CBSE · Class 12 · Chemistry · Chapter 8

Aldehydes, Ketones and Carboxylic Acids — Formula Sheet

Board Formulas

20 formulas

1.Rosenmund Reduction★
$R-COCl + H_{2} Pd-BaSO_{4} R-CHO + HCl$
BaSO₄ poisons the Pd catalyst so the aldehyde is not reduced further to an alcohol. Gives aldehydes, not ketones.
2.Stephen Reaction
$R-CN + SnCl_{2} + HCl \to R-CH=NH H_{3} O^{+} R-CHO$
Nitrile → imine → aldehyde on hydrolysis. Alternatively DIBAL-H reduces nitriles (and esters) to aldehydes: R–CN → (i) AlH(i-Bu)₂, (ii) H₂O → R–CHO.
3.Etard Reaction
$C_{6} H_{5} CH_{3} (i) CrO_{2} Cl_{2}, CS_{2} (ii) H_{3} O^{+} C_{6} H_{5} CHO$
Chromyl chloride converts the –CH₃ of toluene into a chromium complex that hydrolyses to benzaldehyde, so oxidation stops at the aldehyde. Strong oxidants like KMnO₄ would give benzoic acid.
4.Gattermann–Koch Reaction
$C_{6} H_{6} + CO + HCl anhyd. AlCl_{3} / CuCl C_{6} H_{5} CHO$
Puts –CHO directly on the benzene ring using carbon monoxide and HCl.
5.Friedel–Crafts Acylation (aromatic ketones)
$C_{6} H_{6} + R-COCl anhyd. AlCl_{3} C_{6} H_{5} -CO-R + HCl$
CH₃COCl gives acetophenone. Ketones also come from a nitrile + Grignard reagent followed by hydrolysis, e.g. CH₃CH₂CN + C₆H₅MgBr → propiophenone (C₆H₅COCH₂CH₃).
6.Nucleophilic Addition of HCN (cyanohydrin)
$R_{2} C=O + HCN ⇌ OH^{-} R_{2} C(OH)CN$
Base generates CN⁻, the real nucleophile. Reactivity: HCHO > CH₃CHO > CH₃COCH₃ (ketones have two electron-releasing groups and more crowding). Aromatic aldehydes are less reactive than aliphatic ones because of resonance.
7.Condensation with Ammonia Derivatives
$R_{2} C=O + H_{2} N-Z ⇌ R_{2} C=N-Z + H_{2} O$
Z = –OH (oxime), –NH₂ (hydrazone), –NHC₆H₅ (phenylhydrazone), –NHCONH₂ (semicarbazone). 2,4-DNP gives coloured 2,4-dinitrophenylhydrazones, a test for C=O. Acid-catalysed and reversible.
8.Clemmensen Reduction
$R_{2} C=O Zn-Hg, conc. HCl R_{2} CH_{2} + H_{2} O$
C=O → CH₂ in acidic conditions. Suitable for compounds that are stable to acid.
9.Wolff–Kishner Reduction
$R_{2} C=O NH_{2} NH_{2} R_{2} C=NNH_{2} KOH ethylene glycol, Δ R_{2} CH_{2} + N_{2}$
C=O → CH₂ in basic conditions, through the hydrazone. Suitable for compounds that are sensitive to acid.
10.Tollens' Test (silver mirror)
$RCHO + 2 [Ag(NH_{3})_{2}]^{+} + 3 OH^{-} \to RCOO^{-} + 2 Ag ↓ + 2 H_{2} O + 4 NH_{3}$
All aldehydes, aliphatic and aromatic, reduce Tollens' reagent (ammoniacal AgNO₃) and give a silver mirror; ketones do not. The aldehyde is oxidised to the carboxylate.
11.Fehling's Test
$RCHO + 2 Cu^{2 +} + 5 OH^{-} \to RCOO^{-} + Cu_{2} O ↓ + 3 H_{2} O$
Red-brown Cu₂O precipitate with aliphatic aldehydes. Aromatic aldehydes (benzaldehyde) and ketones do NOT respond. Fehling A = aqueous CuSO₄; Fehling B = alkaline sodium potassium tartrate.
