Qualitative tests for aldehydes and ketones MCQs With Answer

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Introduction: Qualitative tests for aldehydes and ketones MCQs With Answer are essential for B.Pharm students to identify carbonyl functional groups, understand reagent-specific reactions, and interpret laboratory observations. This concise guide covers common tests—2,4-DNPH (Brady’s), Tollens’, Fehling’s/Benedict’s, Schiff’s, iodoform, bisulfite, and semicarbazone formation—explaining expected color changes, precipitates, and diagnostic limitations. Mastery of these qualitative reactions aids in structural identification, pharmaceutical impurity analysis, and practical organic lab work. The MCQs focus on mechanisms, reagent behavior, distinguishing features, and real-world pharmaceutical applications to strengthen problem-solving and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which reagent gives an orange to yellow precipitate with most aldehydes and ketones indicating formation of a hydrazone derivative?

  • Fehling’s reagent
  • 2,4-DNPH (Brady’s reagent)
  • Tollens’ reagent
  • Schiff’s reagent

Correct Answer: 2,4-DNPH (Brady’s reagent)

Q2. Which qualitative test produces a silver mirror when positive for aldehydes?

  • Benedict’s reagent
  • Tollens’ reagent
  • 2,4-DNPH
  • Iodoform test

Correct Answer: Tollens’ reagent

Q3. The iodoform test is positive for which structural feature?

  • Aldehydes with aromatic ring
  • Carbonyl compounds with a methyl ketone group (CH3CO–)
  • All ketones
  • Carboxylic acids

Correct Answer: Carbonyl compounds with a methyl ketone group (CH3CO–)

Q4. Which reagent differentiates aldehydes from most ketones by oxidation to carboxylic acids?

  • 2,4-DNPH
  • Chromic acid (Jones reagent)
  • Tollens’ reagent
  • Lucas reagent

Correct Answer: Chromic acid (Jones reagent)

Q5. Schiff’s reagent gives a characteristic color with which functional group?

  • Alcohols
  • Aldehydes
  • Esters

Correct Answer: Aldehydes

Q6. A positive Fehling’s test results in which observable change?

  • Silver mirror
  • Formation of orange-red Cu2O precipitate
  • Formation of yellow precipitate of 2,4-DNPH
  • Formation of a white bisulfite adduct

Correct Answer: Formation of orange-red Cu2O precipitate

Q7. Which test forms crystalline semicarbazone derivatives useful for melting point identification?

  • Schiff’s test
  • Semicarbazide reaction
  • Lucas test
  • Iodoform test

Correct Answer: Semicarbazide reaction

Q8. Which reagent commonly fails to react with aromatic aldehydes like benzaldehyde in Fehling’s test?

  • Fehling’s reagent
  • 2,4-DNPH
  • Tollens’ reagent
  • Schiff’s reagent

Correct Answer: Fehling’s reagent

Q9. The bisulfite addition test forms a water-soluble adduct with which compounds?

  • Most aldehydes and some ketones
  • Only aromatic ketones
  • All esters
  • Alkenes

Correct Answer: Most aldehydes and some ketones

Q10. Which of the following is NOT a correct observation for 2,4-DNPH with carbonyl compounds?

  • Formation of a colored precipitate
  • Used for qualitative detection of C=O
  • Gives silver mirror
  • Forms hydrazone derivatives suitable for mp determination

Correct Answer: Gives silver mirror

Q11. Which reagent is known as Brady’s reagent?

  • Tollens’ reagent
  • 2,4-DNPH
  • Schiff’s reagent
  • Iodoform reagent

Correct Answer: 2,4-DNPH

Q12. Which compound would give a positive iodoform test?

  • Benzaldehyde
  • Acetone
  • Formaldehyde
  • Benzoic acid

Correct Answer: Acetone

Q13. Schiff’s reagent is particularly useful because it gives a rapid pink or magenta color with:

  • Ketones
  • Aldehydes
  • Carboxylic acids
  • Esters

Correct Answer: Aldehydes

Q14. Which test would you choose to confirm presence of a carbonyl group irrespective of aldehyde or ketone?

  • 2,4-DNPH
  • Tollens’ reagent
  • Fehling’s reagent
  • Lucas test

Correct Answer: 2,4-DNPH

Q15. Which aldehyde gives a positive Tollens’ test but a negative 2,4-DNPH test?

  • Formaldehyde
  • None; aldehydes usually react with 2,4-DNPH
  • Benzaldehyde
  • Acetaldehyde

Correct Answer: None; aldehydes usually react with 2,4-DNPH

Q16. The formation of a white crystalline bisulfite adduct is a basis for what application?

