Structure elucidation examples: luteolin, kaempferol, nicotine, caffeine, glycyrrhizin MCQs With Answer

Introduction: This quiz collection focuses on structure elucidation examples of four important phytochemicals—luteolin, kaempferol, nicotine, caffeine—and the triterpenoid glycoside glycyrrhizin. Aimed at M.Pharm students studying Phytochemistry (MPG 103T), the questions integrate structural features, spectral interpretation (UV, IR, NMR, MS), biosynthetic precursors and glycosidic relationships. The set emphasizes practical clues used in laboratory identification and differentiation of structural isomers, positions of hydroxylation, characteristic fragmentation, stereochemistry and common hydrolysis products. These MCQs are designed to reinforce analytical reasoning required for structure elucidation and to connect spectroscopic patterns with chemical structure and activity.

Q1. Which functional group differentiates the flavonol kaempferol from the flavone luteolin?

• A hydroxyl group at C-3 (3-OH) present in kaempferol but absent in luteolin
• An extra carbonyl group at C-4 present in kaempferol but absent in luteolin
• An additional methoxy group on the B-ring in kaempferol
• A saturated C2–C3 bond in kaempferol while luteolin has a double bond

Correct Answer: A hydroxyl group at C-3 (3-OH) present in kaempferol but absent in luteolin

Q2. What is the molecular formula common to both luteolin and kaempferol?

• C15H12O7
• C15H10O6
• C16H12O6
• C14H10O6

Correct Answer: C15H10O6

Q3. In flavonoid UV–Vis spectroscopy, which band (wavelength region) is associated with the B-ring cinnamoyl system (Band I)?

• 240–280 nm
• 310–380 nm
• 200–220 nm
• 400–450 nm

Correct Answer: 310–380 nm

Q4. Which IR absorption is most diagnostic for the conjugated carbonyl (C=O) of flavones and flavonols?

• ~1700–1720 cm⁻¹
• ~1650–1680 cm⁻¹
• ~1550–1600 cm⁻¹
• ~1800–1850 cm⁻¹

Correct Answer: ~1650–1680 cm⁻¹

Q5. Which 1H NMR feature would help identify kaempferol but not luteolin?

• An exchangeable singlet near δ ~12 ppm attributable to a 3-OH proton
• A methoxy singlet at δ ~3.8 ppm
• A singlet for an aliphatic methyl group at δ ~1.2 ppm
• A multiplet at δ ~2.5–3.5 ppm for a methylene chain

Correct Answer: An exchangeable singlet near δ ~12 ppm attributable to a 3-OH proton

Q6. Nicotine is structurally composed of which two heterocyclic rings?

• Pyridine and pyrrolidine
• Imidazole and pyrrole
• Pyrrole and furan
• Pyridine and indole

Correct Answer: Pyridine and pyrrolidine

Q7. What is the nominal molecular ion (M+) m/z value observed for nicotine in electron impact mass spectrometry?

• 146
• 162
• 180
• 194

Correct Answer: 162

Q8. Caffeine is systematically named as which of the following?

• 1,3,7-trimethylxanthine
• 3,7-dimethylxanthine
• 1,3-dimethyluric acid
• 1,7-dimethylhypoxanthine

Correct Answer: 1,3,7-trimethylxanthine

Q9. What is the molecular ion (M+) m/z value commonly reported for caffeine by mass spectrometry?

• 136
• 150
• 194
• 222

Correct Answer: 194

Q10. Glycyrrhizin is best described as which of the following?

• A flavonoid glycoside of luteolin with one glucose unit
• A saponin-like glycoside of glycyrrhetinic acid containing two glucuronic acid units
• An alkaloid glycoside derived from nicotine with a single rhamnose
• A xanthine glycoside related to caffeine with one glucuronic acid

Correct Answer: A saponin-like glycoside of glycyrrhetinic acid containing two glucuronic acid units

Q11. What is the approximate molecular weight of glycyrrhizin (glycyrrhizic acid, aglycone plus sugars)?

• 286 g·mol⁻¹
• 162 g·mol⁻¹
• 822 g·mol⁻¹
• 194 g·mol⁻¹

Correct Answer: 822 g·mol⁻¹

Q12. Upon acid hydrolysis of glycyrrhizin, the major aglycone released is which compound?

• Quercetin
• Glycyrrhetinic acid
• Nicotine
• 7-methylxanthine

Correct Answer: Glycyrrhetinic acid

Q13. A common primary MS fragment of caffeine created by loss of a methyl radical (•CH3) would appear at which m/z?

• 179
• 165
• 149
• 138

Correct Answer: 179

Q14. Luteolin is commonly glycosylated at which hydroxyl position to give abundant naturally occurring glycosides (for example, luteolin-7-O-glucoside)?

• 3-OH
• 5-OH
• 7-OH
• 4′-OH only

Correct Answer: 7-OH

Q15. Kaempferol and luteolin are constitutional isomers. Which spectroscopic technique most reliably differentiates them by showing the presence or absence of the 3-OH substituent?

• Infrared (IR) spectroscopy
• Ultraviolet–visible (UV–Vis) spectroscopy
• Proton nuclear magnetic resonance (1H NMR) spectroscopy
• Thin-layer chromatography (TLC) with visualization

Correct Answer: Proton nuclear magnetic resonance (1H NMR) spectroscopy

Q16. Natural nicotine predominantly occurs as which stereoisomer?

• R-(+)-nicotine
• S-(−)-nicotine
• racemic mixture (50:50)
• meso form

Correct Answer: S-(−)-nicotine

Q17. Which precursor is central to the biosynthetic pathway leading to caffeine formation in plants?

• Shikimic acid
• Geranyl diphosphate
• Xanthosine
• L-phenylalanine

Correct Answer: Xanthosine

Q18. In 1H NMR spectra of flavonoids such as luteolin and kaempferol, the aromatic proton signals typically appear in which chemical shift range?

• 0.5–2.0 ppm
• 2.0–4.5 ppm
• 4.5–6.0 ppm
• 6.0–8.5 ppm

Correct Answer: 6.0–8.5 ppm

Q19. Glycyrrhetinic acid, the aglycone of glycyrrhizin, belongs to which class of natural products?

• Alkaloids
• Triterpenoids (oleanane type)
• Flavonoids
• Monoterpenes

Correct Answer: Triterpenoids (oleanane type)

Q20. To assign alpha versus beta configuration of the sugar linkage in glycyrrhizin’s glucuronic units, which NMR parameter is most informative?

• Chemical shift of methyl groups
• Coupling constant (J) of the anomeric proton
• Integration of aromatic signals
• Observation of carbonyl IR band

Correct Answer: Coupling constant (J) of the anomeric proton

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