Introduction and Structure
Aldehydes and ketones both contain the carbonyl group (C=O). In aldehydes, the carbonyl C is bonded to at least one H (R-CHO). In ketones, it is bonded to two carbon groups (R-CO-R').
The C=O group is polar (oxygen is more electronegative). The carbonyl carbon carries a partial positive charge, making it an electrophile and the site of nucleophilic attack.
IUPAC Naming
| Class | Suffix | Example | IUPAC Name |
|---|---|---|---|
| Aldehyde | -al | CH3CHO | Ethanal (acetaldehyde) |
| Aldehyde (cyclic) | carbaldehyde | C6H5CHO | Benzaldehyde |
| Ketone | -one | CH3COCH3 | Propan-2-one (acetone) |
| Carboxylic acid | -oic acid | CH3COOH | Ethanoic acid (acetic acid) |
| Dicarboxylic acid | -dioic acid | HOOC-COOH | Ethanedioic acid (oxalic acid) |
Preparation of Aldehydes and Ketones
Aldehydes
- Oxidation of primary alcohols with PCC: 1° alcohol + PCC → aldehyde (stopped). The key advantage of PCC is that it does not oxidise the aldehyde further.
- Rosenmund reduction: acid chloride (R-COCl) + H2 (Pd/BaSO4 catalyst, poisoned) → aldehyde (R-CHO). The poisoned catalyst prevents further reduction to alcohol.
- Stephen's reduction: nitrile (R-CN) + SnCl2/HCl → imine salt → hydrolysis → aldehyde. Used for aromatic aldehydes.
- Etard reaction: toluene + CrO2Cl2 (chromyl chloride) → chromium complex → hydrolysis → benzaldehyde.
- Gattermann-Koch reaction: benzene + CO + HCl + anhydrous AlCl3/CuCl → benzaldehyde.
- From Grignard + HCN → imine → aldehyde: RMgX + HCN → RCH=NH → R-CHO (hydrolysis).
Ketones
- Oxidation of secondary alcohols: 2° alcohol + any oxidant → ketone.
- From acid chlorides + organocadmium reagent: R-COCl + R'2Cd → R-CO-R' + CdCl. Organocadmium is milder than Grignard and stops at ketone without over-reaction.
- Friedel-Crafts acylation: ArH + RCOCl + AlCl3 → Ar-CO-R + HCl. Used for aryl ketones (e.g., acetophenone from benzene + CH3COCl).
- From nitriles (Grignard + nitrile): R'MgX + R-CN → ketimine → ketone R-CO-R' (hydrolysis).
- Ozonolysis of alkenes: internal alkene + O3 / Zn-H2O → two ketones (or aldehyde + ketone for unsymmetric alkene).
Nucleophilic Addition Reactions
The carbonyl group undergoes nucleophilic addition. The nucleophile (Nu-) attacks the electrophilic C=O carbon. The pi bond breaks, the O becomes negatively charged (alkoxide), and then a proton is picked up to give the product.
Key Nucleophilic Addition Reactions
| Reagent | Product | Notes |
|---|---|---|
| HCN (cyanide) | Cyanohydrin (R-CH(OH)-CN) | Chain extended by 1C. Nucleophile: CN-. Catalysed by KCN or KOH. |
| NaHSO3 (sodium bisulfite) | Bisulfite addition product | Only for aldehydes and methyl ketones (not bulky). Used to purify aldehydes. |
| 1° amine (RNH2) | Imine (Schiff base, R-CH=NR') | Addition then dehydration. Imines formed from aldehydes + 1° amines. |
| 2° amine (R2NH) | Enamine | Addition then dehydration. No N-H left to form C=N-H, gives C=C-N (enamine). |
| Hydroxylamine (H2N-OH) | Oxime (R-CH=NOH) | Used to identify and characterise aldehydes and ketones (melting point of oxime). |
| Phenylhydrazine (C6H5NHNH2) | Phenylhydrazone | Crystalline solid for identification. |
| 2,4-DNP | 2,4-Dinitrophenylhydrazone (orange ppt) | Confirmatory test for C=O group in aldehydes and ketones. |
| ROH (1 mole) + acid catalyst | Hemiacetal / hemiketal | Unstable intermediate. |
| ROH (2 moles) + acid catalyst | Acetal / ketal + H2O | Stable; used as protecting group for carbonyl in synthesis. |
Reduction of Carbonyls
- H2/Ni: both aldehydes and ketones reduced to alcohols (1° and 2° respectively).
