Aldehydes, Ketones and Carboxylic Acids Class 12 Notes: Full Chapter Revision

Need aldehydes ketones and carboxylic acids class 12 notes that hold together the night before the exam? This page condenses Chapter 8 of the NCERT Chemistry Part II textbook into a revision-ready sequence — carbonyl group structure, IUPAC naming, eleven name reactions, the three distinguishing tests, and the carboxylic acid acidity ladder.

The notes follow the order a teacher builds ideas in, not the textbook’s section order. You start with the carbonyl group, move through nomenclature, preparation, reactions and tests, then the full acids half. Worked examples use original compounds so you practise the pattern, never a memorised answer. NCERT page numbers are cited inline for every key idea.

The Carbonyl Group: Why These Three Families Sit Together

Everything in this chapter flows from one functional group: the carbon-oxygen double bond, \( C=O \), called the carbonyl group. It is one of the most important functional groups in organic chemistry, found in fabrics, flavourings, plastics and drugs (NCERT, p. 228).

The carbonyl carbon is \( sp^2 \)-hybridised and forms three sigma bonds. The fourth valence electron sits in a p-orbital and overlaps with an oxygen p-orbital to make a pi bond.

So the carbonyl carbon and the three atoms attached to it lie in one plane, with the pi-electron cloud above and below that plane; bond angles are about \( 120^\circ \) (NCERT, p. 231).

Because oxygen is more electronegative, the double bond is polarised: the carbonyl carbon becomes an electrophilic (Lewis acid) centre and the carbonyl oxygen a nucleophilic (Lewis base) centre. This polarity is explained by resonance between a neutral form (A) and a dipolar form (B) that carries a positive charge on carbon and a negative charge on oxygen.

What sits next to the \( C=O \) decides the family:

These molecules are widespread in plants and animals and add flavour and fragrance to nature — vanillin (vanilla beans), salicylaldehyde (meadow sweet) and cinnamaldehyde (cinnamon). Acetone is a common industrial solvent, and many members are used to prepare adhesives, paints, resins and perfumes (NCERT, p. 228).

Key Terms of Carbonyl Chemistry: A Quick Lookup

This is your one-mark definition bank. Each term below is defined the way a markscheme expects, with a one-line reaction example (NCERT, pp. 237–238, 245, 253).

Nomenclature: Common and IUPAC Names

Two naming systems run through this chapter, and examiners ask for both. Learn the rules below and the conversion table that follows (NCERT, pp. 229–230, 244).

• Common names of aldehydes: take the acid’s common name and replace -ic acid with -aldehyde (formic → formaldehyde, acetic → acetaldehyde). Positions use Greek letters \( \alpha \), \( \beta \), \( \gamma \), \( \delta \), with \( \alpha \)-carbon directly attached to the \( CHO \).
• Common names of ketones: name the two alkyl or aryl groups attached to \( C=O \) (dimethyl ketone = acetone); alkyl phenyl ketones add the acyl prefix to -phenone.
• IUPAC aldehydes: alkane ending -e → -al; number the chain from the aldehyde carbon; substituents appear alphabetically with locants.
• IUPAC ketones: -e → -one; number from the end nearer the carbonyl carbon.
• Cyclic aldehydes: add the suffix -carbaldehyde; benzene with \( CHO \) is benzenecarbaldehyde, though benzaldehyde is IUPAC-accepted.

For acids, IUPAC replaces -e with -oic acid and numbers the carboxyl carbon as one; poly acids use dicarboxylic/tricarboxylic endings (NCERT, p. 244). Memory hook: formic acid from Latin formica (ant), acetic from acetum (vinegar), butyric from butyrum (butter) — the names record where the acids were first found.

Physical Properties: Boiling Point and Solubility

The repeated exam question “arrange in order of increasing boiling point” is really one reasoning ladder. NCERT gives this ranking for compounds of molecular mass 58–60 (NCERT, p. 235):

Why this order? Three levels of intermolecular force operate. Alkanes rely only on weak van der Waals forces. Aldehydes, ketones and ethers add dipole-dipole interactions, and the carbonyl group makes aldehydes/ketones more polar than ethers. Alcohols — and even more so carboxylic acids — form intermolecular hydrogen bonds, which dominate.

• Hydrogen bonding (alcohols, acids) beats dipole-dipole (aldehydes, ketones, ethers) beats van der Waals (alkanes).
• Carboxylic acids boil higher than alcohols of similar mass because they associate even in the vapour phase, existing largely as dimers.

Solubility: the lower members (methanal, ethanal, propanone) are miscible with water in all proportions because they form hydrogen bonds with it. Solubility drops rapidly as the alkyl chain lengthens. All aldehydes and ketones dissolve in organic solvents like benzene, ether, methanol and chloroform.

