Aldehydes, Ketones and Carboxylic Acids is Unit 8 of NCERT Class 12 Chemistry Part II and part of Organic Compounds Containing Oxygen in both the NEET UG and JEE Main syllabi. These notes cover naming, preparation, nucleophilic addition, Tollens’ and Fehling’s tests, aldol and Cannizzaro reactions, the haloform reaction, acidity of carboxylic acids and HVZ reaction, with a free PDF.
Aldehydes, Ketones and Carboxylic Acids Notes for NEET and JEE Main
Section-wise key points from NCERT Unit 8, name-reaction tables, high-yield facts and common mistakes for NEET and JEE Main 2027 revision.
Aldehydes, Ketones and Carboxylic Acids at a Glance
| Item | Details |
|---|---|
| Exams that cover it | NEET UG and JEE Main (both) |
| NEET UG syllabus unit | Unit 17: Organic Compounds Containing Oxygen |
| JEE Main syllabus unit | Unit 17: Organic Compounds Containing Oxygen (Paper 1 Chemistry) |
| NCERT textbook | Chemistry Part II, Class 12, Unit 8 (rationalised book) |
| Other chapter in the same syllabus unit | Alcohols, Phenols and Ethers (Class 12, Unit 7) |
| Syllabus topics | Preparation, properties, reactions and uses; nature of the carbonyl group; nucleophilic addition to >C=O and relative reactivity of aldehydes and ketones; addition of HCN, NH3 and its derivatives; Grignard reagent; oxidation; reduction (Wolff-Kishner and Clemmensen); acidity of α-hydrogen; aldol condensation; Cannizzaro reaction; haloform reaction; tests to distinguish aldehydes and ketones; carboxylic acids: acidic strength and factors affecting it |
| Official chapter | NCERT Class 12 Chemistry Unit 8 |
| Notes PDF | View PDF |
Topics follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) in December 2025 and the JEE Main 2026 syllabus published by the National Testing Agency (NTA). Both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.
Nomenclature and Structure of the Carbonyl Group
In aldehydes the carbonyl carbon is bonded to a carbon and a hydrogen, in ketones to two carbons, and in carboxylic acids to a carbon or hydrogen and an –OH group.
- IUPAC: replace the ‘-e’ of the alkane with ‘-al’ (aldehydes) or ‘-one’ (ketones). Aldehyde chains are numbered from the –CHO carbon; ketone chains from the end nearer the C=O.
- An aldehyde group on a ring takes the suffix ‘carbaldehyde’; benzenecarbaldehyde is also accepted as benzaldehyde.
- Common names: aldehydes come from the common acid name (formic, acetic) with ‘aldehyde’; substituent positions use Greek letters α, β, γ starting next to the carbonyl group. Alkyl phenyl ketones use the acyl name + ‘phenone’.
- The carbonyl carbon is sp2 hybridised with three σ bonds and one π bond to oxygen; the group is trigonal planar with bond angles of about 120°.
- The C=O bond is polar: the carbon is an electrophilic (Lewis acid) centre and the oxygen a nucleophilic (Lewis base) centre. Carbonyl compounds are more polar than ethers.
| Structure | Common name | IUPAC name |
|---|---|---|
| HCHO | Formaldehyde | Methanal |
| CH3CHO | Acetaldehyde | Ethanal |
| (CH3)2CHCHO | Isobutyraldehyde | 2-Methylpropanal |
| CH2=CHCHO | Acrolein | Prop-2-enal |
| CH3CH2CH2CH2CHO | Valeraldehyde | Pentanal |
| CH3COCH2CH2CH3 | Methyl n-propyl ketone | Pentan-2-one |
| (CH3)2CHCOCH(CH3)2 | Diisopropyl ketone | 2,4-Dimethylpentan-3-one |
| (CH3)2C=CHCOCH3 | Mesityl oxide | 4-Methylpent-3-en-2-one |
How are aldehydes and ketones prepared?
