NEET UG Chemistry · Chapter 18

Aldehydes, Ketones and Carboxylic Acids

Nucleophilic addition, named reactions, tests, and acidity of carboxylic acids.

NCERT Class 12NEET: 2 Qs per paper 8 min read

1. Chapter at a Glance

Aldehydes (RCHO), ketones (RCOR′) and carboxylic acids (RCOOH) all contain the carbonyl group, C=O. The polar C=O makes the carbon an electrophile, so the typical reaction of aldehydes and ketones is nucleophilic addition. Carboxylic acids are the most acidic common organic compounds. This chapter has many named reactions and distinguishing tests, and NEET asks about two questions from it every year.

What NEET asks most
  • Named reactions: Rosenmund, Stephen, Etard, Gattermann–Koch, Clemmensen, Wolff–Kishner, aldol, Cannizzaro, HVZ.
  • Reactivity of aldehydes vs ketones in nucleophilic addition.
  • Tests: Tollens', Fehling's, iodoform, 2,4-DNP.
  • Aldol vs Cannizzaro: which compounds give which.
  • Acidity of substituted carboxylic acids.

2. Structure and Physical Properties

  • The carbonyl carbon is sp²; the C=O bond is polar (Cδ+=Oδ−) and planar with 120° angles.
  • Aldehydes and ketones boil higher than alkanes and ethers (dipole–dipole forces) but lower than alcohols (no hydrogen bonding between their own molecules).
  • Carboxylic acids boil even higher than alcohols because they form hydrogen-bonded dimers.
  • Lower members of all three are water-soluble (H-bonding with water).

3. Preparation of Aldehydes and Ketones

Named reaction / methodChangeNEET Note
Oxidation of alcohols1° → aldehyde (PCC); 2° → ketoneStrong oxidants take 1° to acid
Rosenmund reductionRCOCl + H₂ (Pd/BaSO₄) → RCHOPoisoned catalyst stops at aldehyde
Stephen reactionRCN + SnCl₂/HCl, then H₂O → RCHOVia imine
DIBAL-H reductionRCN or ester → RCHO—
Etard reactionToluene + CrO₂Cl₂, then H₂O → benzaldehydeChromium complex intermediate
Gattermann–KochBenzene + CO + HCl (AlCl₃/CuCl) → benzaldehyde—
Friedel–Crafts acylationArH + RCOCl (AlCl₃) → ArCORAromatic ketones
Ozonolysis of alkenesC=C → two C=O—
From nitriles + GrignardRCN + R′MgX, then H₃O⁺ → RCOR′Ketones

4. Nucleophilic Addition Reactions

  • A nucleophile attacks the carbonyl carbon from above or below the plane, forming a tetrahedral intermediate that is then protonated.
  • Aldehydes are more reactive than ketones: ketones have two electron-donating alkyl groups (less δ+ on carbon) and more steric hindrance. HCHO is the most reactive.
  • Aromatic aldehydes/ketones are less reactive than aliphatic ones (resonance reduces the δ+ on carbon).
ReagentProductNEET Note
HCNCyanohydrinBase-catalysed; gives α-hydroxy acids on hydrolysis
NaHSO₃Bisulphite addition product (crystalline)Used to purify aldehydes and methyl ketones
Grignard reagentAlcohols after hydrolysisHCHO → 1°, RCHO → 2°, R₂CO → 3°
Alcohol (dry HCl)Hemiacetal → acetal (ketal with ketones)Protects the carbonyl group
NH₂–Z (ammonia derivatives)C=N–Z + H₂OHydroxylamine → oxime; hydrazine → hydrazone; 2,4-DNP → orange/yellow precipitate; semicarbazide → semicarbazone

5. Reduction, Oxidation and Condensation

  • Reduction to alcohols: NaBH₄, LiAlH₄ or H₂/Ni.
  • Reduction to hydrocarbons: Clemmensen (Zn–Hg, conc. HCl) or Wolff–Kishner (NH₂NH₂, then KOH in ethylene glycol) convert C=O into CH₂.
  • Oxidation: aldehydes are easily oxidised to acids even by mild reagents; ketones need strong oxidants and break C–C bonds.
  • Tollens' test (ammoniacal AgNO₃): aldehydes give a silver mirror. Fehling's test (Cu²⁺ in alkaline tartrate): aliphatic aldehydes give a red precipitate of Cu₂O; benzaldehyde does not respond to Fehling's test.
  • Iodoform test (I₂/NaOH): compounds with CH₃CO– or CH₃CH(OH)– give yellow CHI₃. Positive for ethanal, ethanol, propanone, propan-2-ol; negative for methanal and propanal.
  • Aldol condensation: aldehydes or ketones with at least one α-hydrogen, in dilute NaOH, give β-hydroxy carbonyl compounds (aldols), which lose water on heating to give α,β-unsaturated carbonyls. Cross-aldol uses two different carbonyls.
  • Cannizzaro reaction: aldehydes with no α-hydrogen (HCHO, C₆H₅CHO) in concentrated NaOH disproportionate into an alcohol and a carboxylate salt.
aldehyde / ketone has α-Hno α-H aldol (dilute NaOH) Cannizzaro (conc. NaOH) CH₃CHO, CH₃COCH₃HCHO, C₆H₅CHO → β-hydroxy carbonyl→ alcohol + acid salt
Presence of an α-hydrogen decides the reaction in base: aldol condensation if present, Cannizzaro reaction if absent.

