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Ch. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis Spectroscopy
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 13 - Mass Spectrometry; Infrared Spectroscopy; UV/Vis SpectroscopyProblem 41
Chapter 14, Problem 41

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

Verified step by step guidance
1
Step 1: Understand the problem. The question is asking about the relative intensities of peaks at m/z = 57 and m/z = 71 in the mass spectra of two isomers: 3-methylpentane and 2-methylpentane. The tallest peak corresponds to the most stable fragment ion formed during fragmentation in the mass spectrometer.
Step 2: Recall the concept of fragmentation in mass spectrometry. When a molecule is ionized, it can break into smaller fragments. The stability of these fragments determines the intensity of the corresponding peaks in the mass spectrum. More stable fragments produce taller peaks.
Step 3: Analyze the fragmentation of 3-methylpentane. For m/z = 57, the fragment corresponds to a tert-butyl cation (C4H9+), which is highly stable due to hyperconjugation and inductive effects. For m/z = 71, the fragment corresponds to a primary carbocation (C5H11+), which is less stable.
Step 4: Analyze the fragmentation of 2-methylpentane. For m/z = 57, the fragment corresponds to a secondary carbocation (C4H9+), which is moderately stable. For m/z = 71, the fragment corresponds to a secondary carbocation (C5H11+), which is also moderately stable.
Step 5: Compare the stability of the fragments. In 3-methylpentane, the peak at m/z = 57 is taller because the tert-butyl cation is more stable than the primary carbocation at m/z = 71. In 2-methylpentane, the peaks at m/z = 57 and m/z = 71 are closer in intensity because both correspond to secondary carbocations with similar stability. Thus, the tallest peak depends on the compound being analyzed.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It helps identify the composition of a sample by generating a mass spectrum, which displays peaks corresponding to different ions. The height of each peak indicates the relative abundance of the corresponding ion, allowing for the comparison of different compounds based on their fragmentation patterns.
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Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart into smaller ions during mass spectrometry. Different compounds will produce characteristic fragments based on their structure, which can be predicted using knowledge of organic chemistry. Understanding these patterns is crucial for interpreting mass spectra and determining which peaks correspond to which compounds.
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Isomeric Compounds

Isomeric compounds, such as 3-methylpentane and 2-methylpentane, have the same molecular formula but different structural arrangements. This structural difference can lead to variations in their fragmentation patterns and, consequently, their mass spectra. Recognizing how isomers can affect peak heights in a mass spectrum is essential for accurately determining which compound produces the tallest peak at a given m/z value.
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Related Practice
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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At pH = 7, one of the ions shown here is purple and the other is blue. Which is which?

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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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Textbook Question

Rank each set of compounds in order of decreasing λmax:

b.

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Textbook Question

Rank the following compounds in order of increasing lmax:

2
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