Aldehydes and ketones

In short

Aldehydes (RCHO) and ketones (RCOR′) contain the carbonyl group C=O, in which a polar, planar, sp²-hybridised carbon carries a partial positive charge and is therefore attacked by nucleophiles while the α-hydrogen next to it is unusually acidic. This single functional group is the hinge of synthetic organic chemistry: almost every carbon–carbon bond-forming reaction taught in a B.Sc course (Grignard, aldol, Wittig, Knoevenagel, benzoin) and almost every functional-group interconversion runs through it. It carries the heaviest weight of any organic chapter in IIT-JAM, GATE Chemistry (CY), CSIR-UGC NET and university CBCS Organic Chemistry papers, usually as "identify the product / propose a mechanism / distinguish between A and B" questions. In industry it underpins formaldehyde resins, acetone and solvent manufacture, vanillin and perfumery aldehydes, paracetamol and ibuprofen intermediates, and caprolactam for nylon — so it is standard interview ground at Deepak Nitrite, Jubilant Ingrevia, Camlin Fine Sciences, Atul, SRF, Pidilite, Asian Paints, Dr Reddy's, Sun Pharma and Divi's Laboratories.

Explained

Structure, nomenclature and physical properties

• Bonding: carbonyl carbon is sp², bond angles ≈120°, planar; C=O is one σ + one π bond, length ≈1.22 Å, bond energy ≈740 kJ/mol (stronger than two C–O single bonds, so C=O does not readily polymerise except in formaldehyde).
• Polarity: oxygen is far more electronegative, so the real description is Cδ+=Oδ−, with resonance contributor C⁺–O⁻. Dipole moment ≈2.3–2.9 D (acetone 2.88 D). Hence carbon is electrophilic, oxygen is basic/nucleophilic.
• IUPAC names: aldehyde = parent alkane + *al* (C1 is the CHO, no locant needed); ketone = alkane + *one* with lowest locant. Cyclic aldehyde side chain → *carbaldehyde* (cyclohexanecarbaldehyde); on a ring as substituent-type → *-carbaldehyde*; when CHO cannot be in the chain → *oxo-* or *formyl-* prefix. Dialdehydes = *-dial* (butanedial); keto-acid → *oxo* prefix (4-oxopentanoic acid) because COOH outranks C=O.
• Trivial names still examined: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propan-2-one), benzaldehyde, acetophenone (1-phenylethanone), benzophenone, cinnamaldehyde, vanillin, citral, glyoxal, acrolein (prop-2-enal), chloral (trichloroacetaldehyde), biacetyl (butane-2,3-dione).
• Seniority order for naming: COOH > SO₃H > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine. An aldehyde always outranks a ketone.
• Boiling points: higher than comparable alkanes and ethers (dipole–dipole) but lower than alcohols (no O–H, so no self hydrogen bonding). Acetone 56 °C, propan-1-ol 97 °C, butane −0.5 °C.
• Solubility: lower members (formaldehyde, acetaldehyde, acetone) are miscible with water because C=O accepts hydrogen bonds from water; solubility falls sharply beyond about C5. Acetone is a universal laboratory solvent precisely for this reason.
• Special forms: formaldehyde is a gas, sold as formalin (37–40% w/v aqueous, with 10–15% methanol as stabiliser) and as the solid trimer trioxane and polymer paraformaldehyde; acetaldehyde trimerises to paraldehyde. Chloral forms the stable crystalline chloral hydrate because three Cl atoms make the carbonyl strongly electron-poor.
• Safety and Indian practice: formaldehyde and acetaldehyde are confirmed/suspected carcinogens and are regulated as hazardous chemicals under the Manufacture, Storage and Import of Hazardous Chemical Rules; FSSAI prohibits formalin as a fish preservative and uses a rapid formalin test kit for enforcement. Acetone and methanol-bearing streams are flammable (acetone flash point −20 °C), handled under the Petroleum and Explosives Safety Organisation (PESO) licensing regime, with workplace exposure and safety duties now under the Occupational Safety, Health and Working Conditions Code (in force 21 November 2025, replacing the Factories Act).

