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Ch. 12 - Infrared Spectroscopy and Mass Spectrometry
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 12 - Infrared Spectroscopy and Mass SpectrometryProblem 9
Chapter 12, Problem 9

Show the fragmentations that give rise to the peaks at m/z 43, 57, and 85 in the mass spectrum of 2,4-dimethylpentane (Figure 12-17).
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Step 1: Begin by understanding the structure of 2,4-dimethylpentane. It is a branched alkane with the molecular formula C7H16. Visualize its structure to identify possible fragmentation patterns.
Step 2: Recall that mass spectrometry involves ionization of the molecule, typically forming a molecular ion (M⁺). Fragmentation occurs when bonds break, forming smaller ions. Each fragment corresponds to a peak in the mass spectrum, with the m/z value representing the mass-to-charge ratio.
Step 3: For the peak at m/z 43, consider the formation of a fragment ion by cleaving a bond near a methyl group. A common fragment with m/z 43 is the propyl cation (C3H7⁺). This can result from the loss of larger alkyl groups from the parent molecule.
Step 4: For the peak at m/z 57, analyze the possibility of forming a butyl cation (C4H9⁺). This can occur by breaking bonds around the central carbon atoms, leading to the loss of smaller alkyl groups while retaining a larger fragment.
Step 5: For the peak at m/z 85, consider the formation of a pentyl cation (C5H11⁺). This fragment is larger and results from cleaving bonds that leave behind a significant portion of the original molecule. Ensure to account for the branching in 2,4-dimethylpentane when identifying the specific bonds that break.

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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 involves ionizing chemical species and sorting the ions based on their mass. The resulting mass spectrum displays peaks corresponding to different ions, where the m/z (mass-to-charge) values indicate the mass of the fragments produced from the original molecule.
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Fragmentation Patterns

Fragmentation patterns refer to the specific ways in which a molecule breaks apart during ionization in mass spectrometry. These patterns are influenced by the molecular structure and stability of the resulting fragments. Understanding these patterns is crucial for interpreting mass spectra, as they help identify the structure of the original compound based on the observed m/z values.
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2,4-Dimethylpentane Structure

2,4-Dimethylpentane is a branched alkane with a specific arrangement of carbon atoms and methyl groups. Its structure influences the fragmentation pathways during mass spectrometry. Recognizing the molecular structure allows for predicting which fragments will form and their corresponding m/z values, essential for explaining the peaks at m/z 43, 57, and 85 in the mass spectrum.
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Related Practice
Textbook Question

Ethers are not easily differentiated by their infrared spectra, but they tend to form predictable fragments in the mass spectrum. The following compounds give similar but distinctive mass spectra.

Both compounds give prominent peaks at m/z 116, 73, 57, and 43. But one compound gives a distinctive strong peak at 87, and the other compound gives a strong peak at 101. Determine which compound gives the peak at 87 and which one gives the peak at 101. Propose fragmentations to account for the ions at m/z 116, 101, 87, and 73.

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Textbook Question

Identify which of these four mass spectra indicate the presence of sulfur, chlorine, bromine, iodine, or nitrogen. Suggest a molecular formula for each.

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Textbook Question

The infrared spectra for three compounds are provided. Each compound has one or more of the following functional groups: conjugated ketone, ester, amide, nitrile, and alkyne. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm–1.

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Textbook Question

Account for the peaks at m/z 87, 111, and 126 in the mass spectrum of 2,6-dimethylheptan-4-ol.

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Textbook Question

Spectra are given for three compounds. Each compound has one or more of the following functional groups: alcohol, amine, ketone, aldehyde, and carboxylic acid. Determine the functional group(s) in each compound, and assign the major peaks above 1600 cm–1.

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Textbook Question

Predict the characteristic infrared absorptions of the functional groups in the following molecules.

(a) cyclohexene

(b) pentan-2-ol

(c) pentan-2-one

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