Aldehydes & Ketones — Named Reactions (Aldol, Cannizzaro, Clemmensen, etc.) Why these named reactions matter for NEET Aldehydes and ketones give a famous set of named reactions that examiners love because they test both concept and recognition. The same structural idea — whether the carbonyl compound has an α-hydrogen — decides between aldol vs Cannizzaro. Similarly, choosing Clemmensen or Wolff–Kishner depends on whether your substrate tolerates strong acid or strong base. Master these patterns and you can solve many ID, completion, and assertion–reason questions quickly. remember In biology, aldol chemistry is central: aldolase in glycolysis interconverts fructose-1,6-bisphosphate with dihydroxyacetone phosphate and glyceraldehyde-3-phosphate — same C=O to C–C bond logic, just enzyme-catalysed. α-Hydrogen acidity and enolate — the switch that decides Aldol vs Cannizzaro α-Hydrogens are the hydrogens on the carbon next to the carbonyl (C=O). They are relatively acidic ( pK a ≈ 20–25) because the conjugate base (enolate) is resonance-stabilised: the negative charge can delocalise between the α-carbon and the oxygen. In base, a small amount of enolate forms — even this tiny fraction is enough to drive powerful C–C bond-forming aldol reactions. If a carbonyl compound has no α-H at all (like formaldehyde or benzaldehyde), it cannot form an enolate; in concentrated base, such aldehydes undergo Cannizzaro (a redox disproportionation). α-Hydrogen (alpha hydrogen) A hydrogen attached to the carbon directly adjacent to a carbonyl carbon (the α-carbon). Its acidity ( pK a ≈ 20–25) enables enolate formation. Enolate ion The resonance-stabilised conjugate base formed when base removes an α-H from a carbonyl compound; nucleophilic at C and O and attacks electrophiles (especially carbonyl carbons). Enolate resonance diagram: acetaldehyde (CH3-CHO) in base OH− abstracts α-H to give enolate. Panel A shows carbanion form, Panel B shows oxyanion form with curved arrows. Labels: 'carbanion resonance', 'oxyanion resonance'. Clean 2D vector style, red curved arrows, atoms black, no internal text beyond labels. 2026-05-26T17:05:38.064Z Resonance of an enolate from acetaldehyde: show base removing α-H, then delocalisation of negative charge between α-carbon and oxygen. gpt-image-2 Aldol addition and aldol condensation Under cold, dilute base, an aldehyde or ketone with at least one α-H undergoes aldol addition: an enolate nucleophilically adds to another carbonyl, giving a β-hydroxy carbonyl (the "aldol": aldehyde + alcohol). With heating or under dehydrating conditions, this β-hydroxy carbonyl eliminates water via E1cb, forming an α,β-unsaturated carbonyl — this second step is aldol condensation. Self-aldol of acetaldehyde: enolate attack to give 3-hydroxybutanal, then dehydration on heating to give 2-butenal (crotonaldehyde). Dil. NaOH, 0–10 C OH− abstracts an α-H from acetaldehyde to give the resonance-stabilised enolate. Base removes α-H; show negative charge delocalised to O. Dil. NaOH The enolate carbon attacks the carbonyl carbon of a second acetaldehyde molecule to form an alkoxide. Curved arrow from enolate C to C=O carbon; C=O opens to O−. O− picks up H from H2O. Aqueous base Protonation of the alkoxide by water regenerates OH− and gives 3-hydroxybutanal (aldol). Form carbanion; expel OH−/H2O to form C=C conjugated to C=O. Heat ( ), basic medium On heating, base removes the acidic H next to the –OH (E1cb), followed by loss of OH−/H2O to give the conjugated alkene (2-butenal). Acetaldehyde enolate adds to acetaldehyde to give 3-hydroxybutanal (aldol). On heating in base, E1cb dehydration gives 2-butenal (crotonaldehyde). ethanal both nucleophile (as enolate) and electrophile acetaldehyde 3-hydroxybutanal β-hydroxy aldehyde intermediate aldol product but-2-enal α,β-unsaturated aldehyde product crotonaldehyde Base-catalysed aldol: acetaldehyde self-aldol then condensation Cold, dilute base gives the aldol (3-hydroxybutanal). Self-aldol addition of acetaldehyde Aldol condensation on heating Heating