Preparation (general methods)

• Oxidation of alcohols: primary alcohol → aldehyde (needs a mild/anhydrous oxidant: PCC in CH₂Cl₂, Collins reagent, Swern, Dess–Martin, or Cu at 573 K); primary → acid with KMnO₄/K₂Cr₂O₇; secondary alcohol → ketone (Jones reagent, K₂Cr₂O₇/H₂SO₄, or PCC — ketones resist further oxidation).
• Dehydrogenation: vapour of alcohol over heated copper at 573 K — primary gives aldehyde, secondary gives ketone, tertiary gives alkene.
• Ozonolysis of alkenes: O₃ then Zn/H₂O (reductive) gives aldehydes/ketones; oxidative work-up (H₂O₂) gives acids. Position of the double bond is deduced from the fragments.
• Hydration of alkynes: acetylene + H₂O/H₂SO₄–HgSO₄ → acetaldehyde (Markovnikov, via enol); higher alkynes give ketones. Anti-Markovnikov hydroboration–oxidation of a terminal alkyne gives the aldehyde.
• From acid derivatives: acid chloride + H₂/Pd-BaSO₄ (Rosenmund reduction) → aldehyde; nitrile or ester + DIBAL-H → aldehyde; nitrile + CH₃MgBr then H₃O⁺ → ketone; acid chloride + R₂CuLi (Gilman) → ketone; Stephen reduction (SnCl₂/HCl on nitrile) → aldehyde.
• From calcium/acid salts: dry distillation of calcium acetate → acetone (older route).
• Aromatic routes: Friedel–Crafts acylation (RCOCl/AlCl₃) → aryl ketone; Gattermann–Koch (CO + HCl, AlCl₃/Cu₂Cl₂) → benzaldehyde; Etard reaction (toluene + CrO₂Cl₂) → benzaldehyde; side-chain chlorination of toluene to benzal chloride then hydrolysis; Reimer–Tiemann (phenol + CHCl₃/NaOH) → salicylaldehyde; Vilsmeier–Haack (DMF/POCl₃) → aldehyde on activated rings; Fries and Houben–Hoesch for phenolic ketones.
• Industrial routes (India): formaldehyde by catalytic oxidation of methanol over silver or Fe–Mo oxide; acetaldehyde by Wacker–Hoechst oxidation of ethylene (PdCl₂/CuCl₂) or ethanol dehydrogenation; acetone and phenol together by the cumene (Hock) process — the backbone of Deepak Nitrite's Dahej plant and Hindustan Organic Chemicals; higher aldehydes by hydroformylation (oxo process) of alkenes with CO/H₂ over Co or Rh.

Nucleophilic addition — the central reaction

• Mechanism (base/neutral): nucleophile attacks the carbonyl carbon → tetrahedral alkoxide (sp³) → protonation gives the addition product. Acid-catalysed: protonation of oxygen first makes carbon far more electrophilic, then a weak nucleophile adds.
• Reactivity order: HCHO > CH₃CHO > higher RCHO > CH₃COCH₃ > R₂CO > ArCHO > ArCOR > Ar₂CO. Two reasons: (i) steric — more/larger groups crowd the approach and the product's 109° geometry; (ii) electronic — alkyl groups donate electrons (+I) and aryl groups donate by resonance, both reducing the positive charge on carbon. Electron-withdrawing groups do the opposite: Cl₃CCHO ≫ CH₃CHO.
• Water (hydration) → gem-diol: equilibrium only. K ≈ 2×10³ for formaldehyde, ≈1 for acetaldehyde, ≈2×10⁻³ for acetone — hence formaldehyde exists in water almost entirely as methylene glycol and chloral hydrate is isolable.
• Alcohols → hemiacetal then acetal (dry HCl, water removed). Acetals are stable to base, nucleophiles and hydride but hydrolyse in dilute acid — so they are the standard protecting group for a carbonyl (ethylene glycol gives the cyclic 1,3-dioxolane). Ketones form ketals less readily; thioacetals (with ethanedithiol) are removed by Raney Ni and give an overall C=O → CH₂.
• HCN → cyanohydrin (base-catalysed, CN⁻ is the true nucleophile). Synthetic value: the nitrile can be hydrolysed to an α-hydroxy acid (acetone → 2-hydroxy-2-methylpropanoic acid, route to methyl methacrylate) or reduced to a β-amino alcohol. Ketones with bulky groups and most aryl ketones do not react.
• NaHSO₃ → bisulphite adduct, a crystalline water-soluble salt used to purify or separate aldehydes and methyl ketones; reversed by dilute acid or base.
• Grignard/organolithium → alcohols: RMgX + HCHO → primary alcohol; + RCHO → secondary; + ketone → tertiary; + CO₂ → carboxylic acid. Anhydrous conditions are compulsory (RMgX is destroyed by any proton source).
• Hydride nucleophiles: NaBH₄ (alcohol/water, mild, does not touch esters, acids or nitro) and LiAlH₄ (dry ether, reduces almost everything) give alcohols. Chemoselectivity is a favourite exam point.
• Nitrogen nucleophiles (addition–elimination, water lost):
  • NH₃/RNH₂ → imine (Schiff base); R₂NH with an α-H → enamine; reductive amination (imine + NaBH₃CN/H₂-Ni) is the standard amine synthesis.
  • NH₂OH → oxime; NH₂NH₂ → hydrazone; 2,4-dinitrophenylhydrazine → 2,4-DNP (Brady's) orange-red crystalline hydrazone; NH₂NHCONH₂ → semicarbazone.
  • pH dependence: rate is maximum near pH 4–5. Too alkaline and the carbonyl is not activated; too acidic and the amine is fully protonated, hence not nucleophilic — a classic bell-shaped rate curve.
• Wittig reaction: R₂C=O + Ph₃P=CHR′ → alkene + Ph₃P=O, via the four-membered oxaphosphetane. Gives an alkene at a defined position (no rearrangement), unlike dehydration of an alcohol; unstabilised ylides favour the Z-alkene, stabilised ylides the E.
• Related: Tebbe/Peterson olefination, and addition of acetylide, enolate or Reformatsky (BrZnCH₂CO₂Et) reagents.

