vibration near 2720-2820 cm⁻¹, which is absent in ketones. Both compounds will show a C=O stretching vibration, typically around 1700 cm⁻¹, but the aldehyde's unique C-H stretch can be used to distinguish it from the ketone.
Step 3: Understand the functional group differences in each pair. IR spectroscopy is particularly useful for identifying functional groups because each type of bond absorbs infrared light at a specific frequency.
Step 4: When analyzing IR spectra, focus on the regions where the functional group vibrations occur. For example, the alkene and alkyne regions are distinct, as are the aldehyde and ketone regions.
Step 5: Use the IR absorption bands mentioned above to identify the compounds in practice. Ensure that the sample is prepared correctly and the IR spectrum is interpreted accurately to confirm the presence of the distinguishing functional groups.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared (IR) Spectroscopy
Infrared spectroscopy is a technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different bonds absorb characteristic frequencies of IR radiation, leading to distinct absorption bands in the IR spectrum. This allows chemists to determine the presence of specific functional groups based on their unique absorption patterns.
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General Features of IR Spect
Functional Groups
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In the context of IR spectroscopy, different functional groups, such as alcohols, carbonyls, and alkenes, exhibit unique absorption bands that can be used to distinguish between compounds. Recognizing these groups is essential for interpreting IR spectra.
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Distinguishing Absorption Bands
Distinguishing absorption bands in IR spectroscopy involves identifying specific peaks in the spectrum that correspond to different functional groups or structural features of the compounds being analyzed. For example, the presence of a carbonyl (C=O) group typically shows a strong absorption around 1700 cm-1, while an alcohol (O-H) shows a broad absorption around 3200-3600 cm-1. These unique bands help differentiate between similar compounds.
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Related Practice
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
Norlutin and Enovid are ketones that suppress ovulation, so they have been used clinically as contraceptives. For which of these compounds would you expect the infrared carbonyl absorption (C=O stretch) to be at a higher frequency? Explain.
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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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Textbook Question
Assuming that the force constant is approximately the same for C–C, C–N, and C–O bonds, predict the relative positions of their stretching vibrations in an IR spectrum.
Textbook Question
Rank the following compounds in order of increasing lmax:
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