- NaBH4: mild, selective — reduces C=O, not C=C or COOH.
- LiAlH4: reduces all C=O groups including COOH, esters, amides. Use in dry ether.
- Clemmensen reduction: C=O → -CH2- using Zn/Hg in conc. HCl. Removes carbonyl oxygen. Used for acid-stable compounds.
- Wolff-Kishner reduction: C=O → -CH2- using N2H4 (hydrazine) + KOH. Used for acid-sensitive compounds (base conditions).
Oxidation of Aldehydes
- Tollens' test: aldehyde + Tollens' reagent (AgNO3 + NH3) → silver mirror (Ag metal deposited). Aldehyde is oxidised to carboxylate. Ketones do NOT give Tollens' test.
- Fehling's test: aliphatic aldehyde + Fehling's solution (Cu2+/tartrate) → brick-red Cu2O precipitate. Note: aromatic aldehydes (e.g., benzaldehyde) do NOT give Fehling's test (too weak a reductant). Ketones also do not.
Track Your NEET Score Across All 90 Chapters
Free 14-day trial. AI tutor, full mock tests and chapter analytics — built for NEET 2027.
Aldehyde vs Ketone Reactivity
Aldehydes are more reactive than ketones towards nucleophilic addition for two reasons:
- Steric effect: Aldehydes have only one alkyl group (H + R) around the carbonyl C. Ketones have two alkyl groups (R + R'), which are bulkier and block nucleophilic approach. Less steric hindrance in aldehydes → faster reaction.
- Electronic effect: Alkyl groups donate electrons by induction (+I effect). This pushes electron density onto the carbonyl C, reducing its electrophilicity. Ketones have two electron-donating alkyl groups; aldehydes have only one. So ketone carbonyl C is less positive, less electrophilic, less reactive toward nucleophiles.
Reactivity order: HCHO > RCHO > RCOR' > RCO-Ar > ArCO-Ar (most to least reactive).
Aldehyde vs Ketone: Nucleophilic Addition Reactivity
Explore why some carbonyl compounds are far more reactive than others. Two factors drive the difference: steric hindrance (how blocked is the carbonyl carbon?) and electronic effect (how electrophilic is it?). Select any compound to see both factors explained.
Formaldehyde
HCHO
Aldol Condensation and Cannizzaro Reaction
Aldol Condensation
When an aldehyde or ketone with at least one alpha-H (C-H adjacent to C=O) is treated with dilute NaOH (base-catalysed) or dilute acid:
- Base removes an alpha-H to form an enolate anion.
- The enolate acts as a nucleophile and attacks the C=O of another molecule (same or different).
- Product: beta-hydroxy aldehyde/ketone (aldol product). The name "aldol" comes from having both aldehyde and alcohol functional groups.
- On heating: dehydration gives an alpha,beta-unsaturated carbonyl compound(conjugated enone). E.g., acetaldehyde → aldol → crotonaldehyde.
Example: 2 CH3CHO + dil NaOH → CH3CH(OH)CH2CHO (3-hydroxybutanal, acetaldol) → on heating → CH3CH=CHCHO (but-2-enal / crotonaldehyde).
Cross-aldol condensation: uses two different carbonyl compounds. Gives a mixture unless one has no alpha-H (e.g., formaldehyde, benzaldehyde) — then only one product forms (directed cross-aldol).
Cannizzaro Reaction
Aldehydes with no alpha-H (formaldehyde, benzaldehyde, trimethylacetaldehyde) undergo disproportionation in concentrated NaOH. One molecule of aldehyde is oxidised to carboxylate; another is reduced to alcohol.
Example: 2 HCHO + NaOH (conc.) → HCOO-Na+ + CH3OH (methanol). Formaldehyde: one molecule oxidised to formate, one reduced to methanol.
Crossed Cannizzaro: formaldehyde + another aldehyde with no alpha-H in NaOH. Formaldehyde is always preferentially oxidised (most easily). The other aldehyde is reduced to alcohol. Used to make benzyl alcohol from benzaldehyde.
