Flavonoids: Isolation, structure determination and quercetin case study MCQs With Answer

Flavonoids: Isolation, structure determination and quercetin case study MCQs With Answer

This collection of multiple-choice questions is designed for M.Pharm students studying MPC 104T: Chemistry of Natural Products. It focuses on practical and analytical aspects of flavonoid chemistry — from extraction and isolation through chromatographic and spectroscopic structure elucidation — with a specific case study on quercetin. The questions emphasize methods used in the laboratory (TLC, HPLC, MS, NMR, IR), interpretation of spectral data, and features that distinguish flavonoid subclasses and glycosides. Use these MCQs to test understanding of experimental strategies, diagnostic tests, and characteristic spectral patterns needed for identifying and characterizing flavonoids in research and quality control.

Q1. What is the core structural skeleton common to flavonoids?

• C6-C2-C6
• C6-C3-C6
• C5-C3-C5
• C6-C4-C6

Correct Answer: C6-C3-C6

Q2. Which solvent system is most commonly used for initial extraction of flavonoids from plant material?

• n-Hexane
• 70% Methanol (aqueous methanol)
• Chloroform
• Diethyl ether

Correct Answer: 70% Methanol (aqueous methanol)

Q3. Acid hydrolysis of flavonoid glycosides typically yields which products?

• Anthocyanidin and methylated sugar
• Aglycone and free sugar
• Only sugar fragments
• Cleaved aromatic rings

Correct Answer: Aglycone and free sugar

Q4. Which TLC spray reagent is highly diagnostic for detecting flavonoids by producing fluorescent complexes with flavonols?

• Dragendorff reagent
• NP/PEG reagent (diphenylboric acid 2-aminoethyl ester followed by PEG)
• Anisaldehyde-sulfuric acid
• ninhydrin

Correct Answer: NP/PEG reagent (diphenylboric acid 2-aminoethyl ester followed by PEG)

Q5. In UV-visible spectroscopic shift reagents for flavonols, which reagent produces a characteristic bathochromic shift indicating a free 3-hydroxyl group?

• AlCl3
• NaOMe (sodium methoxide)
• HCl
• FeCl3

Correct Answer: NaOMe (sodium methoxide)

Q6. Which set of hydroxylation positions correctly describes quercetin (aglycone)?

• 3, 5, 7, 3′, 4′
• 5, 6, 7, 2′, 4′
• 3, 4, 5, 2′, 4′
• 2, 3, 4, 3′, 5′

Correct Answer: 3, 5, 7, 3′, 4′

Q7. What is the approximate molecular weight (MW) of quercetin (aglycone)?

• 302.24 g/mol
• 316.27 g/mol
• 610.52 g/mol
• 286.24 g/mol

Correct Answer: 302.24 g/mol

Q8. In LC-MS analysis of flavonoid glycosides, loss of which nominal mass typically indicates cleavage of a hexose (e.g., glucose)?

• 146 Da
• 132 Da
• 162 Da
• 308 Da

Correct Answer: 162 Da

Q9. The B-ring 1H NMR pattern for quercetin (3′,4′-dihydroxy substitution) typically appears as which pattern?

• A singlet near 7.2 ppm
• Two doublets (orthogonal) around 7.6 and 6.9 ppm
• A triplet and a doublet between 6.0–6.5 ppm
• Complex multiplet centered at 5.5 ppm

Correct Answer: Two doublets (orthogonal) around 7.6 and 6.9 ppm

Q10. In 13C NMR of flavonols, which approximate chemical shift corresponds to the C=O carbon at C4?

• ~120 ppm
• ~145 ppm
• ~176 ppm
• ~200 ppm

Correct Answer: ~176 ppm

Q11. Rutin (quercetin-3-O-rutinoside) has an approximate molecular weight of:

• 302 g/mol
• 610 g/mol
• 480 g/mol
• 450 g/mol

Correct Answer: 610 g/mol

Q12. Which derivatization method is commonly used to render flavonoids volatile for GC-MS analysis?

• Methylation with diazomethane only
• Trimethylsilylation (TMS derivatives using BSTFA or MSTFA)
• Acetylation is always sufficient without silylation
• No derivatization is possible for flavonoids

Correct Answer: Trimethylsilylation (TMS derivatives using BSTFA or MSTFA)

Q13. Which flavonoid subclass is characterized by a saturated C2–C3 bond (no C2=C3 double bond)?

• Flavonol
• Chalcone
• Flavanone
• Anthocyanidin

Correct Answer: Flavanone

Q14. What is the most commonly used stationary phase for normal-phase column chromatography in flavonoid isolation?

• Silica gel
• Ion-exchange resin
• Reverse-phase C18
• Polyethylene glycol

Correct Answer: Silica gel

Q15. For HPLC analysis of quercetin and its glycosides, which mobile phase and column combination is most typical?

• Normal-phase silica with hexane/isopropanol
• Reverse-phase C18 column with water–acetonitrile (0.1% formic acid)
• Ion-pairing HPLC with sodium dodecyl sulfate
• Size-exclusion column with pure water

Correct Answer: Reverse-phase C18 column with water–acetonitrile (0.1% formic acid)

Q16. Which 2D NMR experiment is most useful for detecting long-range (2–3 bond) heteronuclear correlations to help assign flavonoid carbon skeleton?

• HSQC (Heteronuclear Single Quantum Coherence)
• NOESY
• HMBC (Heteronuclear Multiple Bond Correlation)
• COSY

Correct Answer: HMBC (Heteronuclear Multiple Bond Correlation)

Q17. In IR spectroscopy, which band is diagnostic of phenolic OH groups in flavonoids?

• Sharp band near 1700 cm⁻¹
• Broad band around 3200–3550 cm⁻¹
• Strong absorption at 2100 cm⁻¹
• Band at 1450 cm⁻¹ only

Correct Answer: Broad band around 3200–3550 cm⁻¹

Q18. When quercetin forms a complex with AlCl3 for UV or TLC detection, it typically displays what visual response under UV light?

• Red fluorescence
• Yellow fluorescence
• No change
• Green precipitate

Correct Answer: Yellow fluorescence

Q19. Which type of flavonoid glycoside linkage is generally resistant to acid hydrolysis and requires stronger conditions or C–C bond cleavage to be broken?

• O-glycoside
• N-glycoside
• C-glycoside
• S-glycoside

Correct Answer: C-glycoside

Q20. In UV spectra of flavonols like quercetin, Band I (associated with the B-ring cinnamoyl system) typically appears near which wavelength?

• ~255 nm
• ~300 nm
• ~370 nm
• ~210 nm

Correct Answer: ~370 nm

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