onsider the IR spectra for 9-flueorenol that are shown below. Which compound belongs to spectrum A? Which compound belongs to spectrum B? a) in the functional group region of the 9-fluorenone spectrum, identify the absorptions associated with the carbonyl group and with the aromatic rings. b) in the functional group region of the 9-fluorenol spectrum, specify the functional group that is associated with the broad absorption at about 3100-3400 cm^-1.
The Correct Answer and Explanation is:
To correctly assign IR spectra to 9-fluorenol and 9-fluorenone, we must understand the characteristic IR absorption bands of their functional groups.
Correct Answer:
- Spectrum A: Belongs to 9-fluorenone
- Spectrum B: Belongs to 9-fluorenol
Explanation (300+ words):
Infrared (IR) spectroscopy is a powerful tool for identifying functional groups in organic compounds. Each type of bond absorbs IR radiation at specific frequencies, measured in wavenumbers (cm⁻¹). Let’s examine each compound and their expected IR features:
a) 9-Fluorenone (Spectrum A)
9-Fluorenone contains a ketone functional group (C=O) and aromatic rings.
- Carbonyl (C=O) Stretch:
The strong absorption around 1710–1740 cm⁻¹ is characteristic of a C=O bond found in ketones. In aromatic ketones like 9-fluorenone, this usually appears at around 1715 cm⁻¹. This peak is sharp and intense, making it one of the easiest to identify in the IR spectrum. - Aromatic C=C Stretches:
These appear as multiple peaks in the region of 1450–1600 cm⁻¹, due to the conjugated π-system in the aromatic rings. Although weaker than the carbonyl peak, their pattern is typical for aromatic compounds.
Therefore, the presence of a strong C=O peak and aromatic C=C peaks confirms that Spectrum A is 9-fluorenone.
b) 9-Fluorenol (Spectrum B)
9-Fluorenol is an alcohol, with a hydroxyl group (-OH) at the 9-position on the fluorene skeleton.
- Broad O–H Stretch:
A broad absorption between 3200–3600 cm⁻¹ is characteristic of hydrogen-bonded O–H stretching. This is a hallmark of alcohols. In 9-fluorenol, this peak typically appears between 3100–3400 cm⁻¹, confirming the presence of an -OH group. - Aromatic C–H and C=C Stretches:
Similar to 9-fluorenone, 9-fluorenol also has aromatic rings, showing medium-to-weak peaks around 1450–1600 cm⁻¹ (C=C stretching) and around 3030 cm⁻¹ for aromatic C–H stretches.
Importantly, Spectrum B lacks the strong C=O peak, differentiating it from 9-fluorenone.
Conclusion:
- Spectrum A has a strong carbonyl peak → 9-fluorenone
- Spectrum B has a broad O–H stretch → 9-fluorenol
Understanding the position and shape of these peaks allows chemists to reliably determine the identity of compounds using IR spectra.