Here's why:
* The carbonyl group (C=O) is a strong absorber of infrared radiation. The stretching vibration of the C=O bond is a very characteristic and intense absorption in the IR spectrum.
* The position of this absorption band is influenced by the surrounding structure. For aldehydes, the carbonyl group is attached to a hydrogen atom and an alkyl group, which slightly shifts the absorption to a higher frequency (compared to ketones, for example).
Note: This is a general guideline. The exact frequency can vary slightly depending on the specific aldehyde and other factors like conjugation, hydrogen bonding, and steric effects.
