Aldehydes, Ketones & Carboxylic Acids — Notes & 50 MCQs

Advanced Notes — Quick Overview

This concise guide covers important concepts: nucleophilic addition to carbonyl (mechanism and stereochemistry), nucleophilic acyl substitution, oxidation/reduction of carbonyls, special reactions (Tollens, Fehling, Benedict, Cannizzaro, Aldol condensation, Claisen, Wolff–Kishner, Clemmensen), acidity of carboxylic acids, derivatives (acid chlorides, esters, amides), and key reagents (Grignard, LiAlH4, PCC, KMnO4, NaBH4).

Exam-focused concepts

  • Differentiate reagents: NaBH4 reduces aldehydes/ketones; LiAlH4 reduces carboxylic acids & derivatives.
  • Tollens' reagent gives silver mirror with aldehydes (not with ketones usually).
  • Carboxylic acids are more acidic due to resonance stabilization and electronegative oxygen atoms.

Practical tips

  • Practice mechanisms with curved arrows; memorize conditions for named reactions.
  • For multi-step syntheses, identify functional group interconversions (FGI) and protecting groups.

50 Practice MCQs — Answers highlighted

1. Which reagent oxidizes a primary alcohol to an aldehyde without further oxidation to carboxylic acid?

  1. KMnO4
  2. PCC (pyridinium chlorochromate) ✅
  3. HNO3
  4. H2O2

2. Tollens' test gives a silver mirror with which functional group?

  1. Ketones only
  2. Aldehydes ✅
  3. Carboxylic acids
  4. Esters

3. Which reagent reduces ketones to secondary alcohols selectively?

  1. KMnO4
  2. NaBH4 (sodium borohydride) ✅
  3. HNO3
  4. PCC

4. Which reaction couples two aldehyde molecules to give a β-hydroxyaldehyde under basic conditions?

  1. Cannizzaro reaction
  2. Aldol condensation ✅
  3. Wurtz reaction
  4. Grignard formation

5. Which reaction converts an aldehyde to a carboxylic acid using mild oxidant?

  1. Wolff–Kishner reduction
  2. Tollens' or Benedict's oxidation (Tollens' gives Ag, Benedict's gives Cu2O) ✅
  3. Clemmensen reduction
  4. Fischer esterification

6. Which reagent converts carboxylic acid to acid chloride?

  1. NaBH4
  2. SOCl2 (thionyl chloride) ✅
  3. PCC
  4. H2/Pd

7. Which reagent is used to form a Grignard reagent from alkyl halide?

  1. Mg in dry ether ✅
  2. LiAlH4
  3. NaBH4
  4. H2/Pd

8. The Cannizzaro reaction occurs for which type of carbonyl compounds?

  1. Aldehydes with α-hydrogen
  2. Aldehydes without α-hydrogen (e.g., benzaldehyde) ✅
  3. Ketones only
  4. Esters only

9. Which reaction reduces carbonyl to methylene (–CH2–) group?

  1. Wolff–Kishner reduction ✅
  2. Tollens' test
  3. Friedel–Crafts
  4. Grignard formation

10. Which reagent converts ketone to corresponding alkane via reduction (Clemmensen)?

  1. Zn(Hg)/HCl (Clemmensen reduction) ✅
  2. NaBH4
  3. KMnO4
  4. PCC

11. Which reagent gives an oil with a yellow precipitate for aldehydes (Schiff's test)?

  1. Benedict's reagent
  2. Schiff's reagent gives color change with aldehydes ✅
  3. Tollens' reagent only
  4. Grignard reagent

12. Which of the following is a nucleophilic addition to carbonyl?

  1. Aldol condensation (involves nucleophilic addition to carbonyls) ✅
  2. Wurtz coupling only
  3. Friedel–Crafts alkylation only
  4. None

