Aldehydes, Ketones and Carboxylic Acids covers the naming and structure of the polar carbonyl group, methods of preparing these compounds, nucleophilic addition reactions of the carbonyl, reactions involving the alpha-hydrogen such as aldol and Cannizzaro, identification tests, the acidity of carboxylic acids, and conversion of acids into their derivatives. It is a high-yield NEET chapter where questions test reactivity order, named reactions, distinguishing tests like Tollens and iodoform, and the factors affecting acid strength.
Class 12 Chemistry – swipe through all 9 cards to understand the whole chapter.
The Carbonyl Group >C=O
One polar C=O bond is the heart of this whole chapter.
Oxygen is more electronegative, so the carbon is electron-poor (δ+).
- Aldehyde R–CHO (carbonyl at chain end); ketone R–CO–R′ (in the middle); acid R–COOH
- δ+ carbon is electrophilic → open to nucleophiles; oxygen is mildly basic
- This single tilt drives nearly every reaction ahead
IUPAC & Common Names
Number the chain so the carbonyl carbon gets the lowest locant.
Seniority for suffix: carboxylic acid > ester > amide > aldehyde > ketone > alcohol.
- Trivial names: HCHO formaldehyde, CH3CHO acetaldehyde, CH3COCH3 acetone
- HCOOH formic acid, CH3COOH acetic acid
- Winner of seniority becomes the suffix; the rest become prefixes
Making Aldehydes, Ketones & Acids
Match each reagent to its product — the favourite one-word questions.
Rosenmund uses POISONED Pd/BaSO4; plain Pd over-reduces to the alcohol.
- Etard: toluene + CrO2Cl2 → benzaldehyde; Gattermann–Koch: C6H6 + CO/HCl/AlCl3 → benzaldehyde
- Ketones: Friedel–Crafts acylation (RCOCl/AlCl3); nitrile + Grignard then hydrolysis
- Ethyne is the ONLY alkyne giving an aldehyde on hydration (→ acetaldehyde)
Nucleophilic Addition to C=O
Electron-rich nucleophiles attack the δ+ carbonyl carbon — the signature reaction.
NH2–G derivatives: hydroxylamine → oxime, hydrazine → hydrazone, 2,4-DNP → orange ppt.
- 2,4-DNP confirms a carbonyl but CANNOT tell aldehyde from ketone
- NaHSO3 purifies aldehydes & small/methyl ketones; bulky ketones fail
- Flat C=O becomes a tetrahedral carbon carrying two new groups
Aldehydes > Ketones
Fewer alkyl groups means more δ+ and less crowding, so faster addition.
Benzaldehyde is sluggish: the ring donates electrons by resonance, cutting the δ+.
- Two +I alkyl groups in ketones shrink the δ+ on carbon
- Alkyl groups also cause steric hindrance, blocking the nucleophile
- Electron-withdrawing groups raise reactivity; donating groups lower it
Aldol, Cannizzaro & Haloform
The acidic α-hydrogen next to C=O decides which named reaction can run.
Aldol gives a β-hydroxy carbonyl first; on heating it dehydrates to an α,β-unsaturated carbonyl.
- Cannizzaro needs NO α-H (HCHO, C6H5CHO): one oxidised to acid, one reduced to alcohol
- Iodoform (CHI3, yellow): CH3CO– groups, plus CH3CH(OH)– → tests methyl ketones, acetaldehyde, ethanol
- 2 CH3CHO aldol → 3-hydroxybutanal, then → but-2-enal (crotonaldehyde)
Tollens & Fehling
Aldehydes oxidise easily; ketones resist — that gap powers the tests.
NEET trap: benzaldehyde gives Tollens but does NOT give Fehling’s.
- Ketones give neither Tollens nor Fehling
- Aromatic aldehydes do not respond to Fehling’s
- Aldehydes are easily oxidised to acids; ketones cleave only under strong oxidation
Why Carboxylic Acids Are Acidic
The carboxylate ion shares its negative charge equally over two oxygens.
Stronger than phenol: phenoxide spreads charge onto less electronegative carbons.
- EWG (–Cl, –F, –NO2) stabilise the anion → stronger: Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH
- Effect fades with distance: 2-chloro > 3-chloro > 4-chloro acid
- HCOOH > CH3COOH (formic has no electron-donating +I alkyl group)
Reactions of –COOH
Swap the –OH and the single –COOH unfolds into many derivatives.
SOCl2 is preferred — its byproducts SO2 and HCl are gases that escape, leaving a clean product.
- LiAlH4 (or B2H6) reduces –COOH → 1° alcohol; NaBH4 does NOT reduce acids
- HVZ: acid with α-H + X2/red P → α-halo acid; HCOOH & benzoic acid (no α-H) fail
- Decarboxylation: RCOONa + soda lime → alkane (CH3COONa → CH4, loses one carbon)
📝 Practice Aldehydes, Ketones and Carboxylic Acids — 10 NEET PYQs
Real previous-year questions · with answers & solutions
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Chapter Navigation
Previous: Alcohols, Phenols and Ethers Class 12 Notes
Next: Amines Class 12 Notes
Related Chapters in Class 12 Chemistry
- Alcohols, Phenols and Ethers Class 12 Notes
- Amines Class 12 Notes
- Haloalkanes and Haloarenes Class 12 Notes
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Frequently Asked Questions
The carbonyl group is the carbon-oxygen double bond (C=O) found in aldehydes, ketones and carboxylic acids. It is polar because oxygen is more electronegative, giving the carbon a partial positive charge. This makes the carbonyl carbon electrophilic, so it is readily attacked by nucleophiles.
The order is formaldehyde (HCHO) greater than other aldehydes greater than ketones. Reactivity decreases as more or bulkier alkyl groups are attached to the carbonyl carbon, because they cause steric hindrance and reduce the partial positive charge through their electron-donating effect.
The Cannizzaro reaction is a base-induced disproportionation of aldehydes that have no alpha-hydrogen, such as formaldehyde and benzaldehyde. One molecule is oxidised to a carboxylic acid salt and another is reduced to an alcohol.
Carboxylic acids are more acidic because the carboxylate ion formed after losing a proton is stabilised by resonance, with the negative charge delocalised equally over two oxygen atoms. In phenoxide the charge is spread onto less electronegative carbon atoms, so it is less stabilised and phenol is a weaker acid.
Aldehydes are easily oxidised and give positive tests with Tollens’ reagent (forming a silver mirror) and Fehling’s solution (forming a red-brown precipitate), whereas ketones do not. The iodoform test is positive for methyl ketones and acetaldehyde but is not a general aldehyde-versus-ketone test.