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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 15b
Chapter 13, Problem 15b

Choose the bond in each pair that you expect to vibrate at the higher wavenumber.
(b) C―H vs. C―O

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1
Understand that the wavenumber in infrared (IR) spectroscopy is related to the frequency of vibration of a bond. The wavenumber is inversely proportional to the mass of the atoms involved and directly proportional to the bond strength.
Recall that lighter atoms and stronger bonds tend to vibrate at higher frequencies, resulting in higher wavenumbers.
Consider the atomic masses: Hydrogen (H) is lighter than Oxygen (O), so C―H bonds involve a lighter atom compared to C―O bonds.
Evaluate the bond strengths: C―H bonds are generally stronger than C―O bonds due to the higher electronegativity difference and shorter bond length.
Conclude that the C―H bond will vibrate at a higher wavenumber than the C―O bond because it involves a lighter atom and a stronger bond.

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Key Concepts

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

Bond Vibrational Frequency

Bond vibrational frequency refers to the energy required for a bond to stretch or compress during vibration. It is influenced by the mass of the atoms and the strength of the bond. Lighter atoms and stronger bonds typically vibrate at higher frequencies, resulting in higher wavenumbers in infrared spectroscopy.
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Mass of Atoms

The mass of the atoms involved in a bond affects its vibrational frequency. Lighter atoms, such as hydrogen, contribute to higher vibrational frequencies compared to heavier atoms like oxygen. This is because lighter atoms can move more easily, requiring less energy to vibrate, thus increasing the wavenumber.
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Bond Strength

Bond strength is a measure of how strongly two atoms are held together in a molecule. Stronger bonds, such as C-H, typically vibrate at higher frequencies than weaker bonds like C-O. This is due to the greater energy required to stretch or compress a stronger bond, leading to higher wavenumbers in spectroscopic analysis.
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Related Practice
Textbook Question

Choose the bond in each pair that you expect to vibrate at the higher wavenumber.

(a) C―N vs. C = N

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Textbook Question

Given the IR spectrum, suggest what functional groups might correspond to the given molecular formula.

(b) C3H7NO

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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

Calculate the reduced mass for the following bonds.

(a) C―H

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Textbook Question

Calculate the reduced mass for the following bonds.

(c) C―Cl

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Textbook Question

Given the IR spectrum, suggest what functional groups might correspond to the given molecular formula.

(d) C₇H₅O

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