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Ch. 12 - Infrared Spectroscopy and Mass Spectrometry
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoWade 9th EditionCh. 12 - Infrared Spectroscopy and Mass SpectrometryProblema 12a,b,c
Capitolo 12, Problema 12a,b,c

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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1
Identify the functional groups present in each molecule. For cyclohexene, the functional group is a carbon-carbon double bond (C=C). For pentan-2-ol, the functional group is a hydroxyl group (-OH). For pentan-2-one, the functional group is a carbonyl group (C=O).
Recall the characteristic infrared absorption ranges for each functional group. The C=C bond in alkenes typically absorbs in the range of 1600-1680 cm⁻¹. The -OH group in alcohols shows a broad absorption around 3200-3600 cm⁻¹ due to hydrogen bonding. The C=O bond in ketones absorbs strongly in the range of 1700-1750 cm⁻¹.
For cyclohexene, predict the IR absorptions: Look for a peak in the range of 1600-1680 cm⁻¹ for the C=C stretch. Additionally, weak C-H stretching vibrations for sp² hybridized carbons may appear just above 3000 cm⁻¹.
For pentan-2-ol, predict the IR absorptions: Expect a broad, strong absorption around 3200-3600 cm⁻¹ for the -OH group. Also, look for C-H stretching vibrations for sp³ hybridized carbons in the range of 2800-3000 cm⁻¹.
For pentan-2-one, predict the IR absorptions: Look for a strong absorption in the range of 1700-1750 cm⁻¹ for the C=O stretch. Additionally, C-H stretching vibrations for sp³ hybridized carbons will appear in the range of 2800-3000 cm⁻¹.

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Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different functional groups absorb characteristic wavelengths of infrared radiation, leading to distinct peaks in an IR spectrum. Understanding how these absorptions correlate with molecular vibrations is essential for predicting the IR spectra of given compounds.
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The UV-Vis Spectroscopy Concept 1

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Common functional groups include alcohols, ketones, and alkenes, each exhibiting unique IR absorption patterns. Identifying these groups in the provided molecules is crucial for predicting their infrared absorptions.
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Identifying Functional Groups

Molecular Vibrations

Molecular vibrations refer to the oscillations of atoms within a molecule, which occur when energy is absorbed. These vibrations can be stretching or bending motions and are directly related to the frequencies of infrared light absorbed. The type and number of vibrations depend on the molecular structure and functional groups present, influencing the IR spectrum observed for each compound.
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Convert the following infrared wavelengths to cm-1. (a) 6.24 𝜇m, typical for an aromatic C=C (b) 3.38 𝜇m, typical for a saturated C-H bond (c) 5.85 𝜇m, typical for a ketone carbonyl

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(d) pent-1-yne

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(f) pentanoic acid

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