Terpenoids are a vast class of natural products; citral is a prominent monoterpenoid aldehyde widely studied in pharmaceutics. Found in lemongrass and citrus essential oils, citral is a mixture of two geometric isomers, geranial (E‑isomer) and neral (Z‑isomer), with distinct aroma and reactivity. Its chemical properties, analytical identification by GC and HPLC, synthetic routes, stability challenges (oxidation, isomerization), derivatization, spectroscopic characterization (IR, NMR, MS), biological activities (antimicrobial, anti‑inflammatory) and formulation considerations make citral highly relevant for B.Pharm students. Understanding extraction, quality control, metabolism, toxicity and regulatory labeling is essential for safe pharmaceutical use. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What two isomers compose natural citral?
- Geranial and neral
- Citronellal and citronellol
- Geraniol and nerol
- Myrcene and limonene
Correct Answer: Geranial and neral
Q2. Which isomer of citral is the E (trans) isomer?
- Geranial (E‑isomer)
- Neral (E‑isomer)
- Citronellal (E‑isomer)
- Geraniol (E‑isomer)
Correct Answer: Geranial (E‑isomer)
Q3. What is the principal functional group present in citral?
- Aldehyde
- Ketone
- Alcohol
- Carboxylic acid
Correct Answer: Aldehyde
Q4. What is the molecular formula of citral?
- C10H16O
- C8H12O2
- C10H18O2
- C12H20O
Correct Answer: C10H16O
Q5. Which analytical technique is most commonly used to separate and quantify citral isomers in essential oils?
- Gas chromatography (GC)
- Ultraviolet–visible spectroscopy (UV‑Vis)
- Paper chromatography
- Size‑exclusion chromatography
Correct Answer: Gas chromatography (GC)
Q6. Which IR absorption band is most diagnostic for the aldehyde group in citral?
- ~1720 cm⁻¹ (C=O stretch)
- ~3300 cm⁻¹ (O–H stretch)
- ~2100 cm⁻¹ (C≡C stretch)
- ~1600 cm⁻¹ (aromatic C=C)
Correct Answer: ~1720 cm⁻¹ (C=O stretch)
Q7. What are the major chemical stability issues encountered with citral during storage?
- Oxidation and isomerization
- Hydrolysis to inorganic salts
- Photopolymerization to crosslinked networks
- Formation of stable carbocations
Correct Answer: Oxidation and isomerization
Q8. Partial catalytic hydrogenation of citral typically yields which product (major intermediate)?
- Citronellal
- Isoprene
- Acetone
- Citric acid
Correct Answer: Citronellal
Q9. Which extraction method is classically used to obtain citral‑rich essential oil from lemongrass?
- Steam distillation
- Soxhlet extraction with water
- Supercritical CO2 at low pressure
- Cold pressing only
Correct Answer: Steam distillation
Q10. For routine quantitative analysis of citral in essential oil, which detector paired with GC provides reliable quantitation?
- GC with flame ionization detector (GC‑FID)
- GC with thermal conductivity detector (GC‑TCD)
- HPLC with refractive index detector
- Paper chromatography with iodine vapor
Correct Answer: GC with flame ionization detector (GC‑FID)
Q11. Citral is classified as which subclass of terpenoids?
- Monoterpenoid aldehyde
- Diterpenoid ketone
- Triterpenoid alcohol
- Sesquiterpenoid ester
Correct Answer: Monoterpenoid aldehyde
Q12. Which type of reagent reacts with citral to form a Schiff base (imine) derivative?
- Primary amine
- Strong base (NaOH)
- Peracid (mCPBA)
- Hydrogen peroxide
Correct Answer: Primary amine
Q13. From a toxicology standpoint, citral is known primarily as:
- A contact allergen / skin sensitizer
- A nephrotoxin at microgram doses
- A potent mutagen in mammals
- Non‑reactive and non‑sensitizing
Correct Answer: A contact allergen / skin sensitizer
Q14. Which solvent is most suitable for diluting citral for GC analysis?
- n‑Hexane
- Water
- Glycerol
- Methanol/water (50:50)
Correct Answer: n‑Hexane
Q15. What is the approximate boiling point of citral at atmospheric pressure?
- Approximately 229 °C
- Approximately 78 °C
- Approximately 100 °C
- Approximately 40 °C
Correct Answer: Approximately 229 °C
Q16. Which analytical approach is most suitable to determine the E/Z ratio of citral isomers in a sample?