12.Iodoform (Haloform) Reaction★
$R-CO-CH_{3} + 3 I_{2} + 4 NaOH \to R-COONa + CHI_{3} ↓ + 3 NaI + 3 H_{2} O$
Yellow CHI₃ precipitate shows a CH₃CO– group, or a CH₃CH(OH)– group (oxidised to CH₃CO– first). Positive: ethanal, ethanol, propanone, propan-2-ol. Negative: methanal, methanol, propanal, pentan-3-one. The acid formed has one carbon fewer.
13.Aldol Condensation★
$2 CH_{3} CHO dil. NaOH CH_{3} CH(OH)CH_{2} CHO Δ - H_{2} O CH_{3} CH=CHCHO$
Needs at least one α-H. Ethanal → 3-hydroxybutanal (aldol) → but-2-enal on heating. Propanone with Ba(OH)₂ → 4-hydroxy-4-methylpentan-2-one → 4-methylpent-3-en-2-one. Two different carbonyls that both have α-H (cross aldol) give a mixture of four products.
14.Cannizzaro Reaction★
$2 HCHO + conc. KOH Δ CH_{3} OH + HCOOK$
Only aldehydes with NO α-H. Disproportionation: one molecule is reduced to the alcohol, the other oxidised to the carboxylate. Also 2C₆H₅CHO + conc. NaOH → C₆H₅CH₂OH + C₆H₅COONa.
15.Carboxylic Acids by Oxidation of Alkylbenzenes
$C_{6} H_{5} CH_{3} KMnO_{4} -KOH, Δ C_{6} H_{5} COOK H_{3} O^{+} C_{6} H_{5} COOH$
The whole side chain becomes –COOH whatever its length (ethylbenzene also gives benzoic acid). 1° and 2° alkyl groups are oxidised; a 3° alkyl group is not affected.
16.Carboxylic Acids from Grignard Reagents
$R-MgX + CO_{2} dry ether R-COOMgX H_{3} O^{+} R-COOH$
Dry ice (solid CO₂) is used. The acid has one carbon MORE than the alkyl halide. Nitriles also give acids on hydrolysis: R–CN → R–CONH₂ → R–COOH (H⁺ or OH⁻ catalyst).
17.Acid Strength of Carboxylic Acids★
$FCH_{2} COOH > ClCH_{2} COOH > HCOOH > C_{6} H_{5} COOH > CH_{3} COOH > CH_{3} CH_{2} COOH$
Electron-withdrawing groups stabilise the carboxylate ion and raise acidity; electron-releasing alkyl groups lower it. pKa: HCOOH 3.75, C₆H₅COOH 4.19, CH₃COOH 4.76. More halogens, or a halogen closer to –COOH, means a stronger acid (CF₃COOH > CCl₃COOH > CHCl₂COOH).
18.Acid Chlorides from Carboxylic Acids
$RCOOH + SOCl_{2} \to RCOCl + SO_{2} ↑ + HCl ↑$
SOCl₂ is preferred because both by-products are gases, so the acid chloride is easy to purify. Also: RCOOH + PCl₅ → RCOCl + POCl₃ + HCl, and 3RCOOH + PCl₃ → 3RCOCl + H₃PO₃.
19.Decarboxylation with Soda Lime
$R-COONa + NaOH CaO Δ R-H + Na_{2} CO_{3}$
Soda lime = NaOH + CaO (3 : 1). The alkane has one carbon fewer: CH₃COONa gives CH₄. (Kolbe electrolysis of the salt instead gives R–R.)
20.Hell–Volhard–Zelinsky (HVZ) Reaction★
$R-CH_{2} -COOH (i) X_{2} / red P (ii) H_{2} O R-CH(X)-COOH (X = Cl, Br)$
Halogenation at the α-carbon; the acid must have an α-H. α-Halo acids are useful starting materials for other substitutions.