Aldehydes, Ketones and Carboxylic Acids — NEET Notes

The carbonyl group & nomenclature

The carbonyl group, C=O, is the defining feature of aldehydes, ketones and carboxylic acids. In an aldehyde the carbonyl carbon carries at least one hydrogen (R-CHO); in a ketone it carries two alkyl or aryl groups (R-CO-R'); in a carboxylic acid it carries a hydroxyl moiety (R-COOH). Esters, amides, acyl halides and anhydrides are all derivatives of carboxylic acids — but for NEET, the three classes named in this chapter's title dominate.

The carbonyl carbon is sp²-hybridised. It forms three σ-bonds in a trigonal-planar geometry with bond angles near 120°, and the fourth valence electron occupies a p-orbital that overlaps sideways with the p-orbital of oxygen to give the π-bond. Oxygen retains two non-bonding lone pairs. Because oxygen is far more electronegative than carbon, the π-bond is strongly polarised: the carbon becomes a partial-positive electrophilic centre and the oxygen a partial-negative nucleophilic centre. This polarisation — captured in resonance by writing the dipolar structure C⁺–O⁻ alongside the neutral C=O — is the single fact from which every reaction in this chapter follows.

The carbonyl carbon is electrophilic. Nucleophiles attack it. The carbonyl oxygen is nucleophilic. Electrophiles attack it.

The one rule of carbonyl chemistry

Nomenclature — common and IUPAC

Aldehydes take the IUPAC suffix -al on the parent alkane name; the carbonyl carbon is always C-1. Ketones take the suffix -one with the lowest possible locant for the C=O carbon. When the -CHO group is attached to a ring, the suffix carbaldehyde is appended. Common names use Greek letters (α, β, γ…) for substituent positions, with the α-carbon being the one directly next to the carbonyl. Three pairs that NEET expects you to know: HCHO is formaldehyde / methanal; CH₃CHO is acetaldehyde / ethanal; C₆H₅CHO is benzaldehyde (the common name is IUPAC-approved). For ketones: CH₃COCH₃ is acetone / propan-2-one; C₆H₅COCH₃ is acetophenone / 1-phenylethan-1-one; CH₃COCH₂CH₂CH₃ is methyl n-propyl ketone / pentan-2-one.

For carboxylic acids the IUPAC suffix is -oic acid and the carboxyl carbon is C-1. Common names end in -ic acid and often record the natural source: formic acid (HCOOH) from red ants (Latin formica), acetic acid (CH₃COOH) from vinegar (acetum), butyric acid (CH₃CH₂CH₂COOH) from rancid butter (butyrum). Dicarboxylic acids — oxalic (ethanedioic), malonic (propanedioic), succinic (butanedioic), glutaric, adipic — anchor a separate naming convention you should commit to memory.

Preparation of aldehydes & ketones

Six routes cover essentially every NCERT preparation. Memorise the reagent and what it does to each substrate.

1. Oxidation of alcohols. Primary alcohols give aldehydes (controlled oxidation, PCC); over-oxidation produces carboxylic acids. Secondary alcohols give ketones. Vapour-phase dehydrogenation over hot copper or silver is the industrial variant.
2. From hydrocarbons. Ozonolysis of alkenes followed by Zn/H₂O gives aldehydes and/or ketones depending on the substitution. Hydration of alkynes (H₂SO₄, HgSO₄) converts ethyne to acetaldehyde and any higher alkyne to a methyl ketone — Markovnikov places the OH on the more substituted carbon, then tautomerises.
3. From acyl chlorides — Rosenmund reduction. R-COCl + H₂ → R-CHO using Pd / BaSO₄. The poisoned catalyst stops at the aldehyde and prevents further reduction to the alcohol. Aromatic acid chlorides give the corresponding aromatic aldehyde.
4. From nitriles — Stephen reaction. R-CN + SnCl₂ / HCl gives an iminium chloride; aqueous hydrolysis then yields R-CHO. The equivalent modern reagent is DIBAL-H (diisobutylaluminium hydride), which selectively reduces nitriles or esters to aldehydes.
5. From benzene — Friedel-Crafts acylation. Benzene + R-COCl in the presence of anhydrous AlCl₃ gives an aryl alkyl ketone (e.g. acetophenone from acetyl chloride).
6. Aromatic aldehyde routes. Toluene to benzaldehyde by Etard reaction (CrO₂Cl₂ then hydrolysis), by CrO₃ in acetic anhydride (via benzylidene diacetate), or by Gatterman-Koch formylation (CO + HCl + AlCl₃/CuCl). Side-chain chlorination of toluene to benzal chloride (PhCHCl₂) followed by hydrolysis is the commercial route.