6. Carboxylic Acids

  • Preparation: oxidation of 1° alcohols and aldehydes; oxidation of alkylbenzenes with KMnO₄ (any side chain gives benzoic acid); hydrolysis of nitriles and amides; Grignard reagent + CO₂ (dry ice), then acid; hydrolysis of esters.
  • Acidity: carboxylic acids are stronger than phenols and alcohols because the carboxylate ion is stabilised by two equivalent resonance structures. They release CO₂ from NaHCO₃.
  • Electron-withdrawing groups increase acidity (FCH₂COOH > ClCH₂COOH > BrCH₂COOH > CH₃COOH), and more strongly the nearer they are to –COOH. Electron-donating groups decrease acidity. HCOOH is stronger than CH₃COOH.
  • Reactions of –OH group: form acid chlorides (SOCl₂, PCl₅), anhydrides (P₂O₅), esters (alcohol + H⁺) and amides (NH₃, heat).
  • Reduction: LiAlH₄ gives 1° alcohols. Decarboxylation: sodium salt + soda lime gives an alkane with one fewer carbon.
  • Hell–Volhard–Zelinsky (HVZ): acids with α-H + Cl₂/Br₂ and red P give α-halo acids.
  • –COOH on a benzene ring is meta-directing and deactivating; benzoic acid does not undergo Friedel–Crafts reactions.

7. Formula and Reaction Master Table

Named reactionTransformation
RosenmundRCOCl → RCHO (H₂, Pd/BaSO₄)
StephenRCN → RCHO (SnCl₂/HCl, H₂O)
EtardC₆H₅CH₃ → C₆H₅CHO (CrO₂Cl₂)
Gattermann–KochC₆H₆ → C₆H₅CHO (CO, HCl, AlCl₃)
ClemmensenC=O → CH₂ (Zn–Hg, HCl)
Wolff–KishnerC=O → CH₂ (NH₂NH₂, KOH, glycol)
Aldol2CH₃CHO → CH₃CH(OH)CH₂CHO (dil. NaOH)
Cannizzaro2HCHO → CH₃OH + HCOONa (conc. NaOH)
HVZRCH₂COOH → RCHXCOOH (X₂, red P)
Kolbe electrolysis / decarboxylationRCOONa → R–R / RH

8. Master Summary Table

Test / propertyPositive forNEET Note
Tollens' reagentAll aldehydes (and HCOOH)Silver mirror
Fehling's solutionAliphatic aldehydesNot benzaldehyde
Iodoform testCH₃CO–, CH₃CH(OH)–Ethanol yes, methanol no
2,4-DNPAldehydes and ketonesOrange precipitate
NaHCO₃ effervescenceCarboxylic acidsNot phenols
AldolCarbonyls with α-HDilute base
CannizzaroAldehydes without α-HConcentrated base

9. Mind Map

Carbonyl Compounds
Preparation
  • Rosenmund, Stephen
  • Etard, Gattermann–Koch
  • Friedel–Crafts
Addition
  • HCN, NaHSO₃
  • Grignard
  • NH₂–Z derivatives
Redox and condensation
  • Clemmensen, W–K
  • Tollens, Fehling
  • Aldol, Cannizzaro
Carboxylic acids
  • Acidity
  • Derivatives
  • HVZ, decarboxylation

10. Tips and Tricks

Shortcuts
  • Clemmensen uses acid; Wolff–Kishner uses base — choose the one the rest of the molecule can tolerate.
  • In a crossed Cannizzaro with HCHO, HCHO is always oxidised (to formate) and the other aldehyde is reduced (to alcohol).
  • Formic acid behaves like an aldehyde: it reduces Tollens' reagent.
  • Count α-hydrogens: C₆H₅CHO has none, (CH₃)₃CCHO has none, CH₃CHO has three.
Common NEET traps
  • Expecting benzaldehyde to give Fehling's test.
  • Saying propanal gives the iodoform test.
  • Using aldol for HCHO or benzaldehyde (they have no α-H).
  • Thinking ketones are more reactive than aldehydes in nucleophilic addition.