13. Which reagent converts ester to carboxylic acid (hydrolysis)?

  1. NaBH4
  2. Acidic or basic hydrolysis (H+/H2O or NaOH followed by acid) ✅
  3. PCC
  4. Zn(Hg)/HCl

14. Which functional group reacts with 2,4-DNP to give an orange/yellow precipitate?

  1. Alcohols
  2. Aldehydes and ketones (carbonyl compounds) ✅
  3. Carboxylic acids only
  4. Esters only

15. Which reagent converts carboxylic acid to ester (Fischer esterification)?

  1. NaBH4
  2. Alcohol with conc. H2SO4 (acid-catalyzed) ✅
  3. SOCl2 alone
  4. Grignard reagent

16. Which statement is true about carboxylic acid acidity?

  1. They are less acidic than alcohols
  2. They are more acidic than alcohols due to resonance stabilization of carboxylate ion ✅
  3. Acidity independent of substituents
  4. Only inductive effect matters

17. Which reagent converts aldehyde to primary alcohol?

  1. KMnO4
  2. NaBH4 or LiAlH4 (reducing agents) ✅
  3. Tollens' reagent
  4. PCC

18. Which reagent transforms a carboxylic acid into an alcohol (reduction)?

  1. NaBH4 (too mild for acids)
  2. LiAlH4 (strong reducing agent) ✅
  3. PCC
  4. Ag2O

19. Which reaction gives carboxylic acid from alkyl chain terminal primary alcohol via oxidation?

  1. PCC only
  2. Strong oxidants (KMnO4, K2Cr2O7) oxidize primary alcohols to carboxylic acids ✅
  3. NaBH4
  4. Wolff–Kishner

20. Which of the following is a nucleophilic acyl substitution?

  1. Aldol condensation
  2. Hydrolysis of acid chloride to carboxylic acid ✅
  3. Tollens' test
  4. Wolff–Kishner

21. Which compound gives a positive Fehling's test?

  1. Ketones generally
  2. Aldehydes (reducing sugars/aldehydic group) ✅
  3. Esters only
  4. Carboxylic acids only

22. Which reagent converts an acid chloride to an ester?

  1. NaBH4
  2. Alcohol (ROH) in presence of base or pyridine ✅
  3. LiAlH4
  4. PCC

23. Which reagent converts acetone to isopropanol?

  1. NaBH4 or H2/Pd (reduction) ✅
  2. PCC
  3. KMnO4
  4. Tollens' reagent

24. Which statement about 2,4-DNP (2,4-dinitrophenylhydrazine) test is true?

  1. It tests for carboxylic acids
  2. It gives a precipitate with aldehydes and ketones (hydrazone formation) ✅
  3. It oxidizes aldehydes only
  4. It reduces ketones only

25. Which reagent will distinguish between an aldehyde and a ketone?

  1. 2,4-DNP (both give positive)
  2. Tollens' or Fehling's reagent (positive for aldehydes only) ✅
  3. PCC (oxidizes)
  4. LiAlH4

26. Which mechanism is involved in nucleophilic addition to a carbonyl carbon?

  1. SN1
  2. Nucleophilic addition via tetrahedral intermediate ✅
  3. Free radical substitution
  4. Elimination only

27. Which reagent converts carboxylic acid to primary alcohols?

  1. NaBH4
  2. LiAlH4 (strong reducing agent) ✅
  3. PCC
  4. KMnO4

28. Which of the following is an acid derivative more reactive than ester?

  1. Amide
  2. Acid chloride (most reactive) ✅
  3. Carboxylic acid
  4. Alcohol

29. Which reaction involves intramolecular nucleophilic acyl substitution producing cyclic esters/lactones?

  1. Aldol condensation
  2. Lactonization (formation of lactones) ✅
  3. Wurtz reaction
  4. Cannizzaro

30. Which reagent converts α,β-unsaturated carbonyl via conjugate (1,4) addition rather than direct (1,2)?

  1. Strong hard nucleophiles (e.g., Grignard)
  2. Soft nucleophiles (e.g., Gilman reagent, R2CuLi) favor 1,4 addition ✅
  3. Tollens' reagent
  4. NaBH4 always