Preparation of Aldehydes and Ketones: Method Table

“Name the reagent” transformation questions (Example 8.1 style) test exactly this table. Learn the method, the starting material and the one reagent that earns the mark (NCERT, pp. 231–234).

Etard, CrO₃/acetic anhydride, side-chain chlorination and Gatterman-Koch are aromatic-specific — they build benzaldehyde and its derivatives from toluene or benzene. Ozonolysis and alkyne hydration work on aliphatic starting materials.

Nucleophilic Addition: Mechanism and Reactivity

Alkenes undergo electrophilic addition; carbonyl compounds undergo the opposite — nucleophilic addition — because the polar \( C=O \) leaves carbon electron-poor. The mechanism (NCERT, p. 236):

1. A nucleophile attacks the electrophilic carbonyl carbon from a direction roughly perpendicular to the \( sp^2 \) plane.
2. The carbon changes hybridisation \( sp^2 \rightarrow sp^3 \).
3. A tetrahedral alkoxide intermediate forms.
4. The intermediate captures \( H^+ \) from the medium, giving the neutral addition product.

Reactivity order: aldehydes \( \gt \) ketones. Two reasons. Sterically, a ketone carries two fairly large alkyl groups that crowd the approaching nucleophile, while an aldehyde has only one. Electronically, two alkyl groups donate more electron density, lowering the electrophilicity of the carbonyl carbon.

Misconception autopsy — benzaldehyde vs propanal: although benzaldehyde is an aldehyde, it is less reactive than propanal in nucleophilic addition. Resonance with the aromatic ring delocalises the carbonyl pi-electrons, reducing the polarity of the \( C=O \) bond, so the carbonyl carbon is less electrophilic (NCERT, p. 236). This is why aromatic aldehydes behave differently from aliphatic ones in the tests below.

The \( H_2N-Z \) row is Table 8.2: with Z = H you get an imine, Z = R a Schiff’s base, Z = OH an oxime, Z = \( NH_2 \) a hydrazone, Z = \( C_6H_5NH \) a phenylhydrazone, Z = 2,4-dinitrophenyl a 2,4-DNP derivative (yellow/orange/red solids for characterisation), and Z = \( NHCONH_2 \) a semicarbazone (NCERT, p. 237).

Reduction, Oxidation and the Three Distinguishing Tests

Reduction to alcohols: \( NaBH_4 \), \( LiAlH_4 \) or catalytic hydrogenation give 1° alcohols from aldehydes and 2° alcohols from ketones (NCERT, p. 238).

Reduction to hydrocarbons: the carbonyl group becomes a \( CH_2 \) group by Clemmensen reduction (zinc amalgam + concentrated HCl) or Wolff-Kishner reduction (hydrazine, then \( KOH \)/ethylene glycol, heat). Equations appear in the formula sheet below.

Oxidation: aldehydes oxidise easily even with mild agents such as Tollens’ and Fehling’s reagents. Ketones resist oxidation — they need vigorous conditions and strong oxidising agents, and the reaction breaks a C–C bond to give a mixture of smaller carboxylic acids (NCERT, p. 239). That difference is the basis of the three classic tests.

In alkaline Tollens’ and Fehling’s, the product is the carboxylate anion \( RCOO^- \), not the free acid — a common error in equation-writing:

\[ RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag \downarrow + 2H_2O + 4NH_3 \]

\[ RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O \downarrow + 3H_2O \]

Aldol Condensation, Cannizzaro and Aromatic Ring Substitution

Why are \( \alpha \)-hydrogens acidic? The carbonyl group strongly withdraws electron density, and the conjugate base (the enolate) is stabilised by resonance (NCERT, p. 241). That acidity drives the aldol reaction.

• Aldol reaction: aldehydes/ketones with at least one \( \alpha \)-hydrogen react in dilute alkali to give a \( \beta \)-hydroxy aldehyde (aldol) or \( \beta \)-hydroxy ketone (ketol).
• Aldol condensation: on heating, the aldol loses water to give an \( \alpha ,\beta \)-unsaturated carbonyl compound. The dehydration step is what makes it a condensation.
• Cross aldol: two different aldehydes and/or ketones, both with \( \alpha \)-H, give a mixture of four products. With ethanal + propanal, either partner can act as the nucleophile (enolate) adding to the other as electrophile, so all four combinations form.

Cannizzaro reaction: aldehydes with no \( \alpha \)-hydrogen (formaldehyde, benzaldehyde) undergo self oxidation-reduction — a disproportionation — on heating with concentrated alkali. One molecule is reduced to the alcohol, the other oxidised to the carboxylic acid salt (NCERT, p. 242).

Aromatic ring substitution: aromatic aldehydes and ketones undergo electrophilic substitution in which \( -CHO \) / \( -COR \) acts as a deactivating and meta-directing group (NCERT, p. 242).

