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Chapter 17: Aldehydes and Ketones. Nucleophilic Addition
to C=O
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Spectroscopic analyzis of Aldehydes
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Absorbance (cm-1)
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Interpretation
|
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1710 -1750
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C=O stretch
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~2720
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C-H stretch of aldehydes
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~2820
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C-H stretch of aldehydes
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-
1H NMR - the CH=O unit of
aldehydes is the most easily recognised.
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Resonance (ppm)
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Interpretation
|
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9.0 -10.0
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CH=O (deshielding due to O and C=C)
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~2.4
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CH2C=O
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~2.0
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CH3C=O
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-coupling (J, Hz) between protons in HC-CH=O is much smaller than HC-CH=C.
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13C NMR
C=O typically 190-220 ppm (deshielding due to O)
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minimal intensity, characteristic of C's with no attached H's
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UV-VIS
two absorption maxima p®p*
(< 200 nm) n®p*
(> 200 nm)
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p electron
from p
of C=O
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n electron from O lone pair
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p* antibonding
C=O
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Mass Spectrometry
Molecular ion, M+, is prominent and typically an M-1 peak.
Loss of alkyl radical leads to the formation of acyl cations (acylium
ions).