31. Which reagent forms acetals/ketals from aldehydes/ketones protecting the carbonyl?

  1. Excess alcohol with acid catalyst (H+) — acetal/ketal formation ✅
  2. NaBH4
  3. PCC
  4. KMnO4

32. Which reagent converts aldehydes/ketones to oximes?

  1. Hydrazine (gives hydrazone)
  2. Hydroxylamine (NH2OH) gives oximes ✅
  3. PCC
  4. LiAlH4

33. Which test gives a silver mirror with benzaldehyde?

  1. 2,4-DNP
  2. Tollens' test ✅
  3. Br2 water
  4. Lucas test

34. Which reagent is used to convert aldehyde to alditol (sugar alcohol)?

  1. KMnO4
  2. NaBH4 (reduces aldehyde to alcohol) ✅
  3. Tollens' reagent
  4. PCC

35. Which compound reacts with primary amines to give imines (Schiff base)?

  1. Carboxylic acids
  2. Aldehydes and ketones (condensation to imines) ✅
  3. Esters only
  4. Acid chlorides only

36. Which reagent converts an aldehyde to an alkene via Wittig reaction?

  1. Phosphonium ylide (Ph3P=CR2) ✅
  2. NaBH4
  3. LiAlH4
  4. PCC

37. Which of the following acid derivatives is least reactive towards nucleophilic attack?

  1. Acid chloride
  2. Amide (resonance stabilized, least reactive) ✅
  3. Ester
  4. Anhydride

38. Which reagent will convert benzaldehyde to benzyl alcohol?

  1. PCC
  2. NaBH4 (mild reducing agent) ✅
  3. KMnO4
  4. H2O2

39. Which reaction forms an ester from carboxylic acid and alcohol under acid catalysis?

  1. Saponification
  2. Fischer esterification ✅
  3. Wurtz coupling
  4. Grignard formation

40. Which reagent converts acid chloride to amide?

  1. NaBH4
  2. Ammonia (NH3) or amine ✅
  3. PCC
  4. Zn(Hg)/HCl

41. Which of the following gives positive iodoform test?

  1. All aldehydes
  2. Methyl ketones and ethanol (CH3CHO, CH3CO–) ✅
  3. Carboxylic acids only
  4. Esters only

42. Which reagent converts aldehyde to geminal diol in aqueous solution?

  1. Water alone forms hydrate (gem-diol) equilibrium ✅
  2. NaBH4 only
  3. PCC only
  4. LiAlH4

43. Which reaction converts ester to amide (aminolysis)?

  1. Hydrolysis only
  2. Reaction with ammonia/amine (aminolysis) ✅
  3. Wittig reaction
  4. Cannizzaro reaction

44. Which reagent is used to oxidize secondary alcohol to ketone selectively?

  1. LiAlH4
  2. PCC or Jones reagent under controlled conditions ✅
  3. NaBH4
  4. H2/Pd

45. Which reagent converts aldehydes/ketones to hydrazones?

  1. Hydroxylamine
  2. Hydrazine (NH2NH2) ✅
  3. NaBH4
  4. LiAlH4

46. Which of the following is formed by nucleophilic addition of CN- to carbonyl?

  1. Acetal
  2. Cyanohydrin ✅
  3. Hydrazone
  4. Acid chloride

47. Which reagent converts carboxylic acid to aldehyde (via reduction of acid derivative)?

  1. NaBH4 (insufficient)
  2. Partial reduction via LiAlH(OtBu)3 (selective) or via acid chloride then Rosenmund reduction (H2/Pd-BaSO4) ✅
  3. KMnO4
  4. NaOH

48. Which esterification method is reversible and requires removal of water to drive equilibrium?

  1. Saponification
  2. Fischer esterification (acid-catalyzed) ✅
  3. Wurtz reaction
  4. Grignard formation

49. Which reaction converts aldehyde to alkene via phosphorus ylides?

  1. Wolff–Kishner
  2. Wittig reaction ✅
  3. Tollens'
  4. Clemmensen

50. Which of the following is TRUE about acid chlorides?

  1. They are less reactive than esters
  2. They are more reactive than esters and react readily with nucleophiles to give substitution products ✅
  3. They are inert to nucleophilic attack
  4. They are strong reducing agents only

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