Spectroscopic Analysis of Aldehydes

• IR  - presence of , CH=O

 

Absorbance (cm-1)	Interpretation
1710 -1750	C=O stretch
~2720	C-H stretch of aldehydes
~2820	C-H stretch of aldehydes

• ¹H NMR - the CH=O unit of aldehydes is the most easily recognized.

 

Resonance (ppm)	Interpretation
9.0 -10.0	CH=O (deshielding due to O and C=C)
~2.4	CH2C=O
~2.0	CH3C=O

-coupling (J, Hz) between protons in HC-CH=O is much smaller than HC-CH=C.
 
 

• ¹³C NMR

C=O typically 190-220 ppm (deshielding due to O)
• minimal intensity, characteristic of C's with no attached H's

•   UV-VIS

two absorption maxima p®p^* (< 200 nm) n®p^* (> 200 nm)
• p electron from p of C=O
• n electron from O lone pair
• p^* antibonding C=O
  
  
• 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).

Spectroscopic Analysis of Ketones

• IR  - presence of  C=O

Absorbance (cm-1)	Interpretation
1710-1750	C=O stretch

• ¹H NMR

Resonance (ppm)	Interpretation
~2.4	CH2C=O (deshielding due to C=O)
~2.0	CH3C=O  (deshielding due to C=O)


 
• ¹³C NMR

C=O typically 190-220 ppm (deshielding due to O)
• minimal intensity, characteristic of C's with no attached H's

•   UV-VIS

two absorption maxima p®p^* (< 200 nm) n®p^* (> 200 nm)
• p electron from p of C=O
• n electron from O lone pair
• p^* antibonding C=O
• Mass Spectrometry

Molecular ion, M⁺, is prominent.
Loss of alkyl radical leads to the formation of acyl cations (acylium ions), e.g.