Aldehydes, Ketones and Carboxylic Acids
Welcome to HSLC Guru! This article presents complete English-medium notes and question answers for Class 12 Chemistry Chapter 12 — Aldehydes, Ketones and Carboxylic Acids — strictly following the ASSEB syllabus. The chapter explores three closely related families of carbonyl compounds whose chemistry forms the backbone of organic synthesis and biological metabolism.
Chapter Summary
Aldehydes and ketones are carbonyl compounds containing the >C=O group. In aldehydes (R–CHO) the carbonyl carbon bears at least one hydrogen, whereas ketones (R–CO–R′) have it bonded to two carbon atoms. The carbonyl carbon is sp² hybridised, planar with bond angles close to 120°. The C=O bond is highly polar (oxygen δ−, carbon δ+) due to oxygen’s greater electronegativity, making the carbon a centre for nucleophilic attack. IUPAC nomenclature uses the suffix -al for aldehydes and -one for ketones; common names use -aldehyde and the di-alkyl ketone style. Examples: HCHO (methanal/formaldehyde), CH₃CHO (ethanal/acetaldehyde), CH₃COCH₃ (propan-2-one/acetone).
Preparation methods include: (i) controlled oxidation of primary alcohols to aldehydes (PCC) and secondary alcohols to ketones (CrO₃, K₂Cr₂O₇/H⁺); (ii) ozonolysis of alkenes to give aldehydes/ketones; (iii) hydration of alkynes — terminal alkynes give methyl ketones (Markovnikov, HgSO₄/H₂SO₄), acetylene gives acetaldehyde; (iv) Friedel-Crafts acylation of benzene with RCOCl/anhydrous AlCl₃ giving aryl ketones; (v) Etard reaction — toluene with CrO₂Cl₂ gives benzaldehyde; (vi) Gattermann-Koch — benzene with CO + HCl/anhydrous AlCl₃ + CuCl yields benzaldehyde; (vii) Rosenmund reduction — acid chloride with H₂/Pd-BaSO₄ gives aldehyde without further reduction. Physical properties: lower members are polar liquids (acetone, acetaldehyde) miscible with water due to dipole–dipole and H-bond acceptance; boiling points are higher than non-polar hydrocarbons of similar mass but lower than corresponding alcohols (no O–H).
Chemical reactions centre on nucleophilic addition: with HCN giving cyanohydrins; with NaHSO₃ giving crystalline bisulphite adducts (used for purification); with Grignard reagents giving alcohols (HCHO → 1°, RCHO → 2°, ketone → 3°); with alcohols (dry HCl) giving hemiacetals/acetals (and ketals from ketones — important protecting groups); with ammonia derivatives — primary amines give imines (Schiff bases), hydroxylamine gives oximes, hydrazine gives hydrazones, phenylhydrazine gives phenylhydrazones, semicarbazide gives semicarbazones (used for identification). Reactions due to α-H include the aldol condensation (dilute alkali, two molecules of an aldehyde/ketone with α-H combine to give β-hydroxy carbonyl, then dehydrate to α,β-unsaturated carbonyl) and the Cannizzaro reaction (aldehydes without α-H, e.g., HCHO, C₆H₅CHO, with conc. NaOH undergo disproportionation to alcohol + carboxylate). Aldehydes are easily oxidised (Tollens’ silver mirror, Fehling’s red Cu₂O ppt, Benedict’s), ketones are not (except by strong oxidants — C–C cleavage). Reduction: H₂/Ni or NaBH₄/LiAlH₄ to alcohols; Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (NH₂NH₂/KOH/Δ) reduce C=O to CH₂. Methyl ketones (and CH₃CHO, ethanol) give the iodoform test (yellow CHI₃ with I₂/NaOH).
Carboxylic acids (R–COOH) are named with suffix -oic acid (IUPAC) — methanoic, ethanoic, etc. Preparation: oxidation of 1° alcohols/aldehydes (KMnO₄), hydrolysis of nitriles/esters/amides, carbonation of Grignard reagents (R-MgX + CO₂ → RCOOH), oxidation of alkylbenzenes. They form intermolecular H-bonds, existing as cyclic dimers in vapour and non-polar solvents — boiling points are even higher than alcohols of similar mass. Acidity (pKa ≈ 4–5) is far greater than alcohols/phenols because the carboxylate anion is resonance-stabilised over two equivalent oxygens. Acid strength is enhanced by electron-withdrawing groups (–Cl, –NO₂, –CN — inductive effect, falls with distance) and reduced by electron-donating groups (–CH₃, –OCH₃ at appropriate positions). Reactions: with NaHCO₃ release CO₂ (test); esterification with alcohols (conc. H₂SO₄, reversible); formation of acid chlorides (PCl₅, SOCl₂), anhydrides (P₂O₅), amides (NH₃ then heat); reduction by LiAlH₄ to primary alcohols (B₂H₆ also works selectively); Hell-Volhard-Zelinsky (HVZ) reaction — α-halogenation with Cl₂/Br₂ in presence of red P; decarboxylation of sodium salt with soda lime (NaOH+CaO) gives alkane (R–H). These transformations make carboxylic acids the central hubs for synthesising esters, amides, and other acyl derivatives.
1-Mark Questions
Q1. What is the hybridisation of the carbonyl carbon?
Answer: sp² hybridised, with a planar geometry and bond angles of approximately 120°.
Q2. Give the IUPAC name of CH₃COCH₂CH₃.
Answer: Butan-2-one.