Identification Tests for Aldehydes and Ketones
Aldehyde and Ketone Test Identifier
Select a compound and a test to see whether the result is positive or negative, with the chemical reason. Know exactly which tests identify aldehydes, which identify methyl ketones, and the key exceptions.
Tollens' Test
Aliphatic aldehyde — reduces Ag+.
| Test | Reagent | Aldehydes | Ketones | Notes |
|---|---|---|---|---|
| Tollens' test | AgNO3 + NH3 (silver mirror reagent) | Silver mirror / black ppt | No reaction | Both aliphatic and aromatic aldehydes react |
| Fehling's test | Fehling A (CuSO4) + Fehling B (NaOH/KNaC4H4O6) | Brick-red Cu2O ppt (aliphatic aldehydes only) | No reaction | Benzaldehyde and aromatic aldehydes: NO reaction |
| Schiff's test | Schiff's reagent (fuchsin/magenta dye decolourised by SO2) | Pink/magenta colour restored | No reaction (ketones) | Very sensitive; acetone may give weak colour |
| 2,4-DNP test | 2,4-Dinitrophenylhydrazine | Orange/yellow ppt | Orange/yellow ppt | Positive for ALL C=O groups (identifies carbonyl) |
| Iodoform test | I2 + NaOH | Only acetaldehyde (CH3CHO) | Only methyl ketones (CH3CO-R) | Yellow CHI3 precipitate with characteristic smell |
Carboxylic Acids: Structure and Acidity
Carboxylic acids contain the carboxyl group (-COOH). They are much more acidic than alcohols (pKa ~4-5 vs ~15-16) because the carboxylate anion (-COO-) is highly stabilised by resonance — the negative charge is delocalised equally over both oxygen atoms.
Effect of Substituents on Acidity
Electron-withdrawing groups (EWG) on the alpha carbon stabilise the carboxylate anion and increase acidity. Electron-donating groups (EDG) destabilise the carboxylate and decrease acidity.
| Compound | pKa | Effect |
|---|---|---|
| Formic acid (HCOOH) | 3.74 | Simplest — no alkyl group to donate electrons. Most acidic simple acid. |
| Acetic acid (CH3COOH) | 4.74 | CH3 donates electrons (+I), destabilises carboxylate slightly. |
| Chloroacetic acid (ClCH2COOH) | 2.86 | Cl withdraws electrons (-I) from alpha-C, stabilises carboxylate. More acidic than acetic acid. |
| Dichloroacetic acid (Cl2CHCOOH) | 1.48 | Two Cl atoms: stronger -I effect. Even more acidic. |
| Trichloroacetic acid (Cl3CCOOH) | 0.70 | Three Cl atoms: very strong -I, approaching mineral acid strength. |
Distance effect: substituents further from -COOH have less effect. 2-chlorobutanoic acid > 3-chlorobutanoic acid > 4-chlorobutanoic acid in acidity (Cl is closer to COOH in 2-position).
Reactions of Carboxylic Acids
1. Esterification
R-COOH + R'-OH + H2SO4 (cat.) ⇌ R-COO-R' + H2O. Reversible; use excess alcohol or remove water to get good yield. Fischer esterification mechanism: protonation of C=O → nucleophilic attack of alcohol → elimination of water.
2. Formation of Acyl Chloride
R-COOH + SOCl2 (thionyl chloride) → R-COCl + SO2 + HCl. Best method (gaseous byproducts give pure product). Also: PCl5 or PCl3.
3. Reduction
- LiAlH4 (dry ether): R-COOH → R-CH2OH (primary alcohol). Strong reductant needed.
- NaBH4 does NOT reduce COOH (too mild).
4. Decarboxylation
- Dry distillation with soda lime (NaOH + CaO): R-COONa + CaO (heat) → R-H + CaCO3. Removes one carbon. Example: sodium acetate → methane.
- Kolbe electrolysis: 2 R-COOK (aqueous, electrolysis) → R-R + 2CO2 + 2K+ + H2 (at anode, carboxylate → R radical → coupling).
5. Hell-Volhard-Zelinsky (HVZ) Reaction
R-CH2-COOH + Cl2 (or Br2) + red phosphorus → R-CH(Cl/Br)-COOH (alpha-halo acid). Selective alpha-halogenation. Mechanism: P + Cl2 → PCl3; R-COOH + PCl3 → R-COCl (acyl chloride); acyl chloride enolises at alpha-C; Cl2 halogenates alpha position; acyl chloride hydrolysis gives alpha-halo acid.