11. Important One-liners

  • Formalin is a 40% aqueous solution of formaldehyde, used to preserve biological specimens.
  • Aldehydes are better reducing agents than ketones.
  • Carboxylic acids exist as hydrogen-bonded dimers in the vapour and in non-polar solvents.
  • Carboxylic acids are stronger acids than phenols.
  • Oxidation of any alkylbenzene with KMnO₄ gives benzoic acid.
  • Acetals are used to protect carbonyl groups during synthesis.
  • The –COOH group is meta-directing in electrophilic substitution.

12. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

Q1Easy

Which compound undergoes the Cannizzaro reaction?

  1. CH₃CHO
  2. CH₃COCH₃
  3. C₆H₅CHO
  4. CH₃CH₂CHO
Show solution

The Cannizzaro reaction needs an aldehyde without any α-hydrogen. Benzaldehyde has none, because its –CHO is attached directly to the ring. The other three compounds have α-hydrogens, so they undergo aldol condensation instead.

Answer: (C)

Q2Easy

Which gives a positive iodoform test?

  1. Methanol
  2. Propanal
  3. Ethanol
  4. Pentan-3-one
Show solution

Ethanol is oxidised to ethanal, which has a CH₃CO– group, so it gives yellow iodoform. Methanol, propanal and pentan-3-one have no CH₃CO– or CH₃CH(OH)– unit.

Answer: (C)

Q3Medium

Benzoyl chloride is converted into benzaldehyde by:

  1. Stephen reaction
  2. Etard reaction
  3. Rosenmund reduction
  4. Clemmensen reduction
Show solution

The Rosenmund reduction uses H₂ over Pd/BaSO₄ to reduce an acid chloride to an aldehyde. The Stephen reaction starts from a nitrile, the Etard reaction from toluene, and the Clemmensen reduction removes the C=O altogether.

Answer: (C)

Q4Medium

The strongest acid among these is:

  1. CH₃COOH
  2. FCH₂COOH
  3. ClCH₂COOH
  4. BrCH₂COOH
Show solution

F has the strongest −I effect, so it stabilises the carboxylate ion best. The order is FCH₂COOH > ClCH₂COOH > BrCH₂COOH > CH₃COOH.

Answer: (B)

Q5Medium

Which reagent can distinguish between benzaldehyde and acetaldehyde?

  1. Tollens' reagent
  2. Fehling's solution
  3. 2,4-DNP
  4. NaHSO₃
Show solution

Acetaldehyde reduces Fehling's solution and gives a red precipitate, but benzaldehyde does not. Both aldehydes give a silver mirror with Tollens' reagent and both react with 2,4-DNP and NaHSO₃, so those reagents cannot tell them apart.

Answer: (B)

Q6Medium

Acetone is converted into propane by:

  1. NaBH₄
  2. Zn–Hg / conc. HCl
  3. LiAlH₄
  4. H₂ / Pd–BaSO₄
Show solution

The Clemmensen reduction converts C=O into CH₂, so acetone becomes propane. NaBH₄ and LiAlH₄ give propan-2-ol, and Pd–BaSO₄ is the Rosenmund catalyst for acid chlorides.

Answer: (B)

Q7Hard

Formaldehyde and benzaldehyde are heated together with concentrated NaOH. The products are:

  1. Methanol and sodium benzoate
  2. Benzyl alcohol and sodium formate
  3. Benzyl alcohol and methanol
  4. Sodium formate and sodium benzoate
Show solution

In a crossed Cannizzaro reaction, HCHO is the better hydride donor, so it is oxidised to formate. Benzaldehyde is reduced to benzyl alcohol. This is why HCHO is used as a "sacrificial" reductant.

Answer: (B)

Q8Hard

The order of reactivity towards nucleophilic addition is:

  1. HCHO > CH₃CHO > CH₃COCH₃
  2. CH₃COCH₃ > CH₃CHO > HCHO
  3. CH₃CHO > HCHO > CH₃COCH₃
  4. All equal
Show solution

Each alkyl group lowers the positive charge on the carbonyl carbon (+I effect) and adds steric crowding. HCHO has no alkyl groups, CH₃CHO has one and acetone has two, so HCHO reacts fastest and acetone slowest.

Answer: (A)

13. Quick Revision Summary

Remember before the exam
  • Preparation: PCC, Rosenmund, Stephen, Etard, Gattermann–Koch, Friedel–Crafts.
  • Nucleophilic addition: aldehydes > ketones; HCN, NaHSO₃, Grignard, NH₂–Z.
  • Clemmensen (acid) and Wolff–Kishner (base) give CH₂.
  • Tollens (all aldehydes), Fehling (aliphatic only), iodoform (CH₃CO–).
  • Aldol needs α-H; Cannizzaro needs none; acids: −I groups increase acidity; HVZ gives α-halo acids.
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