α-Hydrogen chemistry: tautomerism, enolates, condensations

• Acidity of α-H: pKa ≈19–20 (ketones), ≈17 (aldehydes), 11 (ethyl acetoacetate), 9 (pentane-2,4-dione) — because the resulting carbanion is resonance-stabilised onto oxygen (the enolate). Compare ethane, pKa ≈50.
• Keto–enol tautomerism: acid- or base-catalysed. Acetone is >99.99% keto; phenol is fully "enol"; 1,3-diketones and β-ketoesters carry a high enol content (pentane-2,4-dione ≈80% enol in the vapour or non-polar solvent) because the enol is conjugated and internally hydrogen-bonded. Tautomerism explains racemisation of an α-stereocentre and α-deuterium exchange in D₂O/NaOD.
• Halogenation: acid-catalysed α-halogenation gives mono-substitution (slower after the first halogen); base-catalysed runs away to tri-halo, then C–C cleavage.
• Haloform/iodoform reaction: a methyl ketone (or CH₃CHO, or CH₃CH(OH)R after oxidation by the hypohalite) + I₂/NaOH gives yellow CHI₃ (iodoform) and the carboxylate — the standard test for the CH₃CO– group. Bleaching powder/NaOCl gives chloroform.
• Aldol reaction/condensation: two molecules with α-H; the enolate of one attacks the carbonyl of the other → β-hydroxy aldehyde/ketone (aldol), which on warming loses water to give a conjugated α,β-unsaturated carbonyl. 2 CH₃CHO → 3-hydroxybutanal → but-2-enal (crotonaldehyde).
• Crossed/Claisen–Schmidt: an aldehyde with no α-H (benzaldehyde, formaldehyde) plus a ketone with α-H gives a single clean product — PhCHO + CH₃COCH₃ → benzylideneacetone; with 2 equivalents → dibenzylideneacetone (a standard B.Sc practical).
• Reactions of carbonyls with NO α-hydrogen: Cannizzaro disproportionation in concentrated alkali (2 PhCHO → PhCH₂OH + PhCOO⁻, via hydride transfer from the alkoxide-dianion intermediate); crossed Cannizzaro with excess HCHO reduces the other aldehyde; Tishchenko (Al(OR)₃) gives the ester.
• Other condensations examined: Knoevenagel (active methylene + ArCHO, amine base); Perkin (ArCHO + acetic anhydride/NaOAc → cinnamic acid); Benzoin condensation (2 PhCHO + CN⁻ or thiamine → benzoin; umpolung of the carbonyl); Benzilic acid rearrangement (benzil + OH⁻); Mannich (ketone + HCHO + amine → β-amino ketone); Michael addition (enolate onto an enone, 1,4); Robinson annulation = Michael + intramolecular aldol; Stobbe, Darzens, Wittig–Horner for completeness.
• α,β-Unsaturated carbonyls: 1,2-addition (direct, with strong hard nucleophiles — RMgX, LiAlH₄, organolithiums) versus 1,4/conjugate addition (soft nucleophiles — R₂CuLi, amines, cyanide, enolates). NaBH₄ often reduces both C=C and C=O; Luche conditions (NaBH₄/CeCl₃) give 1,2-selectivity.

