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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 25b
Chapter 14, Problem 25b

Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption bands:
b.

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1
Identify the functional groups in each compound that contain a C=O bond. For example, ketones, aldehydes, esters, carboxylic acids, and amides all have C=O bonds, but their environments differ, affecting the absorption wavenumber.
Recall the general trend for C=O stretching frequencies in IR spectroscopy. The wavenumber is influenced by the electronic environment around the carbonyl group. For example, electron-withdrawing groups increase the wavenumber, while electron-donating groups or resonance effects lower it.
Consider the hybridization and resonance effects. For example, esters typically have a higher wavenumber than ketones because the lone pair on the oxygen in esters participates in resonance, reducing the electron density on the carbonyl carbon less effectively than in ketones.
Account for any conjugation effects. Conjugation with a double bond or aromatic ring lowers the C=O stretching frequency because it delocalizes the electrons, reducing the bond strength and thus the wavenumber.
Rank the compounds based on the above considerations, starting with the compound that has the least resonance or conjugation effects (highest wavenumber) and ending with the compound that has the most resonance or conjugation effects (lowest wavenumber).

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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 infrared radiation at characteristic wavenumbers, which are measured in cm⁻¹. The C=O bond, for example, typically shows strong absorption in the range of 1700-1750 cm⁻¹, making it a key feature in organic compound analysis.
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The UV-Vis Spectroscopy Concept 1

C=O Bond Strength and Wavenumber

The strength of the C=O bond influences the wavenumber of its absorption band in infrared spectroscopy. Stronger bonds generally absorb at higher wavenumbers due to the increased energy required to stretch the bond. Factors such as hybridization and the presence of electron-withdrawing groups can also affect the bond strength and, consequently, the wavenumber of the C=O absorption.
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Functional Group Effects

Different functional groups can influence the wavenumber of C=O absorption bands through electronic effects. For instance, electron-withdrawing groups (like halogens or nitro groups) can increase the wavenumber due to increased bond polarity, while electron-donating groups can lower it. Understanding these effects is crucial for accurately ranking compounds based on their C=O absorption characteristics.
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Related Practice
Textbook Question

Which occurs at a larger wavenumber:

a. the C–O stretch of phenol or the C–O stretch of cyclohexanol?

b. the C=O stretch of a ketone or the C=O stretch of an amide?

c. the C–N stretch of cyclohexylamine or the C–N stretch of aniline?

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Textbook Question

a. Which occurs at a larger wavenumber:

3. a C–N stretch or a C=N stretch?

4. a C=O stretch or a C–O stretch?

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Textbook Question

Which shows an O–H stretch at a larger wavenumber: ethanol dissolved in carbon disulfide or an undiluted sample of ethanol?

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Textbook Question

How can IR spectroscopy be used to distinguish between the following compounds?

a. a ketone and an aldehyde

b. a cyclic ketone and an open-chain ketone

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Textbook Question

A nitrogen-containing compound shows no absorption band at ~3400 cm-1 and no absorption bands between ~1700 cm-1 and ~1600 cm-1 or between 2260 cm-1 and 2220 cm-1. What class of compound is it?

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Textbook Question

Why is the C–O absorption band of 1-hexanol at a smaller wavenumber (1060 cm–1) than the C–O absorption band of pentanoic acid (1220 cm–1)?

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