Carboxylic Acids: Structure, Nomenclature and Acidity

A carboxyl group is a carbonyl attached to a hydroxyl — hence the name. In carboxylic acids the bonds to the carboxyl carbon lie in one plane and are separated by about \( 120^\circ \). Resonance makes the carboxylic carbon less electrophilic than a plain carbonyl carbon (NCERT, p. 245).

Acidity: acid strength is measured by \( K_a \) or, more conveniently, by \( pK_a = -\log K_a \). The smaller the \( pK_a \), the stronger the acid. NCERT quotes (p. 250):

Why is \( RCOOH \) stronger than phenol and alcohols? The conjugate base — the carboxylate ion — is stabilised by two equivalent resonance structures with the negative charge on the more electronegative oxygen atom. The phenoxide ion has more resonating structures, but they are non-equivalent and carry negative charge on less electronegative carbon atoms.

Equivalent, oxygen-centred delocalisation stabilises the carboxylate far more (NCERT, p. 250).

Substituent effect: electron-withdrawing groups (F, Cl, Br, I, CN, NO₂, CF₃) stabilise the carboxylate and strengthen the acid; electron-donating groups (alkyl) destabilise it and weaken the acid. Strengthening order: \( Ph \lt I \lt Br \lt Cl \lt F \lt CN \lt NO_2 \lt CF_3 \).

Direct attachment of a phenyl or vinyl group increases acidity — contrary to what resonance alone would suggest — because the \( sp^2 \) carbon is more electronegative (NCERT, p. 250).

Preparation and Reactions of Carboxylic Acids

Six preparation routes and eight reaction families. The preparation table (NCERT, pp. 245–247):

The reactions (NCERT, pp. 251–253):

• Anhydride formation: heat with \( H_2SO_4 \) or \( P_2O_5 \).
• Esterification: \( RCOOH + R’OH \rightleftharpoons RCOOR’ + H_2O \), catalysed by \( H^+ \).
• Acyl chlorides: \( PCl_5 \), \( PCl_3 \) or \( SOCl_2 \); \( SOCl_2 \) is preferred because \( SO_2 \) and HCl escape as gases.
• Amides: ammonia gives the ammonium salt, which on heating forms the amide.
• Reduction to 1° alcohols: \( LiAlH_4 \) or diborane \( B_2H_6 \). \( NaBH_4 \) does not reduce \( -COOH \); \( B_2H_6 \) leaves ester, nitro and halo groups untouched.
• Decarboxylation: sodium salt + sodalime (\( NaOH \)/\( CaO \), 3:1) on heating gives the hydrocarbon; Kolbe electrolysis doubles the carbons.
• Hell-Volhard-Zelinsky (HVZ): \( Cl_2 \) or \( Br_2 \) with red phosphorus halogenates the \( \alpha \)-position.
• Ring substitution: \( -COOH \) is deactivating and meta-directing; aromatic acids do not undergo Friedel-Crafts because the \( AlCl_3 \) catalyst complexes with the carboxyl group.

Esterification mechanism (nucleophilic acyl substitution): protonation of the carbonyl oxygen activates the carbonyl toward nucleophilic addition of the alcohol. Proton transfer in the tetrahedral intermediate converts \( -OH \) into \( -OH_2^+ \), a better leaving group, which is eliminated as neutral water; the protonated ester then loses a proton (NCERT, p. 251).

Formula Sheet: Key Reactions With General Equations

Here \( R \)/\( R’ \) = alkyl or aryl group, \( X \) = halogen, \( Nu^- \) = nucleophile, and \( Z \) is the substituent in \( H_2N-Z \). Organic equations carry no units — conditions and catalysts above or below the arrow are what earn the mark.

Worked Examples: Naming, Reactivity and Tests

Four original problems that follow the exact marking logic of this chapter. Work through each step — the intermediate is where the marks live.

Example 1: IUPAC naming of a branched aldehyde and ketone

Example 2: Reactivity order toward HCN

Example 3: Distinguishing butanal from butan-2-one

Example 4: Ordering acid strength from pKa values

Common Mistakes in Carbonyl Chemistry

Exam Notes: What Earns the Mark

Observed patterns from this chapter, phrased the way an examiner thinks. No predictions — these are the steps the question itself demands (NCERT objectives, p. 228).