Oxidation, reduction and rearrangements

• Oxidation of aldehydes is easy (they are at an intermediate oxidation level): KMnO₄, K₂Cr₂O₇, HNO₃, Ag⁺ or even air (autoxidation via a peracid) → carboxylic acid. Ketones resist oxidation; forcing conditions cleave the C–C bond (industrial KA-oil → adipic acid for nylon-6,6).
• Mild/selective oxidants: Tollens [Ag(NH₃)₂]⁺ and Fehling's (Cu²⁺–tartrate) oxidise aldehydes only; Benedict's (citrate) likewise, used for reducing sugars.
• Reduction, with the product selected by reagent:
  • to alcohol: NaBH₄, LiAlH₄, H₂/Ni–Pt–Pd, Meerwein–Ponndorf–Verley (Al(OPrⁱ)₃/isopropanol — reversible, selective, leaves C=C alone), Luche.
  • to hydrocarbon (C=O → CH₂): Clemmensen (Zn-Hg/conc. HCl — acidic, for acid-stable substrates) and Wolff–Kishner (NH₂NH₂ then KOH/ethylene glycol, 180 °C — basic, for acid-sensitive substrates); also thioacetal + Raney Ni.
  • bimolecular: Mg/Hg then H₂O gives a pinacol (acetone → pinacol); Bouveault–Blanc (Na/ethanol); acyloin condensation for esters.
  • Pinacol–pinacolone rearrangement: pinacol + H⁺ → pinacolone (3,3-dimethylbutan-2-one), via carbocation and methyl migration.
• Baeyer–Villiger oxidation: ketone + peracid (mCPBA, CF₃CO₃H) → ester (cyclic ketone → lactone), through the Criegee intermediate; migratory aptitude: 3° > 2° ≈ aryl > 1° > methyl; the migrating group retains its configuration.
• Beckmann rearrangement: ketoxime + H₂SO₄/PCl₅ → amide, with the group anti to the OH migrating. Industrially, cyclohexanone oxime → caprolactam for nylon-6 (made in India by Gujarat State Fertilizers & Chemicals, Vadodara).
• Schmidt reaction (HN₃/H₂SO₄) and Wolff rearrangement appear in postgraduate-level papers.
• Cannizzaro vs aldol is the standard discrimination question: α-H present → aldol; α-H absent → Cannizzaro.

Identification: wet tests and spectroscopy

• All aldehydes and ketones: 2,4-DNP → yellow/orange/red precipitate (shade deepens with conjugation); sodium bisulphite adduct; hydroxylamine → oxime.
• Aldehyde only: Tollens (silver mirror), Fehling's (red Cu₂O), Benedict's, Schiff's reagent (magenta restored), chromic acid (green Cr³⁺), and oxidation by mild I₂/NaOH.
• Methyl ketone / CH₃CO– or CH₃CH(OH)–: iodoform test (yellow crystals, antiseptic smell).
• Aromatic vs aliphatic aldehyde: benzaldehyde gives Tollens positive but Fehling negative (a useful discriminator), and no iodoform.
• α-Hydroxy/α-keto: both reduce Tollens; Fehling's distinguishes reducing sugars.
• IR (cm⁻¹): saturated aliphatic aldehyde C=O ≈1720–1740, ketone ≈1710–1720; conjugation lowers it (aryl/enone ≈1685–1700); ring strain raises it (cyclopentanone ≈1745, cyclobutanone ≈1780); the aldehyde C–H doublet at ≈2720 and 2820 is the single most diagnostic band; no broad 3200–3600 O–H.
• ¹H NMR (δ, ppm): CHO 9.5–10.1 (a lone, far-downfield signal); α-H 2.1–2.6; ArCHO ≈9.9. ¹³C: carbonyl 190–205 (aldehyde ≈190–200, ketone ≈195–220) and no proton attached for ketones in DEPT.
• UV-visible: weak, symmetry-forbidden n→π* at ≈270–300 nm (ε ≈10–20) and strong π→π* at ≈180–190 nm; conjugation shifts both to longer wavelength (Woodward–Fieser rules for enones).
• Mass spectrum: α-cleavage (acylium ions, m/z 43 for CH₃CO⁺) and the McLafferty rearrangement (loss of an alkene via a six-membered transition state, m/z 58 for many methyl ketones).