  • Qualitative identification only
  • Purification and isolation of aldehydes
  • Oxidation to acids
  • Polymerization of aldehydes

Correct Answer: Purification and isolation of aldehydes

Q17. Which reagent will oxidize an aldehyde to a carboxylic acid and is used as a qualitative test?

  • Tollens’ reagent
  • 2,4-DNPH
  • Lucas reagent
  • Iodoform reagent

Correct Answer: Tollens’ reagent

Q18. Which of the following carbonyls will NOT give a positive Tollens’ test?

  • Benzaldehyde
  • Glucose (open chain aldehyde form)
  • Formaldehyde
  • Acetone

Correct Answer: Acetone

Q19. In pharmaceutical analysis, 2,4-DNPH derivatives are useful because:

  • They are highly volatile for GC analysis
  • They form crystalline derivatives with characteristic melting points
  • They do not react with ketones
  • They decompose quickly

Correct Answer: They form crystalline derivatives with characteristic melting points

Q20. Which statement about Fehling’s and Benedict’s reagents is TRUE?

  • Both are used to detect tertiary alcohols
  • Both detect reducing aldehydes by forming Cu2O
  • Both give a silver mirror
  • Both react only with ketones

Correct Answer: Both detect reducing aldehydes by forming Cu2O

Q21. Which test can distinguish acetaldehyde from acetone?

  • 2,4-DNPH (both positive)
  • Tollens’ test (acetaldehyde positive, acetone negative)
  • Iodoform test (acetaldehyde negative, acetone positive)
  • Both B and C

Correct Answer: Both B and C

Q22. A positive Schiff’s test gives what color change?

  • Orange precipitate
  • Silver mirror
  • Pink to magenta color
  • Green color

Correct Answer: Pink to magenta color

Q23. Which carbonyl compound forms a stable bisulfite adduct readily?

  • Highly hindered ketones only
  • Simple aldehydes like acetaldehyde
  • Carboxylic acids
  • Alkenes

Correct Answer: Simple aldehydes like acetaldehyde

Q24. Which reagent gives a red or brown precipitate of iodoform when positive?

  • Tollens’ reagent
  • Iodine in alkaline medium (I2/NaOH)
  • 2,4-DNPH
  • Schiff’s reagent

Correct Answer: Iodine in alkaline medium (I2/NaOH)

Q25. Which detection method is commonly used to confirm a carbonyl in trace pharmaceutical samples by derivatization?

  • Lucas test
  • Derivatization with 2,4-DNPH followed by HPLC
  • Tollen’s test alone
  • Iodoform test

Correct Answer: Derivatization with 2,4-DNPH followed by HPLC

Q26. Which reagent would you NOT use to distinguish aldehyde vs ketone?

  • Tollens’ reagent
  • 2,4-DNPH
  • Chromic acid
  • All test both equally

Correct Answer: 2,4-DNPH

Q27. Which compound gives a positive iodoform test despite being an alcohol?

  • 1-Butanol
  • Ethanol
  • Benzyl alcohol
  • Phenol

Correct Answer: Ethanol

Q28. Semicarbazone formation is useful because semicarbazones are:

  • Non-crystalline oils
  • Colorless gases
  • Crystalline solids with characteristic melting points
  • Strong acids

Correct Answer: Crystalline solids with characteristic melting points

Q29. Which reagent is selective for aldehydes and will not oxidize most ketones under mild conditions?

  • Tollens’ reagent
  • 2,4-DNPH
  • Lucas reagent
  • Iodoform reagent

Correct Answer: Tollens’ reagent

Q30. A pharmaceutical sample gives an orange precipitate with 2,4-DNPH and forms a stable bisulfite adduct; this suggests the sample contains:

  • An alkane impurity
  • A carbonyl compound, likely an aldehyde
  • Only an aromatic ring
  • An ester

Correct Answer: A carbonyl compound, likely an aldehyde

Q31. Which of the following carbonyls is most likely to give a negative Fehling’s test?

  • Formaldehyde
  • Glucose (open chain form)
  • Benzaldehyde
  • Acetaldehyde

Correct Answer: Benzaldehyde

Q32. Which test relies on nucleophilic addition to the carbonyl to form a stable adduct used for purification?

  • Iodoform test
  • Bisulfite addition test
  • Tollens’ reagent
  • Schiff’s reagent

Correct Answer: Bisulfite addition test

Q33. In the lab, a carbonyl gives a yellow crystalline 2,4-DNP derivative and a negative Tollens’ test. The compound is likely:

  • An aldehyde
  • A ketone
  • A carboxylic acid
  • An alcohol

Correct Answer: A ketone

Q34. Which reagent is useful to detect alpha-hydroxy ketones because they reduce Tollens’ reagent?

  • Schiff’s reagent
  • Tollens’ reagent
  • 2,4-DNPH
  • Iodoform test

Correct Answer: Tollens’ reagent

Q35. Which diagnostic test would distinguish 2-butanone from 2-butanal?

