Is it correct to say: Applying the Cannizzaro reaction to ethanal.?

Ethanal has α-H, so alkali gives aldol condensation. Cannizzaro needs an aldehyde with NO α-H (HCHO, C₆H₅CHO).

Is it correct to say: Saying all aldehydes give Fehling's test.?

Aromatic aldehydes such as benzaldehyde do not reduce Fehling's solution, though they do give Tollens' test.

Is it correct to say: Thinking only methyl ketones give the iodoform test.?

Ethanal, ethanol and any alcohol with a CH₃CH(OH)– group also give it. Methanol, methanal and propanal do not.

Is it correct to say: Using Clemmensen reduction on an acid-sensitive compound.?

Use Wolff–Kishner (basic conditions) instead. Clemmensen (Zn–Hg/conc. HCl) suits compounds that are stable to acid.

Is it correct to say: Running Rosenmund reduction with ordinary Pd.?

Without the BaSO₄ poison, the aldehyde is reduced further to a primary alcohol.

Is it correct to say: Expecting benzoic acid to undergo Friedel–Crafts reactions.?

–COOH deactivates the ring and binds the AlCl₃ catalyst. Benzoic acid gives meta substitution (e.g. m-nitrobenzoic acid) but no Friedel–Crafts reaction.

📄 Formula sheet (PDF)

Aldehydes, Ketones and Carboxylic Acids

Preparation and reactions of carbonyl compounds and carboxylic acids — Rosenmund, Etard, nucleophilic addition, Tollens', iodoform, aldol, Cannizzaro, acidity and HVZ — NCERT Class 12 Chemistry Ch 8

📐 20 formulas✏️ 4 examples🎯 6 practice⚖️ 8 marks🏫 CBSE📚 Class 12✓ 2025–26 syllabus

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Board Exam Tips

→Name reactions need exact reagents: Rosenmund (H₂, Pd–BaSO₄), Stephen (SnCl₂/HCl), Etard (CrO₂Cl₂), Clemmensen (Zn–Hg/conc. HCl), Wolff–Kishner (NH₂NH₂, then KOH/ethylene glycol), HVZ (X₂/red P).
→Before choosing aldol or Cannizzaro, look for an α-hydrogen: present → aldol (dilute alkali); absent → Cannizzaro (concentrated alkali).
→Distinguishing tests: Tollens' (all aldehydes), Fehling's (aliphatic aldehydes only), iodoform (CH₃CO– or CH₃CH(OH)– group), NaHCO₃ effervescence (carboxylic acids, not phenols).
→Explain reactivity towards nucleophilic addition using both steric and electronic effects: HCHO > other aldehydes > ketones.
→In conversions, track the carbon count: Grignard + CO₂ adds one carbon; soda-lime decarboxylation and the haloform reaction remove one.

📐 Formulas(20)

Rosenmund Reduction★ Board fav

R-COCl + H_{2} Pd-BaSO_{4} R-CHO + HCl

Stephen Reaction

R-CN + SnCl_{2} + HCl \to R-CH=NH H_{3} O^{+} R-CHO

Etard Reaction

C_{6} H_{5} CH_{3} (i) CrO_{2} Cl_{2}, CS_{2} (ii) H_{3} O^{+} C_{6} H_{5} CHO

Gattermann–Koch Reaction

C_{6} H_{6} + CO + HCl anhyd. AlCl_{3} / CuCl C_{6} H_{5} CHO

Friedel–Crafts Acylation (aromatic ketones)

C_{6} H_{6} + R-COCl anhyd. AlCl_{3} C_{6} H_{5} -CO-R + HCl

Nucleophilic Addition of HCN (cyanohydrin)

R_{2} C=O + HCN ⇌ OH^{-} R_{2} C(OH)CN

Condensation with Ammonia Derivatives

R_{2} C=O + H_{2} N-Z ⇌ R_{2} C=N-Z + H_{2} O

Clemmensen Reduction

R_{2} C=O Zn-Hg, conc. HCl R_{2} CH_{2} + H_{2} O

Wolff–Kishner Reduction

R_{2} C=O NH_{2} NH_{2} R_{2} C=NNH_{2} KOH ethylene glycol, Δ R_{2} CH_{2} + N_{2}

Tollens' Test (silver mirror)

RCHO + 2 [Ag(NH_{3})_{2}]^{+} + 3 OH^{-} \to RCOO^{-} + 2 Ag ↓ + 2 H_{2} O + 4 NH_{3}

Fehling's Test

RCHO + 2 Cu^{2 +} + 5 OH^{-} \to RCOO^{-} + Cu_{2} O ↓ + 3 H_{2} O

Iodoform (Haloform) Reaction★ Board fav

R-CO-CH_{3} + 3 I_{2} + 4 NaOH \to R-COONa + CHI_{3} ↓ + 3 NaI + 3 H_{2} O

Aldol Condensation★ Board fav

2 CH_{3} CHO dil. NaOH CH_{3} CH(OH)CH_{2} CHO Δ - H_{2} O CH_{3} CH=CHCHO

Cannizzaro Reaction★ Board fav

2 HCHO + conc. KOH Δ CH_{3} OH + HCOOK

Carboxylic Acids by Oxidation of Alkylbenzenes

C_{6} H_{5} CH_{3} KMnO_{4} -KOH, Δ C_{6} H_{5} COOK H_{3} O^{+} C_{6} H_{5} COOH

Carboxylic Acids from Grignard Reagents

R-MgX + CO_{2} dry ether R-COOMgX H_{3} O^{+} R-COOH

Acid Strength of Carboxylic Acids★ Board fav

FCH_{2} COOH > ClCH_{2} COOH > HCOOH > C_{6} H_{5} COOH > CH_{3} COOH > CH_{3} CH_{2} COOH

Acid Chlorides from Carboxylic Acids

RCOOH + SOCl_{2} \to RCOCl + SO_{2} ↑ + HCl ↑

Decarboxylation with Soda Lime

R-COONa + NaOH CaO Δ R-H + Na_{2} CO_{3}

Hell–Volhard–Zelinsky (HVZ) Reaction★ Board fav

R-CH_{2} -COOH (i) X_{2} / red P (ii) H_{2} O R-CH(X)-COOH (X = Cl, Br)