promotes dehydration to give the α,β-unsaturated aldehyde (crotonaldehyde). Classically, benzaldehyde (no α-H) + acetaldehyde (α-H) → cinnamaldehyde after condensation. Crossed aldol: choose partners so only one forms an enolate (e.g., mix with an aldehyde that lacks α-H). Cross-aldol strategy: pair an aldehyde that has α-H with one that does not. Example: benzaldehyde (benzenecarbaldehyde; SMILES O=CC1=CC=CC=C1) + acetaldehyde (ethanal; SMILES CC=O) under base give an aldol adduct that dehydrates to cinnamaldehyde (3-phenylprop-2-enal; SMILES O=CC=CC1=CC=CC=C1), the main flavour of cinnamon. Cannizzaro reaction — when there is no α-H Aldehydes without any α-hydrogen undergo Cannizzaro reaction in concentrated base: two molecules disproportionate — one is oxidised to a carboxylate, the other reduced to an alcohol. Typical examples: formaldehyde, benzaldehyde, and tert-alkyl aldehydes such as pivaldehyde. Cannizzaro with formaldehyde: two HCHO molecules in conc. NaOH give methanol and sodium formate — classic disproportionation. Cannizzaro reaction (formal example with HCHO) Conc. NaOH OH− adds to the carbonyl carbon to give a tetrahedral alkoxide intermediate. Nucleophilic attack of OH− on C=O. Strongly basic medium Hydride (H−) transfers from the alkoxide-bearing carbon to the carbonyl carbon of a second aldehyde molecule. Show H− migration to the other C=O. Aqueous base Proton transfers generate an alcohol (reduced product) and a carboxylate (oxidised product). Alkoxide gets protonated; the other alkoxide becomes carboxylate. substrate (no α-H) methanal formaldehyde methanol reduction product methanol sodium methanoate oxidation product (as carboxylate) sodium formate Strong base generates a hydroxide-adduct of the aldehyde. A hydride shifts from this adduct to another aldehyde molecule: one reduces to alcohol, the other oxidises to carboxylate. Formal Cannizzaro (disproportionation of formaldehyde) One molecule is reduced; the other is oxidised. α-H required? Yes (must have ≥1 α-H; forms enolate) No (occurs only when there is 0 α-H) Conditions Dilute base; low T (addition); heat for condensation Concentrated base (typically conc. NaOH) Products β-Hydroxy carbonyl (aldol); then α,β-unsaturated carbonyl on dehydration One carboxylate + one alcohol (disproportionation) Classic examples Acetaldehyde → 3-hydroxybutanal → 2-butenal HCHO, benzaldehyde, pivaldehyde Aldol vs Cannizzaro — quick discriminator Feature Aldol (addition/condensation) Cannizzaro Criterion Only aldehydes without any α-hydrogen (e.g., formaldehyde, benzaldehyde, pivaldehyde) show Cannizzaro in conc. base. Those with α-H prefer aldol under dilute base. All aldehydes undergo Cannizzaro reaction. Haloform (Iodoform) reaction — yellow CHI₃ test The haloform test is positive for methyl ketones (–COCH3) and for alcohols that oxidise to them under the test conditions (i.e., ethanol and secondary alcohols with a methyl on the carbinol carbon). Iodine/alkali gives a yellow precipitate of iodoform (CHI3) with an antiseptic smell. Methanol and formaldehyde are negative. Test tube schematic: solution turns pale yellow with fine CHI3 precipitate at bottom. Label reagents 'I2 + NaOH (warm)'. Side bubble shows methyl ketone motif –COCH3. Clean vector, red arrows for steps, no inner text beyond labels. 2026-05-26T17:05:38.585Z Iodoform test: development of a yellow CHI3 precipitate from a methyl ketone with I2/NaOH. gpt-image-2 Positive for CH 3 CO – (methyl ketones) and for ethanol/2° alcohols with –CH(OH)–CH3 (via oxidation to methyl ketone). General iodoform (haloform) reaction Acetone (propan-2-one) and other methyl ketones Acetaldehyde (ethanal) — under test conditions Ethanol (via oxidation to ethanal) All secondary alcohols with a methyl on the carbinol carbon (R–CH(OH)–CH3) Who is positive? No. Only for the CH3CO– group (methyl ketone) or alcohols that become it under the basic, oxidising conditions (e.g., ethanol; certain 2° alcohols). Phenols, primary alcohols without CH3 on the carbinol C, methanol are negative. Iodoform