| Method | Reagents | Product |
|---|---|---|
| Oxidation of alcohols | Oxidising agents such as PCC or CrO3 | Primary alcohol gives aldehyde, secondary gives ketone |
| Dehydrogenation of alcohols | Vapours over Ag or Cu (industrial, volatile alcohols) | Aldehyde or ketone |
| Ozonolysis of alkenes | O3, then Zn dust and water | Aldehydes, ketones or both |
| Hydration of alkynes | H2SO4 + HgSO4 | Ethyne gives acetaldehyde; all other alkynes give ketones |
| Rosenmund reduction | Acyl chloride + H2 over Pd on BaSO4 | Aldehyde |
| Stephen reaction | Nitrile + SnCl2/HCl to imine, then hydrolysis | Aldehyde |
| DIBAL-H reduction | Nitrile or ester + diisobutylaluminium hydride, then hydrolysis | Aldehyde |
| Etard reaction | Toluene + chromyl chloride (CrO2Cl2), then hydrolysis | Benzaldehyde |
| Chromic oxide method | Toluene + CrO3 in acetic anhydride (benzylidene diacetate), then aqueous acid | Benzaldehyde |
| Side-chain chlorination | Toluene chlorinated to benzal chloride, then hydrolysis | Benzaldehyde (commercial method) |
| Gatterman-Koch reaction | Benzene + CO + HCl with anhydrous AlCl3 or CuCl | Benzaldehyde |
| From acyl chlorides | Dialkylcadmium (from CdCl2 + Grignard reagent) | Ketone |
| From nitriles | Grignard reagent, then hydrolysis | Ketone |
| Friedel-Crafts acylation | Benzene + acid chloride + anhydrous AlCl3 | Aromatic ketone |
Physical Properties of Aldehydes and Ketones
- Methanal is a gas at room temperature, ethanal a volatile liquid; the rest are liquids or solids.
- Boiling points are higher than those of hydrocarbons and ethers of similar mass (dipole-dipole attraction) but lower than alcohols (no intermolecular hydrogen bonding).
- Methanal, ethanal and propanone are miscible with water in all proportions because they hydrogen bond with water; solubility falls quickly as the chain grows.
- Lower aldehydes have sharp pungent odours; larger molecules smell less pungent and more fragrant.
| Compound (mass 58 or 60) | Boiling point (K) |
|---|---|
| n-Butane | 273 |
| Methoxyethane | 281 |
| Propanal | 322 |
| Acetone | 329 |
| Propan-1-ol | 370 |
Nucleophilic Addition Reactions
A nucleophile attacks the electrophilic carbonyl carbon roughly perpendicular to the plane of its sp2 orbitals; the carbon becomes sp3, a tetrahedral alkoxide intermediate forms and then picks up a proton.
- Aldehydes are more reactive than ketones: sterically, two groups in a ketone hinder the nucleophile; electronically, two alkyl groups reduce the electrophilicity of the carbonyl carbon more.
- Benzaldehyde is less reactive than propanal because resonance with the ring reduces the polarity of its carbonyl group.
| Reagent | Product | Key point |
|---|---|---|
| HCN (base catalysed) | Cyanohydrin | Pure HCN reacts very slowly; CN– is the stronger nucleophile |
| Sodium hydrogensulphite | Water-soluble addition product | Equilibrium lies right for most aldehydes, left for most ketones; used to separate and purify aldehydes |
| Grignard reagent | Alcohol after hydrolysis | See the alcohols chapter |
| Alcohol with dry HCl | Hemiacetal, then acetal | Ketones with ethylene glycol give ethylene glycol ketals; aqueous mineral acid reverses both |
| Ammonia derivatives H2N–Z (acid catalysed) | >C=N–Z | Reversible; rapid dehydration of the intermediate favours the product |
| Reagent H2N–Z | Product |
|---|---|
| Ammonia | Imine |
| Amine | Substituted imine (Schiff’s base) |
| Hydroxylamine | Oxime |
| Hydrazine | Hydrazone |
| Phenylhydrazine | Phenylhydrazone |
| 2,4-Dinitrophenylhydrazine | 2,4-Dinitrophenylhydrazone (yellow, orange or red solid used to characterise aldehydes and ketones) |
| Semicarbazide | Semicarbazone |
Reduction, Oxidation and Tests for Aldehydes
- NaBH4, LiAlH4 or catalytic hydrogenation reduce aldehydes to primary and ketones to secondary alcohols.
- Clemmensen reduction (zinc amalgam + conc. HCl) and Wolff-Kishner reduction (hydrazine, then heating with NaOH or KOH in ethylene glycol) reduce C=O to CH2.
