Alkaloids: Classification, isolation, purification and structural determination MCQs With Answer

Introduction: Alkaloids: Classification, isolation, purification and structural determination MCQs With Answer is a focused revision resource tailored for M.Pharm students studying natural product chemistry. This set of questions emphasizes structural categories of alkaloids, practical isolation and purification strategies (acid-base extraction, chromatography, crystallization) and advanced structural elucidation techniques (IR, MS, NMR, UV, chemical degradation and derivatization). The MCQs test both theoretical concepts and applied laboratory decision-making, including reagent selection, diagnostic chemical tests and interpretation of spectroscopic data. Use these questions to strengthen core understanding, prepare for exams and refine problem-solving skills in alkaloid analysis and characterization.

Q1. Which biosynthetic precursor class is most commonly associated with indole alkaloids?

• Tryptophan
• Tyrosine
• Ornithine
• Aspartic acid

Correct Answer: Tryptophan

Q2. In acid-base extraction of plant material containing alkaloids, what is the main reason for converting alkaloids to their salts during initial extraction?

• To increase volatility for GC analysis
• To make alkaloids more water-soluble for separation from non‑basic materials
• To prevent oxidation during workup
• To change their UV absorption for easier detection

Correct Answer: To make alkaloids more water-soluble for separation from non‑basic materials

Q3. Which reagent is commonly used as a presumptive test for tertiary alkaloids, producing a red-brown precipitate?

• Mayer’s reagent
• Dragendorff’s reagent
• Wagner’s reagent
• Ferric chloride

Correct Answer: Wagner’s reagent

Q4. Which structural feature distinguishes isoquinoline alkaloids from indole alkaloids?

• Presence of a benzopyrrole (indole) nucleus
• Presence of a benzylisoquinoline or isoquinoline ring system derived from tyrosine
• Presence of a tropane bicyclic system
• Presence of a pyrrolizidine bicyclic ring

Correct Answer: Presence of a benzylisoquinoline or isoquinoline ring system derived from tyrosine

Q5. Which chromatographic technique is most appropriate for preparative isolation of large amounts of a single alkaloid from crude extract?

• Analytical thin-layer chromatography (TLC)
• Preparative column chromatography (silica gel or reverse phase)
• Gas chromatography–mass spectrometry (GC-MS)
• Paper chromatography

Correct Answer: Preparative column chromatography (silica gel or reverse phase)

Q6. Which spectroscopic method gives direct information about the types and numbers of hydrogen atoms and their connectivity in an alkaloid?

• Infrared (IR) spectroscopy
• 1H Nuclear Magnetic Resonance (1H NMR) spectroscopy
• Ultraviolet-visible (UV-Vis) spectroscopy
• Mass spectrometry (MS)

Correct Answer: 1H Nuclear Magnetic Resonance (1H NMR) spectroscopy

Q7. During purification, converting an alkaloid free base to its hydrochloride salt is often performed to:

• Increase its volatility for GC
• Increase water solubility and crystallinity for easier purification
• Cause immediate decomposition for analysis
• Lower the melting point for characterization

Correct Answer: Increase water solubility and crystallinity for easier purification

Q8. Which mass spectrometric fragment information is most useful for determining the presence of a tertiary amine in an alkaloid?

• Base peak at m/z 18 (water)
• Prominent fragment corresponding to loss of an alkyl group (M‑CH3)+ or cleavage alpha to nitrogen
• Strong molecular ion only with no fragments
• Isotopic pattern indicating presence of chlorine

Correct Answer: Prominent fragment corresponding to loss of an alkyl group (M‑CH3)+ or cleavage alpha to nitrogen

Q9. Ehrlich’s reagent is a specific color test primarily used to detect which class of alkaloids?

• Quinoline alkaloids
• Indole alkaloids
• Pyrrolizidine alkaloids
• Tropane alkaloids

Correct Answer: Indole alkaloids

Q10. Which of the following is the best chromatographic choice to separate very polar alkaloids that are difficult to retain on normal-phase silica gel?