6. Anhydride Formation
2 R-COOH (heat with P2O5) → R-CO-O-CO-R + H2O. More commonly via ketene or acyl chloride.
Relative Reactivity of Acyl Derivatives (Hydrolysis)
Acyl chloride > anhydride > ester > amide (most to least reactive toward hydrolysis). Acyl chloride is the most reactive because Cl- is the best leaving group. Amide is the least reactive because the nitrogen lone pair stabilises the C=O through resonance (N-C=O resonance).
Worked NEET Problems
NEET-style problem · Cannizzaro reaction
Question
Solution
NEET-style problem · Identification tests
Question
Solution
NEET-style problem · Acidity order
Question
Solution
Summary Cheat Sheet
Aldehyde vs Ketone (One-line rules)
- Both: nucleophilic addition, form 2,4-DNP hydrazone, reduced by LiAlH4/NaBH4
- Aldehydes only: Tollens' test (silver mirror), Fehling's test (aliphatic only), Schiff's test, oxidised to carboxylic acid
- Ketones only: resistant to Tollens/Fehling oxidation; methyl ketones give iodoform test
Reaction Conditions to Remember
- Clemmensen: C=O → CH2 (Zn/Hg + conc. HCl)
- Wolff-Kishner: C=O → CH2 (N2H4 + KOH)
- Rosenmund: R-COCl + H2 (Pd-BaSO4) → R-CHO
- HVZ: RCOOH + Cl2 + red P → alpha-halo acid
- Aldol: alpha-H present + dilute NaOH → beta-hydroxy carbonyl → heat → unsaturated carbonyl
- Cannizzaro: no alpha-H + conc. NaOH → disproportionation (one → acid, one → alcohol)
Acidity Order (Quick)
CCl3COOH > CHCl2COOH > CH2ClCOOH > HCOOH > CH3COOH > phenol > water > alcohol
Frequently asked questions
Why are aldehydes more reactive than ketones towards nucleophilic addition?
Two factors work together. (1) Steric effect: in an aldehyde (R-CHO), the carbonyl carbon has one H and one alkyl group attached. A nucleophile can approach relatively easily. In a ketone (R-CO-R'), both substituents are bulky alkyl groups, which physically block the nucleophile from approaching the carbonyl carbon. (2) Electronic effect: alkyl groups donate electrons by induction (+I effect), pushing electron density onto the carbonyl carbon. This reduces the partial positive charge on C, making it less electrophilic and less attractive to nucleophiles. Ketones have two electron-donating alkyl groups; aldehydes have only one. The result: formaldehyde (HCHO) is most reactive (H + H), then aliphatic aldehydes (H + R), then aliphatic ketones (R + R), then aryl ketones (Ar + R, ArCO-R) least reactive among common carbonyls.
What is the difference between the Tollens test and Fehling test? Which aldehydes react with each?
Tollens' test: uses Tollens' reagent (ammoniacal silver nitrate, Ag(NH3)2+). The aldehyde reduces Ag+ to metallic silver, which deposits as a silver mirror on the test tube. Both aliphatic AND aromatic aldehydes give a positive Tollens' test. Ketones do NOT react. Fehling's test: uses Fehling's solution (Fehling A = CuSO4, Fehling B = sodium potassium tartrate in NaOH). The aldehyde reduces Cu2+ (deep blue) to Cu+ as brick-red Cu2O precipitate. Only ALIPHATIC aldehydes (formaldehyde, acetaldehyde, propanal, etc.) give a positive Fehling test. Aromatic aldehydes (benzaldehyde, cinnamaldehyde) do NOT give Fehling's test — they are weaker reducing agents and cannot reduce the Cu2+ complex. Ketones give no positive Fehling test. Key distinction: Fehling distinguishes aliphatic from aromatic aldehydes; Tollens does not (both aryl and alkyl react).
What is the aldol condensation? What conditions are needed and what is the product?
