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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 16a
Chapter 12, Problem 16a

Four infrared spectra are shown, corresponding to four of the following compounds. For each spectrum, determine the structure and explain how the peaks in the spectrum correspond to the structure you have chosen.

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Step 1: Analyze the IR spectrum labeled (a). Look for key peaks such as a broad absorption around 3200-3600 cm⁻¹, which indicates the presence of an O-H stretch (alcohol or carboxylic acid). Additionally, check for a sharp peak near 1700 cm⁻¹, which corresponds to a C=O stretch (carbonyl group). These features suggest the compound may be a carboxylic acid.
Step 2: Analyze the IR spectrum labeled (b). Identify the broad absorption around 3200-3600 cm⁻¹, which indicates an O-H stretch. Look for the absence of a sharp peak near 1700 cm⁻¹, which would rule out a carbonyl group. This spectrum likely corresponds to an alcohol.
Step 3: Analyze the IR spectrum labeled (c). Look for sharp peaks near 1600 cm⁻¹ and 1500 cm⁻¹, which are characteristic of aromatic C=C stretches. Additionally, check for a broad absorption around 3200-3600 cm⁻¹, indicating an O-H stretch. These features suggest the compound is a phenol.
Step 4: Analyze the IR spectrum labeled (d). Look for a sharp peak near 1700 cm⁻¹, which indicates a C=O stretch (carbonyl group). Check for the absence of a broad absorption around 3200-3600 cm⁻¹, ruling out an O-H stretch. This spectrum likely corresponds to a ketone.
Step 5: Match each IR spectrum to the provided chemical structures based on the functional groups identified. For example, spectrum (a) corresponds to a carboxylic acid, spectrum (b) to an alcohol, spectrum (c) to a phenol, and spectrum (d) to a ketone. Use the chemical structures provided in the first image to finalize the matches.

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Key Concepts

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

Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify molecular structures by measuring the absorption of infrared light by a compound. Different functional groups absorb characteristic wavelengths of infrared light, resulting in peaks on a spectrum. By analyzing these peaks, one can deduce the presence of specific bonds and functional groups in the molecule.
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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 (–OH), carbonyls (C=O), and alkynes (C≡C). Identifying these groups in a compound is crucial for interpreting infrared spectra, as each group has distinct absorption features.
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Peak Interpretation

In infrared spectroscopy, the position and intensity of peaks in the spectrum provide information about the molecular structure. For example, a broad peak around 3200-3600 cm⁻¹ typically indicates the presence of an –OH group, while sharp peaks in the range of 1700-1750 cm⁻¹ suggest a carbonyl group. Understanding how to correlate these peaks with specific functional groups is essential for accurate structure determination.
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Related Practice
Textbook Question

Predict the masses and the structures of the most abundant fragments observed in the mass spectra of the following compounds.

(c) 4-methylpentan-2-ol

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Textbook Question

Predict the masses and the structures of the most abundant fragments observed in the mass spectra of the following compounds.

(b) 3-methylhex-2-ene

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Textbook Question

Predict the masses and the structures of the most abundant fragments observed in the mass spectra of the following compounds. (a) 2-methylpentane

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Textbook Question

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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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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Textbook Question

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

(d) pent-1-yne

(e) diethylamine

(f) pentanoic acid

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