Question

Which functional group shows very strong absorption band in IR spectroscopy?

• A. \(\text{C-H}\)
• B. \(\text{C-C}\)
• C. \(\text{C=O}\)
• D. \(\text{C=C}\)

Hint:

The carbonyl \(\text{C=O}\) stretch is often called the "king of IR peaks." It is highly reliable, exceptionally sharp, and consistently displays a very strong absorption signature centered around \(\approx 1700\text{ cm}^{-1}\).

Answer 1

The Correct Option is C

Concept: The intensity of an infrared absorption band depends directly on the net change in the dynamic dipole moment of a bond during its vibrational mode. Bonds with a large permanent dipole moment show much larger dipole fluctuations during stretching vibrations, yielding highly intense, sharp absorption bands.

Step 1: Evaluate the Carbonyl group (\(\text{C=O}\)) The carbonyl group (\(\text{C=O}\)) contains a double bond between a carbon atom and a highly electronegative oxygen atom. This electronegativity difference strongly polarizes the bond, localizing significant electron density on the oxygen atom: \[ ^{\delta+}\text{C} = \text{O}^{\delta-} \] Because of this permanent polarization, even a tiny change in the bond length during a stretching vibration causes a massive shift in the overall dipole moment. As a result, the carbonyl stretch produces an exceptionally strong, sharp, signature peak in the \(1650-1750\text{ cm}^{-1}\) region of an IR spectrum, making it one of the easiest functional groups to identify.

Step 2: Evaluate the other choices
• \(\text{C-C}\) and \(\text{C=C}\): These homonuclear bonds share electrons relatively symmetrically. Unless an asymmetric functional group is attached nearby, stretching vibrations produce little to no change in the dipole moment, resulting in weak or completely absent peaks.
• \(\text{C-H}\): While polar enough to produce clear peaks in the \(2850-3000\text{ cm}^{-1}\) region, the electronegativity difference between carbon (2.5) and hydrogen (2.2) is small (\(0.3\)), so its absorption intensity is generally moderate compared to the highly polar carbonyl group.