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Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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All textbooksMullins 1st EditionCh. 14 - Structural Identification I: Infrared Spectroscopy and Mass SpectrometryProblem 48c
Chapter 13, Problem 48c

Identify the important bands you would expect to find in an IR spectrum for the following molecules.
(c) Chemical structure of propargylamine, showing an amine group (H2N) and a terminal alkyne group with a triple bond.

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1
Identify the functional groups present in the molecule. Each functional group has characteristic absorption bands in an IR spectrum.
For each functional group, determine the typical wavenumber range where its absorption band appears. For example, O-H stretches are typically found around 3200-3600 cm-1, C=O stretches around 1700 cm-1, and C-H stretches around 2800-3000 cm-1.
Consider the molecular structure to identify any specific interactions or conjugations that might shift the absorption bands. Conjugation can lower the frequency of C=O stretches, for instance.
Look for any unique or distinguishing features in the molecule that might lead to additional bands or shifts, such as hydrogen bonding, which can broaden and shift O-H stretches.
Compile a list of expected IR bands based on the identified functional groups and any structural considerations, noting the approximate wavenumber ranges for each.

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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 an analytical technique used to identify functional groups in molecules by measuring the absorption of infrared light at different wavelengths. Each functional group absorbs IR radiation at characteristic frequencies, producing a spectrum with peaks that correspond to specific bond vibrations. Understanding these absorption patterns is crucial for interpreting IR spectra.
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General Features of IR Spect

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and reactivity. In IR spectroscopy, different functional groups absorb infrared light at distinct frequencies, allowing for their identification. For example, carbonyl groups (C=O) typically show strong absorption around 1700 cm⁻¹, while hydroxyl groups (O-H) exhibit broad peaks around 3200-3600 cm⁻¹.
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Characteristic IR Absorption Bands

Characteristic IR absorption bands are specific regions in an IR spectrum where particular functional groups absorb infrared light. These bands are used to identify the presence of functional groups in a molecule. For instance, alkanes show C-H stretching vibrations around 2850-2960 cm⁻¹, while alkenes have C=C stretching near 1640-1680 cm⁻¹. Recognizing these bands is essential for analyzing and interpreting IR spectra.
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Related Practice
Textbook Question

Identify the important bands you would expect to find in an IR spectrum for the following molecules.

(a)

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Textbook Question

A carboxylic acid can be converted to an ester using the conditions shown. Explain how a comparison of an IR spectrum of the reactant(s) and product can be used to determine whether the reaction was successful or not.

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Textbook Question

What functional groups might be present in the IR spectra for the molecules with the given molecular formulas. [Be sure to use the molecular formula in your analysis.]

(b) C₃H₄O₄

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Textbook Question

Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber.

(f) C―S vs C=O

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Textbook Question

Hoping to make the following diene, a chemist treated the diol shown with acid. Based on the IR spectrum, was the reaction successful? If not, what compound was made instead?

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Textbook Question

Choose the bond in each pair that you expect to have the more intense stretching band

(b) C=O vs. C=N

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