For ketones specifically: acyl chloride + dialkylcadmium R₂Cd (made from CdCl₂ + Grignard) gives R-CO-R'. Treating a nitrile with a Grignard reagent followed by hydrolysis is the cleanest ketone synthesis from an alkyl halide.

Physical properties

Methanal is a gas at room temperature, ethanal a volatile liquid; higher aldehydes and ketones are liquids or solids. The boiling-point ordering for compounds of comparable molecular mass tells the whole story:

alkane  <  ether  <  aldehyde / ketone  <  alcohol  <  carboxylic acid

Aldehydes and ketones boil higher than alkanes and ethers of similar mass because of dipole-dipole interactions between the polar C=O groups. They boil lower than alcohols because they cannot donate a hydrogen bond — they have no O-H. Lower members (formaldehyde, acetaldehyde, acetone) are miscible with water in all proportions; solubility drops sharply once the alkyl chain exceeds four carbons. Carboxylic acids boil higher than all of them because their hydrogen bonding is reinforced by formation of cyclic dimers — two molecules locked together by two C=O···H-O hydrogen bonds — that persist even in the vapour phase. NEET 2018 and NEET 2022 both tested this ordering directly.

Nucleophilic addition reactions

The defining reaction of aldehydes and ketones. A nucleophile (Nu⁻) approaches the carbonyl carbon from a direction roughly perpendicular to the sp² plane, donating its lone pair into the empty π* orbital. The carbon rehybridises from sp² to sp³, the π-bond breaks, and the electron pair drops onto oxygen as an alkoxide. Protonation gives the neutral product. The net change is addition of Nu and H across the C=O bond.

Reactivity order — aldehydes vs ketones

Aldehydes are more reactive than ketones in nucleophilic addition for two reasons. Sterically, two bulky alkyl groups in a ketone hinder nucleophile approach; an aldehyde has only one. Electronically, two alkyl groups donate electrons to the carbonyl carbon by hyperconjugation and induction, lowering its partial positive charge more than one alkyl group does in an aldehyde. Aromatic aldehydes (benzaldehyde) are less reactive than aliphatic aldehydes (propanal) because resonance with the benzene ring reduces the partial positive charge on the carbonyl carbon.

The five canonical nucleophilic additions

NEET's most-tested cluster. Match each nucleophile to its product:

The 2,4-DNP test deserves its own line — any carbonyl compound, aldehyde or ketone, gives a yellow-to-red precipitate with Brady's reagent (2,4-dinitrophenylhydrazine in methanol / H₂SO₄). It does not distinguish aldehydes from ketones — for that you need Tollens or Fehling — but it confirms whether a C=O is present at all.

Reduction reactions

The carbonyl group can be reduced to either an alcohol (keep the oxygen) or a methylene CH₂ (lose the oxygen). Choose the reagent according to which one you want.

• NaBH₄ or LiAlH₄. Aldehydes → primary alcohols. Ketones → secondary alcohols. LiAlH₄ is the more vigorous reagent; NaBH₄ is milder and tolerates ester or acid groups (it will not reduce -COOH, but LiAlH₄ will).
• Clemmensen reduction. Zn-Hg amalgam + concentrated HCl. C=O → CH₂. Used when the molecule is acid-stable but base-sensitive — e.g. acid-sensitive Friedel-Crafts ketones.
• Wolff-Kishner reduction. NH₂NH₂ + KOH (or NaOH) in ethylene glycol, heated. The hydrazone intermediate loses N₂ to give the CH₂. Used when the molecule is base-stable but acid-sensitive — the complement of Clemmensen.
• Catalytic hydrogenation. H₂ / Ni or Pt reduces C=O to CH(OH); not selective if other unsaturation is present.

Oxidation — Tollens, Fehling, iodoform

Aldehydes are easily oxidised to carboxylic acids; ketones resist oxidation unless the conditions are vigorous enough to cleave a C-C bond. This single difference powers two of the most-asked NEET tests.

Iodoform test — methyl ketones (and three special alcohols / aldehydes)

Sodium hypoiodite (I₂ + NaOH ≡ NaOI) oxidises any carbonyl carrying a CH₃CO- group. The methyl is converted, hydrogen by hydrogen, to -CI₃, and the C-C bond cleaves to give a sodium carboxylate plus the yellow crystalline iodoform CHI₃. The same reagent oxidises certain alcohols first to the methyl ketone, then through the same pathway — so they too give a positive test.