- Gas chromatography with an appropriate column
- Colorimetric pH testing
- Polarimetry only
- Gravimetric analysis
Correct Answer: Gas chromatography with an appropriate column
Q17. In mammalian metabolism, the aldehyde group of citral is primarily biotransformed to what functional group?
- Carboxylic acid (oxidation by aldehyde dehydrogenase)
- Epoxide (via monooxygenase)
- Thioketone
- Nitrile
Correct Answer: Carboxylic acid (oxidation by aldehyde dehydrogenase)
Q18. Which additive is commonly used to retard oxidative degradation of citral in formulations?
- Alpha‑tocopherol (vitamin E)
- Sodium chloride
- Hydrochloric acid
- Sodium hypochlorite
Correct Answer: Alpha‑tocopherol (vitamin E)
Q19. Citral is an important industrial precursor for the synthesis of which perfumery compound?
- Ionone
- Ibuprofen
- Aspirin
- Polyethylene glycol
Correct Answer: Ionone
Q20. Citral exhibits stronger antimicrobial activity against which group of organisms?
- Gram‑positive bacteria
- Obligate anaerobes only
- All viruses selectively
- Prions
Correct Answer: Gram‑positive bacteria
Q21. Is citral a chiral molecule?
- No, citral is achiral (no stereogenic center)
- Yes, both geranial and neral are chiral
- Yes, citral has a single chiral center at C‑2
- Only synthetic citral is chiral
Correct Answer: No, citral is achiral (no stereogenic center)
Q22. How does citral behave in physiological pH regarding ionization?
- Non‑ionizable (neutral aldehyde at physiological pH)
- Strongly acidic (deprotonates at pH 7.4)
- Strongly basic (protonated at pH 7.4)
- Forms stable salts with sodium at pH 7.4
Correct Answer: Non‑ionizable (neutral aldehyde at physiological pH)
Q23. Which reagent is commonly used to reduce the aldehyde group of citral to the corresponding primary alcohol?
- Sodium borohydride (NaBH4)
- Chromic acid (H2CrO4)
- Nitric acid
- Sodium hypochlorite
Correct Answer: Sodium borohydride (NaBH4)
Q24. Under acidic catalytic conditions, citral tends to undergo which transformations?
- Isomerization and polymerization (aldol/condensation reactions)
- Simple dilution with no reaction
- Conversion to stable inorganic salts
- Direct conversion into amino acids
Correct Answer: Isomerization and polymerization (aldol/condensation reactions)
Q25. How is the odor of citral commonly described?
- Lemon‑like, citrus aroma
- Earthy and musty
- Ammonia‑like
- Odorless
Correct Answer: Lemon‑like, citrus aroma
Q26. What is a major formulation challenge when incorporating citral into topical pharmaceutical products?
- High volatility and oxidative instability leading to loss of potency
- Complete insolubility in all organic solvents
- Strong radioactivity requiring shielding
- Inability to mix with oil phases
Correct Answer: High volatility and oxidative instability leading to loss of potency
Q27. Under current EU cosmetic regulation, citral must be:
- Declared on the label as a known fragrance allergen when present above threshold
- Completely banned from all cosmetic products
- Listed as a preservative only
- Exempt from any labeling requirements
Correct Answer: Declared on the label as a known fragrance allergen when present above threshold
Q28. Which isomer of citral is generally perceived as having the stronger lemon odor?
- Geranial
- Neral
- Citronellal
- Myrcene
Correct Answer: Geranial
Q29. Which classical chemical reaction can be used to selectively remove or derivatize the aldehyde group of citral for analytical purposes?
- Bisulfite addition (sodium bisulfite adduct formation)
- Friedel–Crafts acylation
- Wurtz coupling
- Kolbe electrolysis
Correct Answer: Bisulfite addition (sodium bisulfite adduct formation)
Q30. Industrially, citral can be obtained synthetically from which route or precursor?
- Oxidation of geraniol or derivates (and via isomerization of myrcene routes)
- Direct polymerization of ethylene oxide
- Fermentation of glucose to citral by yeast exclusively
- Saponification of long‑chain fatty acids
Correct Answer: Oxidation of geraniol or derivates (and via isomerization of myrcene routes)
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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