Question

which of the structures have a major peak between 1700 and 3000 cm⁻¹? (select all that apply).

Explanation:

Step1: Recall IR - absorption ranges

The carbon - oxygen double bond ($C=O$) in acyl chlorides has an IR absorption around 1700 - 1800 $cm^{-1}$. The $O - H$ bond in phenols has a broad absorption peak in the range of 3200 - 3600 $cm^{-1}$. Nitro groups ($-NO_2$) have absorption peaks around 1500 - 1600 $cm^{-1}$ and 1300 - 1400 $cm^{-1}$.

Step2: Analyze each structure

• The first structure is an acyl chloride ($R - COCl$). The $C=O$ stretch of an acyl chloride is around 1750 $cm^{-1}$, which is in the 1700 - 3000 $cm^{-1}$ range.
• The second structure is a phenol ($Ar - OH$). The $O - H$ stretch of a phenol is above 3000 $cm^{-1}$ (3200 - 3600 $cm^{-1}$).
• The fourth structure is a nitro - compound. The $N=O$ stretches of a nitro - compound are around 1500 - 1600 $cm^{-1}$ and 1300 - 1400 $cm^{-1}$.

Answer:

First structure (acyl - chloride)