Key points

• Carbonyl group – planar sp² carbon, C=O ≈1.22 Å, strongly polarised Cδ+=Oδ−; carbon is the electrophilic site.
• Characteristic reaction – nucleophilic addition (aldehydes and ketones), contrasted with nucleophilic acyl substitution in acids and derivatives.
• Reactivity order – HCHO > CH₃CHO > RCHO > ketones > aryl ketones, from combined steric crowding and electron donation.
• Aldehyde vs ketone – aldehyde has one H on the carbonyl, so it is more reactive to nucleophiles and easily oxidised; the ketone is not.
• Hydration – only formaldehyde and electron-poor carbonyls (chloral) give stable gem-diols; K ≈ 2000, 1, 0.002 for HCHO, CH₃CHO, acetone.
• Acetal – stable to base and hydride, cleaved by dilute acid; the standard carbonyl protecting group.
• α-Hydrogen acidity – pKa ≈17–20 because of enolate resonance; the origin of all condensation chemistry.
• Aldol condensation – needs α-H; gives β-hydroxy carbonyl, then an α,β-unsaturated carbonyl on dehydration.
• Cannizzaro – needs the absence of α-H; disproportionation into alcohol + carboxylate by hydride transfer.
• Haloform/iodoform test – positive only for CH₃CO– (and CH₃CH(OH)–); yields CHI₃ plus the carboxylate.
• Tollens and Fehling – aldehyde-specific; benzaldehyde is Tollens-positive but Fehling-negative.
• 2,4-DNP – positive for every aldehyde and ketone; the hydrazone's melting point identifies the compound.
• Clemmensen (acidic) and Wolff–Kishner (basic) – both convert C=O to CH₂; choose by the substrate's acid/base stability.
• Wittig – puts the C=C exactly where the C=O was, no carbocation and no rearrangement.
• Baeyer–Villiger – ketone to ester, migratory aptitude 3° > 2° ≈ aryl > 1° > CH₃, configuration retained.
• Beckmann – oxime to amide, anti group migrates; the industrial route to caprolactam.
• Conjugate addition – soft nucleophiles add 1,4 to enones; hard organometallics add 1,2.
• Spectroscopic fingerprint – IR ≈1715 plus the 2720/2820 aldehyde C–H doublet; ¹H NMR δ 9.5–10.1; ¹³C δ 190–205.

Formulas and examples

Example 1 — Grignard synthesis of a tertiary alcohol. 11.6 g acetone is treated with excess CH₃MgBr in dry ether and worked up with dilute acid; the isolated 2-methylpropan-2-ol is 11.6 g. Find the percentage yield.

• M(acetone) = 58.08 → n = 11.6/58.08 = 0.1997 ≈ 0.200 mol.
• Stoichiometry is 1:1 → theoretical 0.200 mol of (CH₃)₃COH, M = 74.12 → 0.200 × 74.12 = 14.82 g.
• Yield = 11.6/14.82 × 100 = 78.3%. (Note why acetone cannot be reused as solvent here — it reacts with the reagent.)

Example 2 — Tollens' test, silver recovered. 2.20 g of acetaldehyde is warmed with excess Tollens' reagent. What mass of silver mirror forms?

• M(CH₃CHO) = 44.05 → n = 2.20/44.05 = 0.0499 ≈ 0.0500 mol.
• 1 mol RCHO → 2 mol Ag → 0.100 mol Ag; M(Ag) = 107.87 → 10.79 g silver.
• Acetone under the same conditions gives nothing — the basis of the test.

Example 3 — Iodoform from acetone. 5.80 g acetone with excess I₂/NaOH gives iodoform in 85% yield. Find the mass obtained.

