Step 1: Recall what IR spectroscopy measures.
Infrared (IR) spectroscopy detects the vibration of chemical bonds. Each type of bond absorbs IR radiation at a characteristic wavenumber (cm-1).
Step 2: Identify the carbonyl group.
The carbonyl group (C=O) is a strong, polar double bond between carbon and oxygen. Its stretching vibration is one of the most recognised absorptions in IR spectroscopy.
Step 3: Locate the carbonyl absorption region.
The C=O stretching vibration absorbs strongly around 1700 cm-1. This region is in the middle of the IR spectrum and is called the carbonyl region. The exact position shifts slightly depending on the compound class (ketone ~1715, aldehyde ~1725, amide ~1650, ester ~1735).
Step 4: Rule out the distractors.
1000 cm-1 corresponds to C-O single bond or C-H bending vibrations. 1500 cm-1 corresponds to N-H bending or aromatic C=C ring vibrations. 3000 cm-1 corresponds to C-H or O-H stretching, not C=O.
Step 5: Confirm the answer.
The 1700 cm-1 region is a hallmark for C=O identification in IR analysis, making it an important fact in pharmaceutical analysis for identifying functional groups in drug molecules.
Answer: Option (3) — 1700 cm-1