The normal C=O stretch for aldehydes is 1725, since for ketones the normal stretch is 1715, it is difficult to distinguish the two using a C=O stretch.
To distinguish aldehydes from other carbonyl-containing compounds, look for the doublet in the C-H region, near 2850 and 2750. The 2750 is more useful, because the 2850 can overlap with other C-H stretches.
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| Nonanal. The peak at 1460 is due to bending of CH2. Methylene groups often absorb stronger when attached to a cabonyl. |
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| Crotoaldehyde |
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| Benzaldehyde |
The reason the C-H splits into two peaks is due to fermi-resonance, from a coupling of the C-H stretch with the first overtone of the aldehyde C-H bend at 1400-1350.
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