is positive for any –OH containing compound. Methanol gives iodoform. Methanol (CH3OH) lacks the required CH3CO– or CH3CH(OH)– motif; it does not give the iodoform test. Reducing C=O to CH2 or alcohol: choose the right reagent Reduction choices depend on the substrate and other functional groups present. Acidic Clemmensen (Zn–Hg/HCl) vs strongly basic Wolff–Kishner (hydrazine + KOH, heat) both convert C=O to CH2 (deoxygenation) but in opposite media. Milder hydride donors like NaBH4 and stronger LiAlH4 reduce C=O to alcohols; catalytic hydrogenation (H2/Pt) also reduces C=O to alcohol. Clemmensen (Zn–Hg/HCl, acidic) vs Wolff–Kishner (NH2NH2, KOH, heat, basic): both turn C=O into CH2 but suit different substrates. Hydrazone formation then base-promoted N2 extrusion gives methylene. Wolff–Kishner overall Clemmensen overall Acid-mediated deoxygenation via Zn–Hg amalgam and conc. HCl. Aspect Clemmensen reduction Wolff–Kishner reduction Feature Clemmensen vs Wolff–Kishner — exam-ready comparison Conditions Zn–Hg amalgam, conc. HCl (acidic) NH2NH2 then strong base (KOH), heat (often ethylene glycol) Medium suitability Good for base-sensitive substrates; avoid acid-labile groups Good for acid-sensitive substrates; avoid base-labile groups Scope Aldehydes/ketones → methylene Aldehydes/ketones → methylene (via hydrazone) Mechanistic flag Acidic metal-mediated reduction Hydrazone formation → deprotonation → N2 extrusion (strong base) Reducing agents for C=O — what they reduce (NCERT focus) Reagent Typical action on aldehydes/ketones Notes Agent NaBH4 (sodium borohydride) RCHO, R2C=O → corresponding alcohols Mild; generally does not reduce esters/acids LiAlH4 (lithium aluminium hydride) RCHO, R2C=O → alcohols Strong; also reduces esters and carboxylic acids H2/Pt (catalytic hydrogenation) RCHO, R2C=O → alcohols Reduces C=O; double bonds may also reduce if present/accessible Clemmensen (Zn–Hg/HCl) RCHO, R2C=O → CH2 Strongly acidic conditions Wolff–Kishner (NH2NH2, KOH, heat) RCHO, R2C=O → CH2 Strongly basic, high-temperature conditions Zn–Hg/HCl converts C=O to CH2 under acidic conditions. Hydrazine + strong base + heat converts C=O to CH2 under basic conditions. General set: hydride donors (NaBH4, LiAlH4) and catalytic H2 reduce carbonyls to alcohols. Wolff–Kishner and Clemmensen conditions can be used interchangeably. They are opposite media: Wolff–Kishner is strongly basic and heat; Clemmensen is strongly acidic. Choose based on what the rest of the molecule tolerates. Oxidation and distinguishing tests Aldehydes are easily oxidised; ketones generally resist mild oxidants. This difference powers classic lab tests: Tollens’ (silver mirror), Fehling’s/Benedict’s (Cu2+ to Cu2O brick-red), and Schiff’s (colour change). These help distinguish aldehydes from ketones and detect reducing sugars. 2026-05-26T17:05:39.014Z Test tube schematic with reflective silver mirror on walls, label 'Tollens’ reagent: [Ag(NH3)2]+ in NH3/aq'. Show an aldehyde arrow to carboxylate. Clean vector, neutral palette. gpt-image-2 Tollens’ test: silver mirror on the inner wall of a clean test tube with an aldehyde and ammoniacal AgNO3. Fehling’s test: brick-red Cu2O precipitate forms with an aliphatic aldehyde. gpt-image-2 Two test tubes: blue solution before heating; after, brick-red precipitate at bottom. Labels: 'Fehling’s A/B, heat'. Diagrammatic style. 2026-05-26T17:05:39.320Z Distinguishing tests for carbonyls (what is positive and observation) Test (reagent) Positive for Observation Test Tollens’ reagent (ammoniacal AgNO3) Aldehydes; reducing sugars (e.g., glucose) Shiny silver mirror; aldehyde → carboxylate (RCOO−) Fehling’s solution (alkaline Cu2+–tartrate) Aliphatic aldehydes, reducing sugars Brick-red Cu2O precipitate (benzaldehyde negative) Benedict’s solution (Cu2+–citrate) Reducing sugars; aliphatic aldehydes Red/orange Cu2O precipitate Schiff’s reagent (decolourised fuchsin) Aldehydes Restoration of pink/magenta colour Iodoform test (I2, NaOH) CH3CO– or CH3CH(OH)– motif Yellow CHI3 precipitate 2,4-DNP (Brady’s