Textbook Question
For each of the following pairs of compounds, identify one IR absorption band that could be used to distinguish between them:
i.
j.
15. Analytical Techniques:IR, NMR, Mass Spect
Infrared Spectroscopy Table
6:18 minutesProblem 29a,b
Textbook Question
How can IR spectroscopy be used to distinguish between the following compounds?
a. a ketone and an aldehyde
b. a cyclic ketone and an open-chain ketone
Verified step by step guidance1
Identify the functional groups present in each compound. A ketone contains a carbonyl group (C=O) bonded to two alkyl or aryl groups, while an aldehyde contains a carbonyl group (C=O) bonded to at least one hydrogen atom. Cyclic ketones have the carbonyl group within a ring structure, whereas open-chain ketones have the carbonyl group in a linear or branched chain.
Understand the characteristic IR absorption frequencies for the functional groups. The carbonyl group (C=O) typically absorbs strongly in the range of 1700–1750 cm⁻¹. Aldehydes also exhibit additional characteristic peaks due to the C-H bond stretching of the aldehyde group, which appear as two weak bands around 2700–2900 cm⁻¹.
To distinguish between a ketone and an aldehyde, look for the presence of the aldehyde C-H stretching peaks around 2700–2900 cm⁻¹ in the IR spectrum. These peaks will be absent in the spectrum of a ketone.
To distinguish between a cyclic ketone and an open-chain ketone, examine the exact position of the carbonyl (C=O) stretching peak. The ring strain in cyclic ketones can shift the C=O stretching frequency slightly higher (closer to 1750 cm⁻¹) compared to open-chain ketones, which typically absorb closer to 1715 cm⁻¹. Smaller rings (e.g., cyclopropanone) exhibit even higher frequencies due to increased ring strain.
Compare the IR spectra of the compounds in question, focusing on the regions mentioned above. Use the presence or absence of aldehyde C-H stretching peaks and the position of the carbonyl stretching peak to make the distinctions.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Infrared (IR) Spectroscopy
IR spectroscopy is an analytical technique used to identify functional groups in organic compounds by measuring the absorption of infrared light. Different bonds and functional groups absorb characteristic wavelengths of IR radiation, producing a spectrum that serves as a molecular 'fingerprint' for the compound.
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General Features of IR Spect
Functional Group Identification
In organic chemistry, functional groups are specific groups of atoms within molecules that determine their chemical reactivity and properties. For instance, aldehydes and ketones both contain carbonyl groups (C=O), but they can be distinguished by their unique IR absorption patterns, with aldehydes typically showing a C-H stretch around 2720-2820 cm⁻¹.
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Structural Differences in Ketones and Aldehydes
The structural differences between ketones and aldehydes are crucial for their identification. Aldehydes have a carbonyl group at the end of a carbon chain, while ketones have it within the chain. This positional difference affects their IR spectra, allowing for differentiation based on the presence of specific absorption peaks.
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03:08The difference between aldehydes and ketones.
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