Question

In the IR analysis, aldehydes are distinguished from C=O containing compounds due to _______

• A. \( \text{Low frequency of absorption of aldehydes} \)
• B. \( \text{Doublet at the C-H stretching region} \)
• C. \( \text{Presence of double bond} \)
• D. \( \text{Attachment of alkyl or aryl group to } >\text{C=O} \)

Hint:

Remember the "aldehyde H" gives a special "double tap" signal in the IR spectrum around 2700-2800 cm$^{-1}$.

Answer 1

The Correct Option is B

Aldehydes possess a carbonyl group (C=O), similar to ketones, carboxylic acids, esters, and amides, which all exhibit a strong C=O stretching absorption in the IR spectrum around 1700-1750 cm$^{-1}$. However, aldehydes are unique due to the presence of a hydrogen atom directly bonded to the carbonyl carbon (the aldehyde proton). This C-H bond in aldehydes gives rise to characteristic C-H stretching absorptions in the IR spectrum as a doublet (two bands) around 2720 cm$^{-1}$ and 2820 cm$^{-1}$. These two distinct peaks in the C-H stretching region are a key feature that distinguishes aldehydes from other C=O containing compounds that lack this specific C-H bond directly attached to the carbonyl carbon.