• Normal-phase silica gel column using hexane
• Reverse-phase HPLC (C18) with aqueous mobile phase
• Gas chromatography with nonpolar column
• Paper chromatography with water as solvent

Correct Answer: Reverse-phase HPLC (C18) with aqueous mobile phase

Q11. Which chemical degradation approach is commonly used to determine the ring junctions and substituent positions in complex alkaloids?

• Hydrogenation only
• Partial chemical degradation (e.g., oxidative cleavage, hydrolysis) followed by analysis of fragments
• Simple melting point determination
• Direct sequencing by Edman degradation

Correct Answer: Partial chemical degradation (e.g., oxidative cleavage, hydrolysis) followed by analysis of fragments

Q12. Which optical measurement is most informative when trying to assign absolute configuration of a chiral alkaloid in combination with other methods?

• Melting point
• Specific rotation (optical rotation) and CD (circular dichroism)
• UV-Vis lambda-max only
• Infrared carbonyl stretching frequency

Correct Answer: Specific rotation (optical rotation) and CD (circular dichroism)

Q13. Which isolation solvent is typically preferred to extract free base alkaloids from organic phase after basification of aqueous extracts?

• Water
• Diethyl ether or dichloromethane
• Methanol with 1% acid
• Petroleum ether only

Correct Answer: Diethyl ether or dichloromethane

Q14. In 13C NMR spectra of alkaloids, quaternary carbons are often identified by which technique?

• Proton decoupling only
• DEPT experiments (DEPT-90 and DEPT-135) where quaternary carbons give no signal
• Using only IR spectroscopy
• Measuring melting point shifts

Correct Answer: DEPT experiments (DEPT-90 and DEPT-135) where quaternary carbons give no signal

Q15. Which of the following is a reliable TLC visualization technique for alkaloids after chromatography?

• Exposure to iodine vapor only
• Spraying with Dragendorff’s reagent to reveal alkaloid spots
• Looking under daylight only
• Measuring spot pH with indicator paper

Correct Answer: Spraying with Dragendorff’s reagent to reveal alkaloid spots

Q16. A student isolates an alkaloid and obtains an IR spectrum with a strong absorption at ~1700 cm−1. Which functional group does this most likely indicate in the alkaloid?

• Primary amine
• Carbonyl group (ketone or ester)
• Aromatic C–H stretch
• Aliphatic C–C single bond

Correct Answer: Carbonyl group (ketone or ester)

Q17. Which structural class of alkaloids contains the characteristic bicyclic tropane skeleton?

• Isoquinoline alkaloids
• Tropane alkaloids (e.g., atropine, cocaine)
• Pyrrolizidine alkaloids
• Indole alkaloids

Correct Answer: Tropane alkaloids (e.g., atropine, cocaine)

Q18. When elucidating structure by NMR, which 2D experiment helps correlate proton and directly bonded carbon atoms, aiding assignment in alkaloids?

• NOESY
• HSQC (or HMQC)
• IR spectroscopy
• Melting point determination

Correct Answer: HSQC (or HMQC)

Q19. Which impurity-removal step is most effective for removing colored polyphenolic compounds from an alkaloid-containing aqueous acidic extract?

• Washing with dilute acid
• Treating with activated charcoal or decolorizing carbon
• Adding sodium chloride to salt out polysaccharides
• Drying over anhydrous magnesium sulfate

Correct Answer: Treating with activated charcoal or decolorizing carbon

Q20. For structural confirmation of an alkaloid molecular formula and high-resolution mass, which instrument/technique is most appropriate?

• Low-resolution GC only
• High-resolution mass spectrometry (HRMS) or HR‑ESI‑MS
• Thin-layer chromatography (TLC)
• Polarimetry alone

Correct Answer: High-resolution mass spectrometry (HRMS) or HR‑ESI‑MS

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