- Aldehydes are oxidised easily to carboxylic acids, even by mild agents; ketones need strong oxidants and high temperature, and break C–C bonds to give acids with fewer carbons.
| Test | Reagent | Positive result |
|---|---|---|
| Tollens’ test | Freshly prepared ammoniacal silver nitrate (alkaline medium) | Bright silver mirror; aldehyde oxidised to carboxylate anion |
| Fehling’s test | Fehling A (aqueous CuSO4) + Fehling B (alkaline sodium potassium tartrate, Rochelle salt) in equal amounts | Reddish brown precipitate; aromatic aldehydes do not respond |
| Haloform reaction | Sodium hypohalite | Methyl ketones (and aldehydes with CH3CO–) give the sodium salt of an acid with one carbon less plus haloform; a C=C bond is not affected |
| Iodoform test | Sodium hypoiodite (I2 + NaOH) | Yellow iodoform precipitate; detects CH3CO– and CH3CH(OH)– groups |
| 2,4-DNP test | 2,4-Dinitrophenylhydrazine | Yellow, orange or red precipitate from both aldehydes and ketones |
Reactions Due to α-hydrogen: Aldol and Cannizzaro
α-Hydrogens are acidic because the carbonyl group withdraws electrons strongly and the conjugate base is resonance stabilised.
| Reaction | Condition | Product |
|---|---|---|
| Aldol reaction | At least one α-hydrogen; dilute alkali | β-Hydroxy aldehyde (aldol) or β-hydroxy ketone (ketol) |
| Aldol condensation | Aldol or ketol loses water | α,β-Unsaturated carbonyl compound |
| Cross aldol condensation | Two different carbonyl compounds | If both have α-hydrogens, a mixture of four products (e.g. ethanal + propanal) |
| Cannizzaro reaction | No α-hydrogen; heat with concentrated alkali | Disproportionation: one molecule reduced to alcohol, another oxidised to carboxylic acid salt |
| Ring substitution | Aromatic aldehydes and ketones with electrophiles | Carbonyl group is deactivating and meta-directing |
Uses of Aldehydes and Ketones
- Formalin (40% formaldehyde) preserves biological specimens; formaldehyde is used for bakelite (phenol-formaldehyde resin) and urea-formaldehyde glues.
- Acetaldehyde is a starting material for acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.
- Benzaldehyde is used in perfumery and dyes; acetone and ethyl methyl ketone are common industrial solvents.
- Vanillin, acetophenone, camphor and butyraldehyde are known for odours and flavours.
Carboxylic Acids: Nomenclature, Structure and Preparation
- Common names come from natural sources: formic acid from red ants (formica), acetic acid from vinegar (acetum), butyric acid from rancid butter (butyrum).
- IUPAC: replace ‘-e’ with ‘-oic acid’; the carboxyl carbon is C-1. Higher acids (C12 to C18) in natural fats are called fatty acids.
- Bonds to the carboxyl carbon lie in one plane about 120° apart; resonance makes the carboxyl carbon less electrophilic than a carbonyl carbon.
| Structure | Common name | IUPAC name |
|---|---|---|
| HCOOH | Formic acid | Methanoic acid |
| CH3COOH | Acetic acid | Ethanoic acid |
| HOOC–COOH | Oxalic acid | Ethanedioic acid |
| HOOC–CH2–COOH | Malonic acid | Propanedioic acid |
| HOOC–(CH2)2–COOH | Succinic acid | Butanedioic acid |
| HOOC–(CH2)4–COOH | Adipic acid | Hexanedioic acid |
| C6H5COOH | Benzoic acid | Benzenecarboxylic acid |
| C6H4(COOH)2 | Phthalic acid | Benzene-1,2-dicarboxylic acid |
| Preparation | Reagents | Key point |
|---|---|---|
| From primary alcohols | KMnO4 (neutral, acidic or alkaline), or K2Cr2O7 / CrO3 in acid (Jones reagent) | Carboxylic acid |
| From aldehydes | Mild oxidising agents | Carboxylic acid |
| From alkylbenzenes | Chromic acid or acidic/alkaline KMnO4 | Whole side chain becomes –COOH whatever its length; tertiary alkyl groups are not affected |
| From nitriles and amides | Hydrolysis with H+ or OH– | Mild conditions stop at the amide |
| From Grignard reagents | CO2 (dry ice), then mineral acid | Acid with one carbon more than the alkyl halide (as with nitriles) |
| From acyl halides and anhydrides | Water or aqueous base | Acid or carboxylate |
| From esters | Acidic or basic hydrolysis | Acid directly (acidic); carboxylate, then acid (basic) |
Why are carboxylic acids more acidic than phenols?