Reactions due to α-hydrogen — aldol condensation

The α-hydrogen — the one on the carbon directly attached to C=O — is unusually acidic (pKa ≈ 17 for an aldehyde, ≈ 20 for a ketone). Two reasons: the C=O group strongly withdraws electrons inductively, and the conjugate base (an enolate) is stabilised by resonance with the carbonyl. Mild base (dilute NaOH) generates the enolate; the enolate then acts as a carbon nucleophile toward another molecule of carbonyl. This is the aldol reaction.

Cross aldol condensation happens when two different carbonyls react. If both have α-H, you get four products (two self-aldols + two cross-aldols) — usually a synthetic nightmare. NEET 2022 Q.88 tested this for acetone + 2-pentanone. The case is cleaner when only one of the two has an α-H — then only that one can be the enolate, and the cross-aldol is selective. The reaction of benzaldehyde (no α-H) with acetophenone (one α-H) under dilute NaOH is the classic cross-aldol giving chalcone. NEET 2020 Q.139 tested this exact pair.

Cannizzaro reaction

What happens to an aldehyde that has no α-hydrogen? It cannot form an enolate, so the aldol pathway is closed. Instead, in the presence of concentrated alkali, it undergoes a disproportionation — one molecule is oxidised to the carboxylate, another is reduced to the alcohol. Both products come from the same starting aldehyde.

2 HCHO  +  conc. NaOH  →  CH₃OH  +  HCOO⁻Na⁺

Cannizzaro: one aldehyde reduces, another oxidises

The aldehydes that undergo Cannizzaro are exactly those without α-H: HCHO (formaldehyde), C₆H₅CHO (benzaldehyde), (CH₃)₃C-CHO (2,2-dimethylpropanal / trimethylacetaldehyde), and similar tertiary-substituted aldehydes. Mechanistically, hydroxide adds to one aldehyde's C=O to give a tetrahedral intermediate; this intermediate transfers a hydride (H⁻) to a second aldehyde molecule. Hydride transfer makes the first molecule a carboxylate and the second an alkoxide — protonation completes the products. NEET has tested both the identification of Cannizzaro-capable substrates and the disproportionation principle multiple times.

Carboxylic acids — nomenclature & preparation

The carboxyl group, -COOH, is a hybrid: a carbonyl C=O fused to a hydroxyl -OH on the same carbon. The carbon is sp²-hybridised, planar, with bond angles near 120°. The C-O bond lengths are equalised (between single- and double-bond values) because of resonance — the lone pair on the -OH oxygen is conjugated with the C=O π-system. This resonance is also why the carboxyl carbon is less electrophilic than an aldehyde or ketone carbonyl — there is already electron density pushed back onto the C=O carbon from the -OH oxygen.

Six preparation routes

The six standard NCERT preparations:

1. From primary alcohols. R-CH₂-OH + KMnO₄ (acidic or alkaline) or K₂Cr₂O₇ / CrO₃ (Jones) → R-COOH. The most common laboratory route.
2. From aldehydes. R-CHO + mild oxidant (Tollens, Fehling, even acidified KMnO₄) → R-COOH.
3. From alkylbenzenes. The whole side-chain is oxidised to -COOH by hot acidic or alkaline KMnO₄, regardless of its length — provided the benzylic carbon bears at least one H. Toluene, ethylbenzene, propylbenzene all give benzoic acid. tert-Butylbenzene (no benzylic H) is unreactive.
4. Hydrolysis of nitriles. R-CN + H₂O / H⁺ or OH⁻ → R-CONH₂ (amide) → R-COOH. Mild conditions stop at the amide; harsher conditions push through to the acid. Combined with the SN2 conversion of alkyl halide to nitrile, this route extends the chain by one carbon.
5. Grignard + CO₂. R-MgX + dry ice → R-COO⁻Mg⁺X (NEET 2022 Q.53 tested this intermediate exactly); aqueous acid liberates R-COOH. Also a one-carbon homologation.
6. Hydrolysis of acid derivatives. Acyl halides, anhydrides, and esters all hydrolyse (acidic or basic) to the parent acid.

Acidity & effect of substituents

Carboxylic acids are weak acids (pKa typically 3–5) but they are far stronger than alcohols (pKa ≈ 16) or simple phenols (pKa ≈ 10). The reason is the stability of the conjugate base — the carboxylate ion has two equivalent resonance structures, distributing the negative charge symmetrically over two electronegative oxygens. Phenoxide has non-equivalent resonance structures (negative charge on less-electronegative ring carbons), and alkoxide has no resonance at all. Better delocalisation → more stable base → stronger acid.

