Chemical tests for Benzoin MCQs With Answer

Chemical tests for Benzoin MCQs With Answer are essential for B. Pharm students learning qualitative analysis and identification of organic compounds. Benzoin is an alpha‑hydroxy ketone (secondary alcohol plus carbonyl) whose characteristic reactions—2,4‑DNP, oxidation to benzil, acetylation, phenylhydrazone/semicarbazone formation and bisulfite addition—help confirm its identity. Understanding reagent choices, expected color changes, derivative formation, and complementary spectroscopic confirmation (IR, NMR) strengthens lab competency and drug‑analysis skills. This focused set of multiple‑choice questions emphasizes reaction mechanisms, diagnostic tests, and practical interpretation relevant to pharmaceutical analysis and natural benzoin resin identification. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which functional groups are present in benzoin?

• Primary alcohol and aldehyde
• Alpha-hydroxy ketone (secondary alcohol and ketone)
• Phenol and ester
• Carboxylic acid and amide

Correct Answer: Alpha-hydroxy ketone (secondary alcohol and ketone)

Q2. Which result is expected when benzoin is treated with 2,4‑dinitrophenylhydrazine (2,4‑DNP)?

• No reaction
• Evolution of gas
• Formation of an orange/red precipitate (2,4‑DNP hydrazone)
• Blue coloration only

Correct Answer: Formation of an orange/red precipitate (2,4‑DNP hydrazone)

Q3. Which oxidizing reagent commonly converts benzoin into benzil?

• Sodium borohydride (NaBH4)
• Nitric acid (HNO3)
• Pyridine
• Hydrochloric acid (HCl)

Correct Answer: Nitric acid (HNO3)

Q4. What is the expected result of Tollens’ test with benzoin?

• Silver mirror formation (positive)
• Formation of a black precipitate
• No silver mirror (negative)
• Immediate gas evolution

Correct Answer: No silver mirror (negative)

Q5. How does benzoin typically respond to Fehling’s or Benedict’s reagent?

• Strong positive (brick-red precipitate)
• Weak positive with green color
• Negative (no typical aldehyde oxidation)
• Forms a yellow complex immediately

Correct Answer: Negative (no typical aldehyde oxidation)

Q6. The sodium bisulfite test is used to detect many carbonyl compounds. What is the expected interaction with benzoin?

• Formation of a soluble bisulfite addition product
• Immediate charring
• No reaction at all
• Evolution of hydrogen gas

Correct Answer: Formation of a soluble bisulfite addition product

Q7. Acetylation of benzoin (reaction with acetic anhydride/pyridine) primarily modifies which site?

• The ketone carbonyl (forming a ketal)
• The aromatic rings (nitration)
• The hydroxyl group, giving benzoin acetate (ester)
• The alpha carbon, forming an alkyl halide

Correct Answer: The hydroxyl group, giving benzoin acetate (ester)

Q8. Reaction of benzoin with phenylhydrazine yields which diagnostic derivative?

• Oxime
• Phenylhydrazone (carbonyl derivative)
• Acetal
• Carboxylate salt

Correct Answer: Phenylhydrazone (carbonyl derivative)

Q9. Which IR absorptions are characteristic for benzoin?

• Strong carbonyl stretch near 1680 cm⁻¹ and broad O–H band around 3200–3600 cm⁻¹
• No carbonyl band; only O–H at 2500 cm⁻¹
• Sharp N–H stretch and no aromatic peaks
• Only C–H stretches below 1000 cm⁻¹

Correct Answer: Strong carbonyl stretch near 1680 cm⁻¹ and broad O–H band around 3200–3600 cm⁻¹

Q10. In proton NMR, which signals help identify benzoin?

• Aliphatic methyl singlet only
• Aromatic multiplets around 7–8 ppm, benzylic CH and OH signals in the 4–6 ppm region
• Only a single peak at 0 ppm
• Large broad peak at 12–13 ppm only

Correct Answer: Aromatic multiplets around 7–8 ppm, benzylic CH and OH signals in the 4–6 ppm region

Q11. Which classic test can distinguish benzoin (a carbonyl-containing compound) from benzyl alcohol?

• Lucas test (instant turbidity)
• 2,4‑DNP test (forms a hydrazone with carbonyls)
• Iodoform test (positive for benzoin only)
• Biuret test

Correct Answer: 2,4‑DNP test (forms a hydrazone with carbonyls)

Q12. How does benzoin typically react with ferric chloride (FeCl3) test for phenols?

• Gives a strong purple color (positive phenol)
• Gives a green precipitate
• No characteristic color change (negative for phenol)
• Instant formation of a blue complex

Correct Answer: No characteristic color change (negative for phenol)

Q13. What result is expected when benzoin is treated with Lucas reagent (ZnCl2/HCl) at room temperature?

• Immediate cloudiness (typical tertiary alcohol)
• Slow reaction or no immediate turbidity, consistent with a secondary alcohol
• Instant precipitation of a heavy salt
• Explosive reaction

Correct Answer: Slow reaction or no immediate turbidity, consistent with a secondary alcohol

Q14. Is benzoin expected to give a positive iodoform test?

• Yes, a strong positive with yellow precipitate
• Only after heating
• No, typically negative (not a methyl ketone or ethanol-type structure)
• Gives a blue coloration instead

Correct Answer: No, typically negative (not a methyl ketone or ethanol-type structure)

Q15. The natural benzoin resin used in pharmacy is obtained from which genus?