The carboxylate ion has two equivalent resonance structures with the negative charge on two electronegative oxygen atoms, while the phenoxide ion’s structures are non-equivalent and put charge on less electronegative carbons.
- Carboxylic acids react with metals (H2), alkalis and, unlike phenols, with carbonates and hydrogencarbonates, releasing CO2; this is a test for the carboxyl group.
- pKa values from NCERT: HCl –7.0, trifluoroacetic acid 0.23 (the strongest carboxylic acid), benzoic acid 4.19, acetic acid 4.76; ethanol about 16 and phenol 10.
- Smaller pKa means a stronger acid: below 1 strong, 1 to 5 moderately strong, 5 to 15 weak, above 15 extremely weak.
- Electron-withdrawing groups increase acidity; electron-donating groups decrease it. Effect order: Ph < I < Br < Cl < F < CN < NO2 < CF3.
- A phenyl or vinyl group directly on –COOH increases acidity because the sp2 carbon is more electronegative.
- Substituted benzoic acids: 4-nitrobenzoic acid 3.41, benzoic acid 4.19, 4-methoxybenzoic acid 4.46.
Acidity order from NCERT: CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC–CH2COOH > FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH > ClCH2CH2COOH > C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH.
Reactions of Carboxylic Acids
| Reaction | Reagent and conditions | Product and key point |
|---|---|---|
| Anhydride formation | Heat with H2SO4 or P2O5 | Acid anhydride |
| Esterification | Alcohol or phenol with conc. H2SO4 or HCl gas | Ester; nucleophilic acyl substitution, water or ester removed as formed |
| With PCl5, PCl3 or SOCl2 | SOCl2 preferred | Acyl chloride; with SOCl2 the other two products are gases, so purification is easy |
| With ammonia | NH3, then strong heating | Ammonium salt, then amide |
| Reduction | LiAlH4 or, better, diborane | Primary alcohol; diborane does not easily reduce ester, nitro or halo groups; NaBH4 does not reduce –COOH |
| Decarboxylation | Sodium salt + soda lime (NaOH:CaO = 3:1), heat | Hydrocarbon with one carbon less |
| Kolbe electrolysis | Electrolysis of aqueous alkali metal salt | Hydrocarbon with twice the carbons of the alkyl group |
| Hell-Volhard-Zelinsky (HVZ) | Cl2 or Br2 with a little red phosphorus | α-Halocarboxylic acid (needs an α-hydrogen) |
| Ring substitution | Electrophiles | –COOH is deactivating and meta-directing; no Friedel-Crafts reaction because AlCl3 binds to the carboxyl group |
Uses: methanoic acid in rubber, textile, dyeing, leather and electroplating; ethanoic acid as solvent and vinegar; hexanedioic acid for nylon-6,6; benzoate esters in perfumery; sodium benzoate as a food preservative; higher fatty acids for soaps and detergents.
Most-asked Facts for NEET and JEE Main: Aldehydes, Ketones and Carboxylic Acids
Revise these one-line facts from the NCERT chapter just before the exam.
- Rosenmund: acyl chloride + H2 over Pd-BaSO4 gives an aldehyde.
- Stephen: nitrile + SnCl2/HCl, then hydrolysis, gives an aldehyde.
- Etard: toluene + CrO2Cl2 gives benzaldehyde.
- Gatterman-Koch: benzene + CO + HCl with anhydrous AlCl3 or CuCl gives benzaldehyde.
- Ketones from acyl chlorides use dialkylcadmium, not Grignard reagent directly.
- Hydration of ethyne gives acetaldehyde; every other alkyne gives a ketone.
- Aldehydes are more reactive than ketones in nucleophilic addition.
- 2,4-DNP derivatives are yellow, orange or red solids.
- Tollens’ gives a silver mirror; Fehling’s gives a reddish brown precipitate and fails with aromatic aldehydes.
- Clemmensen uses Zn-Hg/conc. HCl; Wolff-Kishner uses hydrazine and KOH in ethylene glycol.
- Aldol needs an α-hydrogen and dilute alkali; Cannizzaro needs no α-hydrogen and concentrated alkali.
- Iodoform test detects CH3CO– and CH3CH(OH)– groups.
- Trifluoroacetic acid (pKa 0.23) is the strongest carboxylic acid in NCERT.