reagent) Carbonyls (aldehydes/ketones) Yellow–orange precipitate of hydrazone (derivative formation) Aldehydes reduce [Ag(NH3)2]+ to Ag (mirror), forming carboxylate. Aliphatic aldehydes reduce Cu2+ to Cu2O (brick-red). Aromatic aldehydes like benzaldehyde usually negative. Ketones also give Tollens’ silver mirror test. Ordinary ketones are negative with Tollens’. Tollens’ is a reliable positive for aldehydes (and some reducing sugars). Perkin reaction, Benzoin condensation, Baeyer–Villiger oxidation (overview) Perkin reaction forms α,β-unsaturated acids by condensing an aromatic aldehyde with an acid anhydride in the presence of the anhydride’s sodium salt (e.g., benzaldehyde + acetic anhydride + sodium acetate → cinnamic acid). Benzoin condensation couples two benzaldehyde molecules with catalytic cyanide (KCN) to give benzoin — it works via umpolung (temporary polarity inversion at carbonyl carbon). Baeyer–Villiger oxidation inserts an oxygen next to the carbonyl of a ketone using a peracid (e.g., mCPBA), converting a ketone to an ester (or forming a lactone from a cyclic ketone). Benzaldehyde + acetic anhydride + sodium acetate → cinnamic acid (α,β-unsaturated acid). 2 Benzaldehyde + KCN (cat.) → benzoin via umpolung at carbonyl. gpt-image-2 Perkin reaction scheme: benzaldehyde + acetic anhydride (NaOAc) → cinnamic acid. 2026-05-26T17:05:39.584Z One-panel reaction scheme with structures: benzaldehyde + acetic anhydride, catalyst sodium acetate above arrow, product cinnamic acid with highlighted C=C–CO2H. Vector style, labelled reagents, clean layout. Industrial and daily-life links Cinnamaldehyde and cinnamic acid (Perkin/aldol sequences) in flavours and perfumes Cannizzaro for handling formaldehyde in certain waste streams Iodoform once used as an antiseptic; characteristic odour Tollens’ silver mirror used in decorative silvering of glassware Where you meet these reactions neet-alert High-yield patterns: (1) α-H present → aldol; α-H absent → Cannizzaro (conc. base). (2) Iodoform positive only for CH3CO– or CH3CH(OH)–. (3) Fehling usually fails for aromatic aldehydes (benzaldehyde negative). (4) Choose Clemmensen (acid) vs Wolff–Kishner (base) to protect other groups. Pick the deoxygenation: "Clem likes acid, Wolf likes base" — Clemmensen (acidic Zn–Hg/HCl), Wolff–Kishner (basic NH2NH2/KOH, heat). α-Hydrogen Hydrogen on the carbon adjacent to C=O; its acidity enables enolate formation. Enolate Resonance-stabilised anion from α-H deprotonation; nucleophilic at C/O. Base-catalysed addition of an enolate to a carbonyl, giving a β-hydroxy carbonyl. Aldol addition Aldol condensation Dehydration of a β-hydroxy carbonyl to an α,β-unsaturated carbonyl (often via E1cb). Cross aldol Aldol between two different carbonyls; often choose one partner without α-H to control products. Disproportionation of aldehydes lacking α-H in conc. base to carboxylate + alcohol. Cannizzaro reaction A redox where the same species is simultaneously oxidised and reduced. Disproportionation Haloform (iodoform) test Reaction of methyl ketones (or precursors) with halogen/alkali to give haloform (e.g., CHI3, yellow). Clemmensen reduction Zn–Hg/HCl converts C=O to CH2 in acidic medium. Hydrazine + base + heat converts C=O to CH2 in basic medium (via hydrazone). Wolff–Kishner reduction Tollens’ reagent Ammoniacal AgNO3; aldehydes give silver mirror. Fehling’s solution Alkaline Cu2+–tartrate; aliphatic aldehydes reduce to Cu2O (brick-red). Benedict’s solution Cu2+–citrate; detects reducing sugars; gives red/orange Cu2O. Decolourised fuchsin; aldehydes restore pink colour. Schiff’s reagent Baeyer–Villiger oxidation Peracid inserts O adjacent to C=O: ketone → ester; cyclic ketone → lactone. Perkin reaction Aromatic aldehyde + anhydride (Na salt) → α,β-unsaturated acid. Benzoin condensation 2 Benzaldehyde + CN− (cat.) → benzoin; involves umpolung (polarity inversion). Temporary reversal of the natural polarity of a functional group to enable otherwise forbidden bond formation. Umpolung Glossary — key terms at a glance