✏️ Solved Examples

1Solved Exampleeasy2 steps

Convert propanone into propene.

Reduce the ketone to a 2° alcohol

CH_{3} COCH_{3} NaBH_{4} CH_{3} CH(OH)CH_{3}

2Solved Exampleboard2 steps

Convert benzoic acid into benzaldehyde.

Direct reduction with LiAlH₄ would go all the way to benzyl alcohol, so first make the acid chloride

C_{6} H_{5} COOH + SOCl_{2} \to C_{6} H_{5} COCl + SO_{2} + HCl

3Solved Exampleboard2 steps

Convert bromobenzene into benzoic acid.

Make the Grignard reagent in dry ether

C_{6} H_{5} Br + Mg dry ether C_{6} H_{5} MgBr

4Solved ExampleHOTS3 steps

Compound A (C₂H₄O) gives a silver mirror with Tollens' reagent and a yellow precipitate with I₂/NaOH. With dilute NaOH, A forms B (C₄H₈O₂), which loses water on heating to give C (C₄H₆O). Identify A, B and C.

Tollens' positive ⇒ aldehyde. Iodoform positive ⇒ CH₃CO– group. The only C₂H₄O aldehyde is ethanal

A = CH_{3} CHO

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Applying the Cannizzaro reaction to ethanal.
✓Ethanal has α-H, so alkali gives aldol condensation. Cannizzaro needs an aldehyde with NO α-H (HCHO, C₆H₅CHO).
2
Saying all aldehydes give Fehling's test.
✓Aromatic aldehydes such as benzaldehyde do not reduce Fehling's solution, though they do give Tollens' test.
3
Thinking only methyl ketones give the iodoform test.
✓Ethanal, ethanol and any alcohol with a CH₃CH(OH)– group also give it. Methanol, methanal and propanal do not.
4
Using Clemmensen reduction on an acid-sensitive compound.
✓Use Wolff–Kishner (basic conditions) instead. Clemmensen (Zn–Hg/conc. HCl) suits compounds that are stable to acid.
5
Running Rosenmund reduction with ordinary Pd.
✓Without the BaSO₄ poison, the aldehyde is reduced further to a primary alcohol.
6
Expecting benzoic acid to undergo Friedel–Crafts reactions.
✓–COOH deactivates the ring and binds the AlCl₃ catalyst. Benzoic acid gives meta substitution (e.g. m-nitrobenzoic acid) but no Friedel–Crafts reaction.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Give a simple chemical test to distinguish propanal from propanone.
Q2
Arrange in increasing order of reactivity towards nucleophilic addition: propanone, methanal, ethanal.
Q3
Write the products of the Cannizzaro reaction of benzaldehyde with concentrated NaOH.
Q4
Arrange in increasing order of acid strength: CH₃COOH, ClCH₂COOH, FCH₂COOH, HCOOH.
Q5
Which of these undergo aldol condensation: methanal, ethanal, benzaldehyde, propanone?
Q6
What is formed when ethanoic acid is treated with Cl₂ and red phosphorus, followed by water?

📝 Notes

Aldehydes, Ketones and Carboxylic Acids

All three contain the carbonyl group, C=O. In aldehydes and ketones it is the main reactive site. In carboxylic acids it is joined to –OH, which makes the O–H acidic.

The carbonyl carbon is the target

The C=O bond is polar, so nucleophiles attack the carbon. This explains addition of HCN, condensation with H₂N–Z reagents (oximes, hydrazones, semicarbazones), and the Grignard reaction. Aldehydes react faster than ketones because they have less crowding and only one electron-releasing group. Aldehydes are also easy to oxidise, which is why Tollens' and Fehling's tests tell them apart from ketones.

The α-hydrogen decides the pathway

α-H present (CH₃CHO, CH₃COCH₃): dilute alkali gives the aldol, which loses water on heating to an α,β-unsaturated carbonyl compound.
No α-H (HCHO, C₆H₅CHO): concentrated alkali gives the Cannizzaro reaction, producing an alcohol and a carboxylate.
A CH₃CO– group gives the iodoform test.

Acidity of carboxylic acids

The carboxylate ion is stabilised by resonance: its two C–O bonds are equivalent. So carboxylic acids are stronger than phenols and alcohols, and they release CO₂ from NaHCO₃. Electron-withdrawing groups (F > Cl > Br, and more of them, closer to –COOH) raise acidity; alkyl groups lower it. Benzoic acid (pKa 4.19) is stronger than acetic acid (pKa 4.76).

Building a conversion

Choose the reaction that gives the right carbon count and the right oxidation level. Rosenmund, Stephen and Etard stop at the aldehyde. Grignard + CO₂ adds a carbon; decarboxylation removes one. Clemmensen and Wolff–Kishner remove the C=O completely.