• n(acetone) = 5.80/58.08 = 0.0999 ≈ 0.100 mol → theoretical 0.100 mol CHI₃.
• M(CHI₃) = 12.01 + 1.01 + 3 × 126.90 = 393.73 g mol⁻¹ → theoretical 39.37 g.
• Actual = 0.85 × 39.37 = 33.5 g iodoform. Iodine consumed = 3 × 0.100 = 0.300 mol = 76.1 g I₂.

Example 4 — Cannizzaro mass balance. 21.2 g benzaldehyde is refluxed with 50% NaOH. Calculate the maximum masses of the two products.

• M(C₆H₅CHO) = 106.12 → n = 21.2/106.12 = 0.200 mol.
• 2:1:1 stoichiometry → 0.100 mol each product.
• Benzyl alcohol, M = 108.14 → 10.81 g; sodium benzoate, M = 144.10 → 14.41 g. Half the aldehyde is reduced and half oxidised — so the theoretical maximum of either product is only 50 mol%.

Example 5 — identifying an unknown from its 2,4-DNP derivative. An unknown neutral liquid gives an orange 2,4-DNP derivative of molar mass 238.2, is Tollens-negative, and gives a yellow precipitate with I₂/NaOH.

• M(carbonyl) = 238.2 − 180.12 = 58.1 g mol⁻¹.
• Tollens-negative rules out an aldehyde; iodoform-positive demands CH₃CO–; C₃H₆O = 58.08 fits.
• Therefore the compound is acetone (propan-2-one). Confirmation: IR ≈1715 cm⁻¹, no 2720 cm⁻¹ band; ¹H NMR a single peak at δ 2.1; ¹³C at δ ≈206.

Where it is used

• Resins, adhesives and laminates: formaldehyde with urea, melamine and phenol gives the glues for plywood, particle board and decorative laminates — Pidilite, Greenply, Century Plyboards, Merino; free-formaldehyde limits in plywood adhesive are set by the relevant Indian Standards for synthetic resin adhesives (quote the current IS edition from BIS, as several older numbers have been revised).
• Bulk solvents and intermediates: acetone (cumene process, Deepak Nitrite, Hindustan Organic Chemicals) for pharma solvents, MIBK, bisphenol-A and methyl methacrylate; acetaldehyde (Jubilant Ingrevia) for acetic acid, acetic anhydride, pentaerythritol and pyridines. Industrial chemicals generally fall in the 18% GST slab (slabs since 22 September 2025 are 5%, 18% and 40%).
• Pharmaceutical synthesis: reductive amination, aldol and Knoevenagel steps in paracetamol, ibuprofen, salbutamol and chloramphenicol routes; chloral hydrate as a sedative; formaldehyde as a vaccine inactivating agent and laboratory fixative — roles at Sun Pharma, Dr Reddy's, Divi's, Aurobindo and Cipla.
• Flavour, fragrance and food: vanillin (Camlin Fine Sciences is among the world's largest producers), cinnamaldehyde, citral, benzaldehyde, hexanal and the "aldehydic" notes of perfumery; flavour purity and permitted use are governed by FSSAI regulations on food additives and flavourings.
• Polymers and fibres: cyclohexanone → oxime → caprolactam → nylon-6 (GSFC); KA-oil oxidation → adipic acid → nylon-6,6; formaldehyde → polyacetal (POM) engineering plastic.
• Analysis, QC and jobs: QC chemist and analytical roles running carbonyl value/aldehyde content titration (hydroxylamine hydrochloride method), GC purity of solvent-grade acetone, HPLC of carbonyls as DNPH derivatives in air monitoring, and FSSAI formalin screening; R&D and process-chemistry roles in CRAMS/CDMO companies.

