Quercetin (C15H10O7) is a naturally occurring flavonoid widely distributed in fruits, vegetables, and medicinal plants. Its structure has been elucidated using various spectroscopic techniques and chemical reactions.
1. Molecular Formula
The molecular formula of Quercetin is C15H10O7, indicating a flavonoid skeleton with multiple hydroxyl groups.
2. Hydrolysis Reaction
Quercetin glycosides undergo hydrolysis in the presence of acid to yield Quercetin and a sugar molecule (rhamnose):
This reaction confirms that Quercetin exists as an aglycone part of its glycoside.
3. UV-Visible Spectroscopy
Quercetin shows characteristic absorption bands in the UV region due to its conjugated aromatic system:
- Band I (~370 nm): Associated with the cinnamoyl system.
- Band II (~255 nm): Associated with the benzoyl system.
4. IR Spectroscopy
IR spectrum confirms the presence of functional groups:
- Broad band around 3400 cm-1: O–H stretching (hydroxyl groups).
- Strong band near 1660 cm-1: C=O stretching (carbonyl group).
- Bands around 1600 cm-1: Aromatic C=C stretching.
5. NMR Spectroscopy
¹H NMR and ¹³C NMR provide detailed insights:
- Aromatic proton signals confirm the flavonoid ring system.
- Signals corresponding to hydroxyl protons are observed downfield due to hydrogen bonding.
- Carbon signals confirm the presence of conjugated aromatic carbons and carbonyl carbon.
6. Mass Spectrometry
The molecular ion peak at m/z 302 confirms the molecular weight of Quercetin. Fragmentation patterns support the flavonoid skeleton.
7. Position of Hydroxyl Groups
Several chemical reactions help define the hydroxyl content and positions:
- Acetylation & Methylation: Quercetin forms both penta-acetyl and penta-methyl derivatives, indicating the presence of five hydroxyl groups.
- Phenolic Nature: The solubility of quercetin in alkali solutions confirms the presence of acidic phenolic hydroxyl groups.
- KOH Fusion Degradation: Degradation via fusion with KOH breaks the molecule into two distinct fragments, which provides definitive evidence for its core structural components:
| Product | Structural Insight |
|---|---|
| Phloroglucinol | Confirms the A-ring structure with hydroxyl groups at the 5 and 7 positions. |
| Protocatechuic Acid | Confirms the B-ring structure as a 3', 4'-dihydroxy substituted phenyl ring. |
Based on chemical degradation and spectral data, Quercetin contains hydroxyl groups at positions 3, 5, 7, 3′, and 4′.
8. Final Structure
Based on the combined evidence, Quercetin is identified as 3,3′,4′,5,7-pentahydroxyflavone, a flavonol with hydroxyl groups at positions 3, 5, 7, 3′, and 4′.
Frequently Ask Questions
What is the molecular formula of Quercetin?
The molecular formula of Quercetin is C15H10O7.
What happens when Quercetin glycosides undergo hydrolysis?
On hydrolysis with acid, Quercetin glycosides yield Quercetin and rhamnose sugar.
Which spectroscopic techniques are used in structural elucidation of Quercetin?
UV-Visible, IR, NMR, and Mass Spectroscopy are used to confirm the structure of Quercetin.
Where are the hydroxyl groups located in Quercetin?
Quercetin has hydroxyl groups at positions 3, 5, 7, 3′, and 4′.