Effect of substituents

An electron-withdrawing group (EWG) near the carboxyl pulls electron density away from the carboxylate oxygen, stabilises the negative charge, and strengthens the acid. An electron-donating group (EDG) does the opposite. The effect drops sharply with distance — a halogen on the α-carbon matters far more than the same halogen on the γ-carbon. Multiple halogens compound the effect: CCl₃COOH (pKa 0.66) is dramatically stronger than CHCl₂COOH (1.30), which is stronger than ClCH₂COOH (2.85).

For the four common α-haloacetic acids, NCERT gives the order:

F-CH₂-COOH > Cl-CH₂-COOH > Br-CH₂-COOH > CH₃-COOH

This tracks the inductive electron-withdrawing power of the halogen, which goes as F > Cl > Br > I > H (electronegativity). Note that the resonance/lone-pair effect of halogens does not matter here because the halogen is attached to a saturated carbon, not the carboxyl directly.

Aromatic acids and the ortho effect

For substituted benzoic acids, an EWG on the ring (-NO₂, -CN, -X) strengthens the acid; an EDG (-OCH₃, -CH₃, -NH₂) weakens it. The numbers: benzoic acid pKa 4.19; 4-nitrobenzoic acid 3.41 (stronger — NO₂ withdraws); 4-methoxybenzoic acid 4.46 (weaker — OCH₃ donates by resonance to the ring). But beware the ortho effect — any substituent (electron-withdrawing or donating) at the ortho position increases acidity, because of a combination of steric strain in the un-ionised acid and proximity field effects. NEET problems on benzoic acid pKa often hinge on this anomaly.

Reactions of carboxylic acids

Four behavioural classes, organised by which bond of the -COOH group breaks.

1. Reactions involving the O-H bond — acid behaviour

Carboxylic acids react with active metals (Na, K, Mg) to liberate H₂ and with alkali metal hydroxides, carbonates, and hydrogen carbonates to form carboxylate salts. The reaction with NaHCO₃ is diagnostic — brisk effervescence of CO₂ is given by carboxylic acids but not by phenols (phenols are too weak to protonate HCO₃⁻).

2. Reactions involving the C-OH bond

• Esterification (Fischer). R-COOH + R'-OH ⇌ R-COO-R' + H₂O, catalysed by concentrated H₂SO₄ or HCl gas. The equilibrium is unfavourable; water must be removed (or the ester distilled out) to push the reaction forward. Mechanism: protonation of carbonyl oxygen, nucleophilic addition of alcohol, proton transfer, loss of water, deprotonation. NEET 2021 placed this in a match-list.
• Anhydride formation. Two molecules of acid + P₂O₅ or H₂SO₄, heated, lose one water between them to give (R-CO)₂O.
• Conversion to acid chloride. R-COOH + SOCl₂ → R-COCl + SO₂ + HCl. PCl₃ and PCl₅ also work but SOCl₂ is preferred because the by-products are gases.
• Amide formation. R-COOH + NH₃ → R-COONH₄, then dehydration on heating → R-CONH₂.

3. Reactions involving the -COOH group as a whole

• Reduction by LiAlH₄ (or diborane B₂H₆). R-COOH → R-CH₂-OH. NaBH₄ is too mild to touch -COOH. Diborane is selective — it reduces -COOH but leaves esters, nitro and halo groups untouched.
• Decarboxylation (soda-lime). R-COONa + NaOH / CaO, heated → R-H + Na₂CO₃. NEET 2023 Q.69 used this exact reaction (sodium ethanoate → methane, 16 g/mol).
• Kolbe electrolysis. Electrolysis of aqueous sodium carboxylate gives the symmetric alkane R-R + 2 CO₂ + H₂.

4. Substitution in the hydrocarbon part — HVZ reaction

The α-hydrogen of a carboxylic acid is much less acidic than the α-H of an aldehyde or ketone, so direct base-promoted enolisation fails. The Hell-Volhard-Zelinsky (HVZ) reaction gets around this by using red phosphorus as a catalyst with Cl₂ or Br₂. The mechanism: red P reacts with X₂ to give PX₃; PX₃ converts the acid to its acid bromide (R-COBr); the acid bromide enolises far more readily, reacts with X₂ to give an α-halo acid bromide; finally exchange with another molecule of acid regenerates the catalyst and gives R-CH(X)-COOH. NEET 2021 Q.100 tested the HVZ name-match.