SaitechAI – Key Sheet

Aldehydes, Ketones & Carboxylic Acids • 100 One-Mark Questions

Answer Key
  1. Propanal.
  2. OHC–CH₂–CO–CH₂–CH₃ (3-oxopentanal).
  3. Ammoniacal silver nitrate, [Ag(NH₃)₂]⁺OH⁻ (Tollens’ reagent).
  4. It is used as an oxidising reagent to detect aldehydes by forming a silver mirror.
  5. Cl–CH₂–CH₂–CH₂–CO–CH₃ (4-chloropentan-2-one).
  6. Carbonyl carbon (C=O) bonded to two alkyl/aryl groups (–CO–).
  7. Due to less steric hindrance and stronger +I of two alkyl groups in ketones, carbonyl carbon in aldehydes is more δ⁺ and hence more reactive.
  8. Use Tollens’ or Fehling’s test: aldehydes give positive test, ketones generally do not.
  9. Methanoic acid.
  10. Ethanoic acid.
  11. They form strong intermolecular hydrogen bonds (dimerisation), raising boiling point.
  12. Intermolecular hydrogen bonding.
  13. p-Methylbenzaldehyde (–CHO at 1, –CH₃ at 4 position on benzene ring).
  14. Hydrogen atom attached to the α-carbon (carbon adjacent to C=O).
  15. Canizzaro reaction.
  16. Aromatic and aliphatic aldehydes without α-hydrogen (e.g. benzaldehyde, formaldehyde).
  17. Because the –OH group donates electrons (+R/mesomeric effect) and the carboxyl group is resonance-stabilized, making carbonyl carbon less electrophilic.
  18. Carbon dioxide (CO₂).
  19. Benzoic acid effervesces with NaHCO₃ (CO₂ evolution); phenol does not react.
  20. CH3–CH(OH)–CH₂–CO-CH₃ (4-hydroxypentan-2-one).
  21. R–CHO.
  22. R–CO–R'.
  23. R–COOH.
  24. CH₃–CH(CH₃)–CH₂–CHO (2-methylbutanal).
  25. An alkane (decarboxylation) + sodium carbonate (Na₂CO₃).
  26. Decarboxylation.
  27. LiAlH₄ or NaBH₄ (any one).
  28. 1-Phenylpentan-1-one.
  29. Iodoform test; product is iodoform (CHI₃) + sodium salt of acid.
  30. Methyl ketones (R–CO–CH₃) and ethanol/ethanal-type carbonyl compounds.
  31. Acetone has a –CO–CH₃ group (methyl ketone) while benzophenone does not.
  32. Butanone < propanone < propanal < ethanal.
  33. ClCH₂COOH is more acidic.
  34. –Cl exerts –I effect, stabilising the carboxylate ion more than –CH₃.
  35. Acetaldehyde.
  36. Acetone (dimethyl ketone).
  37. Aldehyde (–CHO) group.
  38. Yes.
  39. No.
  40. Acetone, ethanol/ethanal, etc. (any methyl ketone, e.g. CH₃COCH₃).
  41. They are harder to oxidise; Tollens’ reagent is a mild oxidant suitable mainly for aldehydes.
  42. Canizzaro reaction; two molecules give a salt of carboxylic acid and an alcohol.
  43. Tollens’/Fehling’s/Schiff’s test (e.g. Tollens’: aldehyde positive, ketone negative).
  44. Methanoic acid reduces Tollens’ reagent but ethanoic acid does not.
  45. HCOOH (formic acid).
  46. 3-Hydroxybutanoic acid.
  47. Carbon atom directly attached to the C=O carbon.
  48. C₆H₅–CO-CH₂–CH₂–CH2–CH₃ (1-phenylpentan-1-one).
  49. Aldehyde.
  50. Because the –OH group strongly deactivates the carbonyl carbon via –R and –I effects and exists mainly in the resonance-stabilised –COO⁻ form.