- Carboxylic acids, not phenols, release CO2 from hydrogencarbonates.
- Carboxylic acids exist as dimers in the vapour phase and in aprotic solvents.
- Diborane reduces –COOH but not ester, nitro or halo groups easily; NaBH4 does not reduce –COOH.
- HVZ reaction uses Cl2 or Br2 with red phosphorus and gives α-halo acids.
- Benzoic acid does not undergo Friedel-Crafts reaction.
Common Mistakes to Avoid
Watch for these mix-ups when you answer questions on this chapter.
- Writing α-hydroxy aldehyde as the aldol product. Aldol gives a β-hydroxy aldehyde or ketone.
- Running Cannizzaro on aldehydes with α-hydrogens (like ethanal). They undergo aldol instead.
- Expecting benzaldehyde to give a positive Fehling’s test. Aromatic aldehydes do not respond.
- Thinking the haloform reaction attacks a C=C bond in the molecule. It does not.
- Ranking phenol above carboxylic acids in acidity because phenoxide has more resonance structures. The carboxylate structures are equivalent and keep the charge on oxygen.
- Assuming electron-donating groups raise acidity. They destabilise the carboxylate and lower it.
- Using NaBH4 to reduce a carboxylic acid. Use LiAlH4 or diborane.
- Forgetting that side-chain oxidation of alkylbenzenes converts the whole chain to one –COOH, whatever its length.
- Mixing up Clemmensen (acidic, Zn-Hg/HCl) and Wolff-Kishner (basic, hydrazine/KOH).
- Saying HVZ halogenates the β-carbon. It halogenates the α-carbon.
Aldehydes, Ketones and Carboxylic Acids Notes PDF and Related Pages
Download the AglaSem notes for Aldehydes, Ketones and Carboxylic Acids (key notes and important questions with answers) free: View PDF
- All chapters: NEET and JEE Main Chemistry notes
- Full topic list: NEET syllabus and JEE Main syllabus
- Practice: NEET question papers and JEE Main question papers
- Source text: NCERT Class 12 Chemistry Unit 8 (NCERT, Class 12 Chemistry Part II)
Chemistry Notes for NEET and JEE Main: All Chapters
- Some Basic Concepts of Chemistry
- Structure of Atom
- Chemical Kinetics
- Equilibrium
- Organic Chemistry: Some Basic Principles and Techniques
- Electrochemistry
- Amines
- Solutions
- Thermodynamics
- Coordination Compounds
- Haloalkanes and Haloarenes
- p-Block Elements
- Alcohols, Phenols and Ethers
- Hydrocarbons
- Aldehydes, Ketones and Carboxylic Acids
All chapters with key points and notes PDFs are on the Chemistry notes for NEET and JEE Main page.
Aldehydes, Ketones and Carboxylic Acids Notes for NEET and JEE Main FAQs
Which NCERT chapter is Aldehydes, Ketones and Carboxylic Acids?
It is Unit 8 of the NCERT Class 12 Chemistry Part II textbook (rationalised book), between Alcohols, Phenols and Ethers (Unit 7) and Amines (Unit 9).
Is Aldehydes, Ketones and Carboxylic Acids in the NEET 2027 and JEE Main 2027 syllabus?
It is in the latest syllabi of both exams: Unit 17, Organic Compounds Containing Oxygen, of the NEET UG 2026 syllabus (NMC) and of the JEE Main 2026 syllabus (NTA). The 2027 syllabi are expected later; check the syllabus pages once they are out.
How can you distinguish an aldehyde from a ketone?
Aldehydes reduce Tollens’ reagent (silver mirror) and Fehling’s reagent (reddish brown precipitate, aliphatic aldehydes only); ketones do not. Both give a coloured 2,4-DNP derivative, so that test alone cannot tell them apart.
When does an aldehyde undergo aldol condensation and when Cannizzaro reaction?
Aldehydes with at least one α-hydrogen undergo aldol reaction with dilute alkali. Aldehydes with no α-hydrogen, such as methanal and benzaldehyde, undergo Cannizzaro reaction with concentrated alkali, giving an alcohol and a carboxylic acid salt.
Are these notes enough for NEET and JEE Main questions on this chapter?
No. Read NCERT Unit 8 line by line first, including every name reaction and its conditions, then use these notes and the PDF for quick revision and practise previous year questions of both exams.