• Name reactions are the highest-yield items. A one-line general equation with the correct reagent and condition is enough: Rosenmund, Stephen, Etard, Gatterman-Koch, Friedel-Crafts acylation, Aldol, Cannizzaro, Clemmensen, Wolff-Kishner, HVZ, Kolbe.
• “Name the reagent” transformation questions (Example 8.1 style) test the preparation table. Writing just “PCC” or “DIBAL-H” above the arrow gets the mark — no mechanism needed (NCERT, p. 234).
• In distinguishing tests, the observation is the marks-earning part — silver mirror, red-brown precipitate, yellow precipitate — not the reagent name alone.
• Mechanism answers need all three steps: nucleophilic attack, \( sp^2 \rightarrow sp^3 \) tetrahedral intermediate, proton transfer. Missing any one costs the mark.
• pKa inversion trap: smaller \( pK_a \) = stronger acid. Students consistently invert this in the acidity question.
• Physical chemistry style tables — boiling point ranking, pKa ordering — are the chapter’s most predictable numerical items.

Aldehydes Ketones and Carboxylic Acids Class 12 Notes: Revision Summary

The night-before version of the whole chapter. First, the reaction map — every named reaction grouped by starting material, reagent and product (NCERT, pp. 254–255):

Test-observation micro-table:

Memory devices: “Kishner and KOH both start with K — Wolff-Kishner needs basic KOH; Clemmensen uses acidic Zn-Hg/HCl.” For reactivity, picture the carbonyl carbon as the north pole of a magnet: nucleophiles are attracted to it, and two bulky alkyl groups on a ketone are like two guards blocking the approach.

Five-line night-before checklist:

• Nomenclature: \( -al \), \( -one \), \( -oic \) acid, number from the functional group end.
• Reactivity: aldehydes \( \gt \) ketones; benzaldehyde is the exception (resonance).
• Three tests: silver mirror (Tollens’), red-brown \( Cu_2O \) (Fehling’), yellow \( CHI_3 \) (iodoform).
• Acidity: smaller \( pK_a \) = stronger; EWG strengthens, EDG weakens.
• Name reactions with reagent + condition on the arrow.

These carbonyl reactions connect forward to the functional-group conversions covered in our alcohols, phenols and ethers revision notes (Grignard additions, oxidation of alcohols) and our amines class 12 notes (imines, amides, reductive routes). For the wider set, browse the full Class 12 chemistry notes and the complete Class 12 notes collection on CBSE notes.

To verify any equation or page reference against the official source, open the NCERT Class 12 Chemistry Part II textbook PDF.

FAQs on Aldehydes, Ketones and Carboxylic Acids

Why are aldehydes more reactive than ketones in nucleophilic addition reactions?

Aldehydes are more reactive for two reasons. Sterically, a ketone has two alkyl groups that crowd the approaching nucleophile, while an aldehyde has only one. Electronically, two alkyl groups donate more electron density to the carbonyl carbon, making it less electrophilic (NCERT, p. 236).

Why does benzaldehyde reduce Tollens’ reagent but not Fehling’s solution?

Both reagents oxidise aldehydes, but Fehling’s solution responds only to aliphatic aldehydes. Aromatic aldehydes such as benzaldehyde do not give the red-brown cuprous oxide precipitate with Fehling’s, yet they still produce the silver mirror with Tollens’ reagent (NCERT, p. 239).

What is the difference between aldol condensation and Cannizzaro reaction?

Aldol condensation needs at least one \( \alpha \)-hydrogen and uses dilute alkali as catalyst; the product is a β-hydroxy aldehyde/ketone that dehydrates to an \( \alpha ,\beta \)-unsaturated carbonyl. Cannizzaro reaction happens only for aldehydes without an \( \alpha \)-hydrogen (formaldehyde, benzaldehyde) and uses concentrated alkali, giving a disproportionation into an alcohol and a carboxylate salt (NCERT, pp. 241–242).

Why is a carboxylic acid a stronger acid than phenol even though the phenoxide ion has more resonating structures?

Resonance stability depends on the equivalence of the structures, not their number. The carboxylate ion is stabilised by two equivalent resonance structures with the negative charge on oxygen. The phenoxide ion has more structures, but they are non-equivalent and place negative charge on less electronegative carbon atoms, so they stabilise it far less (NCERT, p. 250).

How would you distinguish propanal from propanone in the laboratory?

Warm each with Tollens’ reagent. Propanal gives a bright silver mirror; propanone gives none. Alternatively, use Fehling’s solution — propanal gives a red-brown \( Cu_2O \) precipitate and propanone does not — or run the iodoform test, which propanone (a methyl ketone) would pass with a yellow \( CHI_3 \) precipitate and propanal would not.

Which reagent reduces a carboxylic acid to a primary alcohol and why is sodium borohydride not used?

Lithium aluminium hydride, \( LiAlH_4 \), or diborane, \( B_2H_6 \), reduce a carboxylic acid to a primary alcohol. Sodium borohydride is not used because it does not reduce the carboxyl group — \( NaBH_4 \) is selective for aldehydes and ketones (NCERT, p. 253).

Reference: NCERT Class 12 Chemistry textbook, chapter Aldehydes, Ketones and Carboxylic Acids.