  • 2,4-DNPH (both positive)
  • Tollens’ test (2-butanal positive, 2-butanone negative)
  • Iodoform test (both positive)
  • Both A and C

Correct Answer: Tollens’ test (2-butanal positive, 2-butanone negative)

Q36. Which reagent is most often used to detect carbonyl impurities before HPLC in pharmaceutical QC?

  • Lucas reagent
  • 2,4-DNPH derivatization
  • Iodoform test
  • Semicarbazide reaction without derivatization

Correct Answer: 2,4-DNPH derivatization

Q37. Which carbonyl compound will give both a positive Tollens’ and Fehling’s test?

  • Acetone
  • Benzaldehyde
  • Acetaldehyde
  • Propanone

Correct Answer: Acetaldehyde

Q38. Which qualitative test produces a color change by formation of an imine-like colored species from aldehydes?

  • 2,4-DNPH
  • Schiff’s reagent
  • Tollens’ reagent
  • Iodoform test

Correct Answer: Schiff’s reagent

Q39. Which of the following is a limitation of the Tollens’ test in pharmaceutical labs?

  • It cannot detect any aldehydes
  • It may give false positives with enediol-forming sugars or alpha-hydroxy ketones
  • It reacts with all ketones
  • It forms volatile derivatives unsuitable for analysis

Correct Answer: It may give false positives with enediol-forming sugars or alpha-hydroxy ketones

Q40. Which reagent would you use to specifically test for a methyl ketone in a drug impurity?

  • Schiff’s reagent
  • Iodoform test
  • Tollens’ reagent
  • Bisulfite addition

Correct Answer: Iodoform test

Q41. Which test gives a colored hydrazone or azine suitable for mp comparison and is stable?

  • 2,4-DNPH
  • Tollen’s reagent
  • Iodoform test
  • Benedict’s reagent

Correct Answer: 2,4-DNPH

Q42. Which observation indicates a positive semicarbazone test?

  • Formation of a gas
  • Formation of crystalline solid
  • Silver mirror
  • Formation of a red precipitate

Correct Answer: Formation of crystalline solid

Q43. Which reaction is commonly used to confirm the identity of an aldehyde by converting it to a more easily characterized derivative?

  • Iodoform reaction
  • Formation of 2,4-DNPH derivative
  • Tollens’ reagent without isolation
  • Lucas test

Correct Answer: Formation of 2,4-DNPH derivative

Q44. Which of the following will give a positive iodoform test?

  • Propanal
  • Isopropyl alcohol (2-propanol)
  • Benzaldehyde
  • Cyclohexanone

Correct Answer: Isopropyl alcohol (2-propanol)

Q45. A carbonyl gives a yellow 2,4-DNPH precipitate and positive iodoform; the likely structure is:

  • Acetaldehyde or methyl ketone
  • Benzaldehyde
  • Formic acid
  • tert-Butyl ketone

Correct Answer: Acetaldehyde or methyl ketone

Q46. Which reagent would detect conjugated aldehydes less sensitively due to resonance stabilization?

  • 2,4-DNPH (still reacts but may be slower)
  • Tollens’ reagent (always fast)
  • Iodoform test
  • Lucas reagent

Correct Answer: 2,4-DNPH (still reacts but may be slower)

Q47. Which functional group can be mistaken for an aldehyde in some tests because it can be oxidized or give similar adducts?

  • Tertiary alcohols
  • Alpha-hydroxy ketones
  • Ethers
  • Alkanes

Correct Answer: Alpha-hydroxy ketones

Q48. For pharmaceutical quality control, why are derivative melting points of 2,4-DNPH and semicarbazones preferred?

  • The derivatives are volatile
  • The derivatives provide reproducible, characteristic melting points for identification
  • The derivatives are invisible
  • The derivatives decompose easily

Correct Answer: The derivatives provide reproducible, characteristic melting points for identification

Q49. Which reagent would you use to detect a reducing sugar contaminant that can open to an aldehyde form?

  • Fehling’s or Benedict’s reagent
  • 2,4-DNPH only
  • Iodoform test
  • Lucas reagent

Correct Answer: Fehling’s or Benedict’s reagent

Q50. In distinguishing carbonyls, which sequence of tests is most diagnostic in a lab setting?

  • Do iodoform, then semicarbazone, then stop
  • Derivatize with 2,4-DNPH, test with Tollens’/Fehling’s, and confirm with bisulfite or semicarbazone
  • Only use Tollens’ reagent
  • Use Lucas test followed by iodoform only

Correct Answer: Derivatize with 2,4-DNPH, test with Tollens’/Fehling’s, and confirm with bisulfite or semicarbazone

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