Aldol condensation occurs when an aldehyde or ketone with at least one alpha-H (H on the carbon adjacent to C=O) reacts under dilute NaOH (base catalyst) or dilute acid catalyst. Step 1: base removes an alpha-H to form an enolate anion. Step 2: the enolate attacks the C=O carbon of another molecule nucleophilically. Step 3: protonation gives a beta-hydroxy aldehyde or ketone (the "aldol product" — named from having both ALDehyde and alcohOL groups). On gentle heating, the beta-hydroxy carbonyl undergoes dehydration to give an alpha,beta-unsaturated carbonyl compound (conjugated enone or enal). The reaction is very important in synthesis. If one of the two components has no alpha-H (e.g., benzaldehyde, formaldehyde), a crossed-aldol condensation gives only one product because only the alpha-H-containing partner can act as the nucleophile.
What is the Cannizzaro reaction and which aldehydes undergo it?
The Cannizzaro reaction is a disproportionation reaction — one molecule is oxidised while another is reduced. It occurs with aldehydes that have NO alpha-H, in the presence of concentrated NaOH. Examples: formaldehyde (HCHO), benzaldehyde (C6H5CHO), and trimethylacetaldehyde (pivaldehyde). Mechanism: NaOH adds to one aldehyde to form an alkoxide. This alkoxide transfers a hydride (H-) to the carbonyl carbon of a second aldehyde molecule (intermolecular hydride transfer). Products: one molecule is oxidised to carboxylate (salt), the other is reduced to alcohol. Example: 2 HCHO + NaOH → HCOONa + CH3OH. Why does it not happen with aldehydes that have alpha-H? Because those aldehydes preferentially undergo aldol condensation, which is much faster than Cannizzaro for alpha-H-bearing aldehydes.
Why is formic acid (HCOOH) more acidic than acetic acid (CH3COOH)?
Both are carboxylic acids. Their acidity depends on how stable the carboxylate anion (RCOO-) is after losing a proton. Formic acid (HCOOH, pKa 3.74) has no alkyl group — the carboxylate HCOO- has only H attached to the carbonyl carbon. No electron donation occurs. Acetic acid (CH3COOH, pKa 4.74) has a methyl group (-CH3) that donates electrons to the ring by induction (+I effect). This electron donation increases the electron density on the carboxylate oxygen, destabilising the negative charge slightly, making it slightly harder to form and making acetic acid weaker. In general: as alkyl chain length increases, acidity decreases (more electron donation). Formic acid is the most acidic among simple aliphatic carboxylic acids because it has the least electron donation.
What is the Hell-Volhard-Zelinsky reaction?
The Hell-Volhard-Zelinsky (HVZ) reaction selectively halogenates the alpha position of a carboxylic acid. Reagents: Cl2 (or Br2) + a small amount of red phosphorus (catalyst). The red P reacts with Cl2 to form PCl3, which converts the carboxylic acid to an acyl chloride (more reactive toward enolisation). The acyl chloride enolises at the alpha carbon. Cl2 (or Br2) then halogenates the alpha carbon. The acyl chloride intermediate is hydrolysed back to give the alpha-halo carboxylic acid. Product: R-CHCl-COOH (2-chloroalkanoic acid) or R-CHBr-COOH. The reaction is important because it installs a halogen at the alpha position, which can then be displaced by -OH, -NH2, -CN, etc. to make alpha-substituted carboxylic acids.
How many questions from this chapter appear in NEET and which topics are most important?
Aldehydes, Ketones and Carboxylic Acids is one of the highest-weightage organic chapters in NEET Class 12 Chemistry. You can expect 3-6 questions every year. The most tested topics are: (1) Identification tests — Tollens vs Fehling vs Schiff, which aldehyde reacts with which. Know the exceptions (benzaldehyde: Tollens yes, Fehling no). (2) Aldol condensation — what product forms, conditions, and whether crossed-aldol is clean or gives a mixture. (3) Cannizzaro reaction — recognise aldehydes with no alpha-H; know products are one acid + one alcohol. (4) Acidity order — formic acid most acidic among simple acids; EWG on alpha-C increases acidity; remember the chloroacetic acid series. (5) Named reactions — Clemmensen, Wolff-Kishner, Rosenmund, HVZ. Know conditions and products precisely.
Continue with the next chapter notes
Stay in NCERT order — the next chapter's notes are one click away.
Track Your NEET Score Across All 90 Chapters
Free 14-day trial. AI tutor, full mock tests and chapter analytics — built for NEET 2027.
Free 14-day trial · No credit card required