Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 14 - Structural Identification 1: Infrared Spectroscopy and Mass Spectrometry

문제 18a

13장, 문제 18a

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

검증된 단계별 안내

Understand the concept of wavenumber in IR spectroscopy: Wavenumber is inversely proportional to wavelength and is directly related to the energy of the bond vibration. Higher wavenumbers correspond to higher energy vibrations.

Identify the types of C―H bonds in question: An acetylenic C―H bond is found in alkynes, where the carbon is sp-hybridized. In ethene, the C―H bond is found in alkenes, where the carbon is sp2-hybridized.

Consider the hybridization of the carbon atoms: sp-hybridized carbons (as in alkynes) have more s-character (50%) compared to sp2-hybridized carbons (33% s-character) in alkenes. More s-character results in a stronger, shorter bond.

Relate bond strength to wavenumber: Stronger bonds typically absorb at higher wavenumbers because they require more energy to vibrate.

Conclude based on bond strength and hybridization: Since the acetylenic C―H bond is stronger due to higher s-character, it will absorb at a higher wavenumber compared to the C―H bond in ethene.

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이 영상 해법은 위 문제에 도움이 된다고 튜터들이 추천한 것입니다.

주요 개념

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Infrared Spectroscopy

Infrared spectroscopy is a technique used to identify functional groups in organic molecules by measuring the absorption of infrared light, which causes molecular vibrations. Different bonds absorb at characteristic wavenumbers, allowing for the identification of specific types of bonds, such as C―H, C=C, and C≡C, based on their unique vibrational frequencies.

Bond Strength and Wavenumber

The wavenumber at which a bond absorbs infrared light is related to the bond strength and the masses of the atoms involved. Stronger bonds, such as triple bonds, typically absorb at higher wavenumbers compared to double or single bonds. This is because stronger bonds require more energy to vibrate, resulting in higher frequency and wavenumber absorption.

Acetylenic vs. Ethylenic C―H Bonds

Acetylenic C―H bonds, found in alkynes, are associated with sp-hybridized carbon atoms, while ethylenic C―H bonds, found in alkenes, involve sp2-hybridized carbons. The sp-hybridized C―H bond is stronger and shorter than the sp2-hybridized C―H bond, leading to a higher wavenumber absorption for acetylenic C―H in infrared spectroscopy compared to ethylenic C―H.