• Pinus
• Styrax
• Aloe
• Curcuma

Correct Answer: Styrax

Q16. Regarding solubility, benzoin is best described as:

• Highly soluble in water at room temperature
• Insoluble in common organic solvents
• Slightly soluble in cold ethanol, more soluble in hot ethanol and organic solvents like ether
• Only soluble in concentrated acids

Correct Answer: Slightly soluble in cold ethanol, more soluble in hot ethanol and organic solvents like ether

Q17. Which of the following is closest to the melting point of pure benzoin?

• 25–30 °C
• 60–70 °C
• Approximately 137 °C
• Above 300 °C

Correct Answer: Approximately 137 °C

Q18. Which reagent, besides 2,4‑DNP, is commonly used to form a stable derivative to confirm the carbonyl in benzoin?

• Semicarbazide to form a semicarbazone
• Sodium chloride to form a halide
• Magnesium sulfate to form a complex
• Hydrochloric acid to form an ester

Correct Answer: Semicarbazide to form a semicarbazone

Q19. For quick laboratory identification and purity check of benzoin, which simple technique is most commonly applied?

• Thin-layer chromatography (TLC) with UV visualization
• Mass spectrometry only
• Gel electrophoresis
• Polarimetry exclusively

Correct Answer: Thin-layer chromatography (TLC) with UV visualization

Q20. The benzoin condensation (to form benzoin from benzaldehyde) is typically catalyzed by which species?

• Carbanion of acetone
• Cyanide ion (CN–)
• Strong oxidizing agent
• Acidic proton only

Correct Answer: Cyanide ion (CN–)

Q21. Which starting material is used in the classical benzoin synthesis?

• Benzaldehyde
• Benzyl alcohol
• Toluene
• Benzoic acid

Correct Answer: Benzaldehyde

Q22. Why is 2,4‑DNP a preferred qualitative test for benzoin’s carbonyl compared with simple acid tests?

• It oxidizes benzoin to aromatic acids
• It forms a stable, often brightly colored hydrazone precipitate that confirms presence of C=O
• It removes the hydroxyl group selectively
• It converts benzoin into an amide

Correct Answer: It forms a stable, often brightly colored hydrazone precipitate that confirms presence of C=O

Q23. Which reagent combination is typically used to acetylate the hydroxyl group of benzoin in a qualitative acetylation test?

• Hydrochloric acid and sodium chloride
• Acetic anhydride and pyridine
• Potassium permanganate
• Sodium hydroxide in water

Correct Answer: Acetic anhydride and pyridine

Q24. After oxidizing benzoin to benzil, which spectroscopic change is expected in the IR spectrum?

• Appearance of strong C=O stretches characteristic of diketone and loss of O–H band
• Disappearance of all aromatic bands
• Emergence of a strong N–H stretch
• Appearance of a broad O–H band only

Correct Answer: Appearance of strong C=O stretches characteristic of diketone and loss of O–H band

Q25. Which statement explains why benzoin gives a positive derivatization with semicarbazide?

• Benzoin contains a reducible nitro group
• The ketone carbonyl reacts with semicarbazide to form a semicarbazone derivative
• The aromatic ring undergoes addition with semicarbazide
• Semicarbazide esterifies the hydroxyl group

Correct Answer: The ketone carbonyl reacts with semicarbazide to form a semicarbazone derivative

Q26. For pharmaceutical analysis, which complementary technique is most useful to confirm benzoin after performing chemical tests?

• Infrared spectroscopy (IR) combined with melting point and TLC
• Paper chromatography alone
• Benedict’s test only
• Colorimetric test for proteins

Correct Answer: Infrared spectroscopy (IR) combined with melting point and TLC

Q27. Which of the following best describes a practical sign that the hydroxyl group has been acetylated on benzoin?

• Increase in solubility in water
• Loss of broad O–H IR band and a new ester C=O band near 1740 cm⁻¹
• Immediate precipitation of salt
• Formation of a silver mirror

Correct Answer: Loss of broad O–H IR band and a new ester C=O band near 1740 cm⁻¹

Q28. When confirming benzoin identity from natural resin, which additional test helps identify resin origin?

• Microbial culture
• Botanical source verification (Styrax species) combined with characteristic resin tests and chromatography
• Elemental analysis for nitrogen only
• Protein electrophoresis

Correct Answer: Botanical source verification (Styrax species) combined with characteristic resin tests and chromatography

Q29. Which property makes TLC a useful first-line test for benzoin in the lab?

• It consumes large sample quantities
• Ability to separate benzoin from related impurities quickly and visualize under UV or after staining
• It gives molecular weight directly
• It replaces all spectroscopic methods

Correct Answer: Ability to separate benzoin from related impurities quickly and visualize under UV or after staining

Q30. Which safety or practical precaution is important when performing chemical tests on benzoin in teaching labs?

• Ignore reagent hazards for quick results
• Use minimal protective equipment and no fume hood
• Follow waste disposal guidelines, use appropriate PPE, and work in a fume hood for volatile or corrosive reagents
• Always heat sealed samples in closed containers

Correct Answer: Follow waste disposal guidelines, use appropriate PPE, and work in a fume hood for volatile or corrosive reagents

G S Sachin

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.

Mail- Sachin@pharmacyfreak.com