  51. Acidic.
  52. Ethanoic acid.
  53. Methanoic acid (or methanol; methanoic acid is most strongly hydrogen-bonded).
  54. Due to hydrogen bonding between the lone pair of oxygen of water and δ⁺ carbonyl carbon and/or δ⁻ oxygen.
  55. Because the hydrophobic hydrocarbon part increases and predominates over the polar group.
  56. Pentan-3-one.
  57. No.
  58. Yes.
  59. CH₃–CH(CH₃)–CH₂–CH₂–CHO.
  60. Propanoic acid.
  61. Reduction (catalytic hydrogenation) – a redox/addition reaction.
  62. No reaction (propanone is resistant to mild oxidation; under strong conditions it cleaves to smaller acids/ketones).
  63. Propanal is oxidised to propanoic acid.
  64. Oxidation of ketones involves cleavage of C–C bond and is difficult; aldehydes oxidise easily to acids.
  65. K₂Cr₂O₇/H⁺ or KMnO₄/H⁺ (any suitable oxidant).
  66. Acid chloride (R–COCl).
  67. –COCl (acyl chloride/acid chloride group).
  68. Benzoic acid gives effervescence with NaHCO₃; benzaldehyde does not.
  69. Negative inductive effect of substituents which withdraw electrons through σ-bond.
  70. –NO₂ withdraws electrons, stabilising the carboxylate anion and increasing acidity.
  71. A ketone containing C=C (double bond) in conjugation with C=O.
  72. Example: CH₃–CO–CH=CH₂ (but-3-en-2-one) or CH₂=CH–CO–CH₃ (but-2-en-1-one).
  73. The C=C and/or C=O bonds are reduced to saturated alcohol/alkane (hydrogen adds across double bond).
  74. It increases acidity by stabilising the conjugate base via –I effect.
  75. A ketone having at least one –CO–CH₃ group.
  76. Yes (it behaves like an aldehyde/methyl group system).
  77. Butan-2-one.
  78. Nucleophilic addition (or nucleophilic addition followed by protonation).
  79. CN⁻, H⁻ (from NaBH₄/LiAlH₄), NH₂OH, NH₂NH₂, etc. (any two).
  80. Ketones have no hydrogen on carbonyl carbon and C–C bonds are difficult to cleave; aldehydes have H and oxidise easily to acids.
  81. Benzoic acid.
  82. Ethanoic acid (and silver is reduced to metallic Ag).
  83. α-Bromination and formation of bromo-ketone (haloform pathway).
  84. Hell–Volhard–Zelinsky (for acids) or generally “haloform-type α-halogenation” reaction.
  85. Propanal (lower molar mass and weaker intermolecular forces) is more volatile.
  86. CH₃–CH(CH₃)–CO–CH(CH₃)₂ (2,4-dimethylpentan-3-one).
  87. Benzaldehyde (benzenecarbaldehyde; commonly benzaldehyde).
  88. Acetone (propanone).
  89. 2,4-dinitrophenylhydrazone derivative (orange/yellow precipitate) indicating presence of carbonyl group.
  90. No.
  91. Hydrocarbon (alkane) + sodium carbonate (Na₂CO₃) or sodium oxide (from strong heating).
  92. Reduction of carbonyl group to CH₂ in presence of Zn–Hg/HCl.
  93. Aldehydes and ketones (especially those with no base-sensitive groups).
  94. CH₃–CO–CH₂–CH₃.
  95. CH₃–CH₂–CO–CH₂–CH₃.
  96. Ketones having same molecular formula but different arrangement of carbon atoms (e.g. pentan-2-one and pentan-3-one).
  97. Aldehyde with only single bonds (e.g. ethanal, propanal – saturated aliphatic aldehydes).
  98. Further oxidised to CO₂ and H₂O (complete oxidation).
  99. HCOOH is strongest (then CH₃COOH > CH₃CH₂COOH).
  100. –I group increases acidic strength by stabilising the carboxylate anion.