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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 21a(1,2)
Chapter 14, Problem 21a(1,2)

a. Which occurs at a larger wavenumber:
1. a C=C stretch or a C=C stretch?
2. a C–H stretch or a C–H bend?

Verified step by step guidance
1
Step 1: Understand the concept of wavenumber in IR spectroscopy. Wavenumber is inversely proportional to the wavelength of light absorbed and is directly related to the energy of the vibration. Higher energy vibrations correspond to larger wavenumbers.
Step 2: Compare the bond strengths and masses of the atoms involved in the C=C stretch and C=C stretch. Stronger bonds and lighter atoms result in higher vibrational frequencies and thus larger wavenumbers.
Step 3: Analyze the C-H stretch and C-H bend. Stretching vibrations generally occur at higher wavenumbers than bending vibrations because stretching involves a direct change in bond length, which requires more energy compared to the angular deformation in bending.
Step 4: Recall typical IR absorption ranges. For example, C=C stretches typically appear around 1600-1680 cm⁻¹, while C-H stretches are found around 2800-3100 cm⁻¹. Bending vibrations for C-H are usually in the range of 600-1000 cm⁻¹.
Step 5: Conclude that the C=C stretch occurs at a larger wavenumber than the C=C stretch due to bond strength differences, and the C-H stretch occurs at a larger wavenumber than the C-H bend due to the energy required for stretching versus bending.

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Key Concepts

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

Wavenumber in Infrared Spectroscopy

Wavenumber is a measure of the frequency of a vibration in infrared spectroscopy, expressed in reciprocal centimeters (cm⁻¹). It is directly related to the energy of molecular vibrations, with higher wavenumbers corresponding to higher energy transitions. Understanding wavenumbers is essential for interpreting IR spectra and identifying functional groups in organic compounds.
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Types of Vibrational Modes

Molecules can undergo different types of vibrational motions, including stretching and bending. Stretching involves changes in bond lengths, while bending involves changes in bond angles. Recognizing these modes is crucial for predicting which vibrations will occur at higher wavenumbers, as stretching vibrations typically require more energy than bending vibrations.
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Comparison of C=C and C-H Vibrations

In organic compounds, the C=C double bond exhibits a stronger and more energetic stretching vibration compared to the C-H bond. Similarly, C-H stretching vibrations occur at higher wavenumbers than C-H bending vibrations. This distinction is important for analyzing IR spectra, as it helps in identifying the presence and type of functional groups in a molecule.
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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?

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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Textbook Question

Using curved arrows, show the principal fragments you would expect to see in the mass spectrum of each of the following compounds:

f.

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Textbook Question

The reaction of (Z)-2-pentene with water and a trace of H2SO4 forms two products. Identify the products from their mass spectra.

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Textbook Question

Using curved arrows, show the principal fragments you would expect to see in the mass spectrum of each of the following compounds:

d.

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