Textbook Question
Which of the following compounds has a vibration that is infrared inactive?
1-butyne, 2-butyne, H2, H2O, Cl2, ethene
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15. Analytical Techniques:IR, NMR, Mass Spect
Infrared Spectroscopy
4:05 minutesProblem 21a(3,4)
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?
Verified step by step guidance1
Understand the concept of bond stretching frequencies in IR spectroscopy: The wavenumber of a bond stretch in IR spectroscopy depends on the bond strength and the masses of the atoms involved. Stronger bonds and lighter atoms result in higher wavenumbers.
For part 3: Compare the bond strengths of C-N and C=N. A double bond (C=N) is stronger than a single bond (C-N), so the C=N stretch will occur at a larger wavenumber than the C-N stretch.
For part 4: Compare the bond strengths of C=O and C-O. A double bond (C=O) is stronger than a single bond (C-O), so the C=O stretch will occur at a larger wavenumber than the C-O stretch.
Relate the bond strength to the vibrational frequency: Stronger bonds vibrate at higher frequencies, which correspond to larger wavenumbers in the IR spectrum.
Summarize the findings: The C=N stretch occurs at a larger wavenumber than the C-N stretch, and the C=O stretch occurs at a larger wavenumber than the C-O stretch.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Wavenumber and Infrared Spectroscopy
Wavenumber is a measure of the frequency of a wave, typically expressed in cm⁻¹, and is commonly used in infrared (IR) spectroscopy to identify molecular vibrations. In IR spectroscopy, different types of chemical bonds absorb infrared light at characteristic wavenumbers, allowing for the identification of functional groups in a molecule.
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Bond Strength and Stretching Frequencies
The strength of a chemical bond influences the frequency at which it vibrates. Stronger bonds, such as double bonds (C=N, C=O), generally stretch at higher wavenumbers compared to weaker bonds, like single bonds (C-N, C-O). This is due to the greater energy required to stretch stronger bonds, resulting in higher vibrational frequencies.
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Functional Group Identification
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Identifying functional groups through their IR absorption peaks allows chemists to deduce the structure and reactivity of organic compounds, making it essential for understanding molecular behavior.
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