Interview questions

• Q: Why are aldehydes more reactive than ketones towards nucleophiles? A: A ketone has two alkyl groups that both crowd the carbonyl carbon sterically and donate electron density inductively, so its carbon is less electrophilic and the tetrahedral product more strained.
• Q: How will you distinguish acetaldehyde from acetone using one reagent? A: Tollens' reagent — acetaldehyde gives a silver mirror, acetone does not; alternatively Fehling's or Schiff's reagent. The iodoform test would fail to distinguish them, as both are positive.
• Q: Why does benzaldehyde give a Tollens test but not a Fehling test? A: The Cu²⁺–tartrate complex of Fehling's is a weaker, more selective oxidant and the aryl aldehyde is less easily oxidised; the ammoniacal silver complex is strong enough.
• Q: Why does formaldehyde undergo Cannizzaro but acetaldehyde does not? A: Cannizzaro needs the absence of α-hydrogen; acetaldehyde has three α-H, so the faster base-catalysed aldol pathway takes over.
• Q: Why is oxime formation fastest near pH 4.5? A: Acid is needed to protonate the carbonyl oxygen and assist dehydration of the carbinolamine, but at lower pH the amine is protonated and loses its nucleophilicity — so the rate peaks at weakly acidic pH.
• Q: Chloral hydrate is a stable gem-diol while acetone hydrate is not. Why? A: Three electronegative chlorines strongly withdraw electrons, destabilising the carbonyl and stabilising the sp³ diol; intramolecular hydrogen bonding adds further stability.
• Q: How would you convert cyclohexanone to cyclohexane? A: Clemmensen reduction (Zn-Hg/conc. HCl) or Wolff–Kishner (hydrazone then KOH at 180 °C); or form the thioacetal and desulphurise with Raney nickel.
• Q: Why use a Wittig reaction instead of dehydrating an alcohol? A: The Wittig places the double bond exactly where the carbonyl was and involves no carbocation, so there is no rearrangement or mixture of regioisomers.
• Q: Which group migrates in a Baeyer–Villiger oxidation, and does it keep its stereochemistry? A: The more substituted/better electron-donating group migrates (3° > 2° ≈ aryl > 1° > methyl), and it migrates with retention of configuration.
• Q: Why does a 1,3-diketone show far more enol than acetone? A: Its enol is conjugated with the second carbonyl and stabilised by a six-membered intramolecular hydrogen bond, lowering its energy sharply.
• Q: A compound shows IR bands at 1725, 2720 and 2820 cm⁻¹ and a ¹H NMR singlet at δ 9.8. What is it? A: An aldehyde — 1725 is the C=O, the 2720/2820 doublet is the aldehyde C–H, and δ 9.8 is the CHO proton; a ketone would show neither the doublet nor that signal.
• Q: Why must a carbonyl group be protected as an acetal during a Grignard reaction elsewhere in the molecule? A: The Grignard reagent would add to the free carbonyl; the acetal has no electrophilic carbonyl, survives the basic/organometallic conditions, and is restored by dilute acid afterwards.
• Q: Acetone and propan-1-ol have similar molar masses, but very different boiling points. Explain. A: Propan-1-ol self-associates by O–H hydrogen bonding (b.p. 97 °C); acetone has only dipole–dipole attraction (56 °C), though it still dissolves in water because it accepts hydrogen bonds.

Common mistakes

• Treating the carbonyl as electron-rich: the carbon is electrophilic, so the attacking species is a nucleophile, never an electrophile at carbon.
• Attempting an aldol on a carbonyl without α-hydrogen (benzaldehyde, benzophenone) or a Cannizzaro on one with α-hydrogen.
• Claiming the iodoform test proves "an aldehyde or ketone" — it proves a CH₃CO– (or CH₃CH(OH)–) group only, and acetaldehyde is the sole positive aldehyde.
• Using NaBH₄ and LiAlH₄ interchangeably: LiAlH₄ also reduces esters, acids, amides and nitro groups and is destroyed by water, while NaBH₄ in alcohol is selective for aldehydes and ketones.
• Writing a Grignard reaction in an alcohol, water or acetone solvent, or forgetting the acidic work-up step.
• Mixing up Clemmensen (acidic) with Wolff–Kishner (basic) and so choosing the one that destroys the substrate; also forgetting that both give CH₂, not CH–OH.
• Numbering an aldehyde carbon other than C1, or giving the CHO a locant; and ranking a ketone above an aldehyde when choosing the principal functional group.
• Dropping the 2:1 stoichiometry in Cannizzaro or Tollens calculations, so the computed yield or silver mass is out by a factor of two.
• Reporting "gem-diol formed" as a complete reaction when hydration is only an equilibrium, overwhelmingly on the carbonyl side for ketones.

Related topics

• Organic reaction mechanisms · Alcohols, phenols and ethers · Carboxylic acids and derivatives · Amines and nitrogen compounds · Stereochemistry · Organic nomenclature and isomerism