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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 20d
Chapter 13, Problem 20d

For each pair, choose the molecule that you expect to have the highest wavenumber for its C=O stretch.
(d) Two molecular structures compared: a six-membered ring with C=O vs. a six-membered ring with C=O and NH.

Verified step by step guidance
1
Understand that the C=O stretch in infrared spectroscopy is influenced by factors such as bond strength and the surrounding chemical environment. A stronger bond typically results in a higher wavenumber.
Consider the electronic effects on the carbonyl group. Electron-withdrawing groups can increase the bond strength of the C=O bond, leading to a higher wavenumber.
Evaluate the resonance effects. If a molecule has resonance structures that delocalize the electrons away from the carbonyl group, this can weaken the C=O bond and lower the wavenumber.
Assess the steric effects. Bulky groups near the carbonyl can affect the bond angle and length, potentially influencing the wavenumber.
Compare the molecules in the pair based on these factors to determine which one has the stronger C=O bond and thus the higher wavenumber for its C=O stretch.

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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 functional groups in molecules by measuring the absorption of infrared light, which causes molecular vibrations. The C=O stretch is a common feature in IR spectra, typically appearing around 1700 cm⁻¹. The exact position can vary based on molecular environment and bonding.
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Wavenumber and Bond Strength

Wavenumber, measured in cm⁻¹, is inversely related to wavelength and directly related to the energy of the bond vibration. A higher wavenumber indicates a stronger bond, as more energy is required to stretch it. In the context of C=O bonds, factors like resonance, inductive effects, and hybridization can influence bond strength and thus the wavenumber.
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Resonance and Inductive Effects

Resonance and inductive effects can alter the electron density around a C=O bond, affecting its strength and the corresponding IR wavenumber. Resonance can delocalize electrons, often weakening the bond and lowering the wavenumber. Inductive effects, from electronegative atoms or groups, can increase electron withdrawal, strengthening the bond and increasing the wavenumber.
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Related Practice
Textbook Question

Would you expect an acetylenic C―H to absorb at a higher or lower wavenumber than the C―H in ethene?

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Textbook Question

The IR spectrum for anisole contains two C―O stretching bands in the fingerprint region. Match the band to the bond that gives rise to it. Why are these bands so intense?

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Textbook Question

The molecule that gave the mass spectrum shown here contains a halogen. Which halogen is present?

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Textbook Question

Match each of the four IR spectra to one of the given compounds. [One of the compounds does not match a spectrum.]

(c) <IMAGE>

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Textbook Question

Match each of the four IR spectra to one of the given compounds. [One of the compounds does not match a spectrum.]

(d) <IMAGE>

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Textbook Question

Calculate the reduced mass for the following bonds.

(c) C―Cl

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