Q3. Name the reagent used in the Rosenmund reduction.
Answer: Hydrogen gas with palladium supported on barium sulphate (H₂/Pd-BaSO₄), poisoned with sulphur or quinoline.
Q4. Why do aldehydes have lower boiling points than alcohols of similar molecular mass?
Answer: Aldehydes lack an O–H bond, so they cannot form intermolecular hydrogen bonds among themselves; only weaker dipole–dipole interactions exist.
Q5. Which aldehyde does not undergo the aldol reaction?
Answer: Aldehydes without α-hydrogen, such as HCHO (formaldehyde), C₆H₅CHO (benzaldehyde), and (CH₃)₃CCHO (trimethylacetaldehyde).
Q6. What is the product of the iodoform test on acetone?
Answer: A yellow precipitate of iodoform (CHI₃) along with sodium acetate.
Q7. Name the catalyst used in Friedel-Crafts acylation.
Answer: Anhydrous aluminium chloride (AlCl₃).
Q8. Which is more acidic — formic acid or acetic acid?
Answer: Formic acid (HCOOH, pKa ≈ 3.75) is more acidic than acetic acid (CH₃COOH, pKa ≈ 4.76) because the methyl group in acetic acid donates electrons (+I), destabilising the carboxylate anion.
Q9. Write the product of the reaction of CH₃CHO with HCN.
Answer: CH₃CH(OH)CN — 2-hydroxypropanenitrile (acetaldehyde cyanohydrin).
Q10. What is HVZ reaction?
Answer: Hell-Volhard-Zelinsky reaction — α-halogenation of carboxylic acids with Cl₂ or Br₂ in the presence of red phosphorus, producing α-halocarboxylic acids.
2-3 Mark Questions
Q11. Distinguish between aldehydes and ketones using two chemical tests.
Answer: (i) Tollens’ test: aldehydes reduce Tollens’ reagent [Ag(NH₃)₂]⁺ to a shiny silver mirror; ketones do not. (ii) Fehling’s test: aldehydes give a brick-red precipitate of Cu₂O on heating with Fehling’s solution; ketones give no reaction. The difference arises because aldehydes have an H on the carbonyl carbon, allowing easy oxidation to carboxylic acids, whereas ketones lack this H.
Q12. Explain the mechanism of nucleophilic addition to a carbonyl group.
Answer: The polar C=O bond carries a δ+ on carbon. A nucleophile (Nu⁻) attacks the electrophilic carbon, breaking the π-bond and pushing electrons onto oxygen to give a tetrahedral alkoxide intermediate. The intermediate is then protonated by water or another acid source, giving the neutral addition product. The reaction is faster in aldehydes than ketones because ketones have two alkyl groups providing steric hindrance and electron donation, both of which slow nucleophilic attack.
Q13. Write the products of: (i) ozonolysis of 2-methylbut-2-ene; (ii) hydration of propyne.
Answer: (i) (CH₃)₂C=CHCH₃ on ozonolysis (O₃, then Zn/H₂O) gives acetone (CH₃COCH₃) and acetaldehyde (CH₃CHO). (ii) CH₃C≡CH + H₂O / HgSO₄ / H₂SO₄ gives propan-2-one (CH₃COCH₃) by Markovnikov addition through an enol intermediate.
Q14. Why do carboxylic acids exist as dimers? Show the structure.
Answer: Each –COOH has both an H-bond donor (O–H) and acceptor (C=O). Two molecules associate via two strong O–H···O=C hydrogen bonds, giving a stable cyclic eight-membered-ring dimer. Because of this, in vapour and non-polar solvents the apparent molecular mass is doubled, and boiling points are unusually high.
Q15. Compare the acidity of CH₃COOH, ClCH₂COOH and Cl₂CHCOOH.
Answer: Order of acidity: Cl₂CHCOOH > ClCH₂COOH > CH₃COOH. Chlorine is electron-withdrawing (–I), it disperses the negative charge of the carboxylate ion, stabilising it. Two chlorines are stronger than one, while methyl is electron-donating and destabilises the anion. pKa values: ~1.3, 2.85, 4.76 respectively.
Q16. Describe the Cannizzaro reaction with an example.
Answer: Aldehydes that lack α-hydrogen, on treatment with concentrated alkali (NaOH), undergo intermolecular disproportionation: one molecule is oxidised to a carboxylate, while another is reduced to an alcohol. Example: 2 HCHO + NaOH → HCOONa + CH₃OH. The reaction occurs because there is no α-H available for the alternative aldol pathway.
5-7 Mark Questions
Q17. Discuss the methods of preparation of aldehydes and ketones from (a) alcohols, (b) hydrocarbons, (c) acid chlorides.
Answer: (a) From alcohols: Primary alcohols undergo controlled oxidation with PCC (pyridinium chlorochromate) in CH₂Cl₂ to give aldehydes, avoiding overoxidation to acids. Stronger oxidants (KMnO₄, K₂Cr₂O₇/H⁺) push to the carboxylic acid. Secondary alcohols are oxidised by CrO₃ or K₂Cr₂O₇/H⁺ to ketones. Catalytic dehydrogenation (Cu/300°C) also converts 1° alcohols to aldehydes and 2° alcohols to ketones.
(b) From hydrocarbons: (i) Ozonolysis of alkenes with O₃ followed by reductive work-up (Zn/H₂O) gives aldehydes and/or ketones depending on substitution pattern. (ii) Hydration of alkynes — acetylene with H₂O/HgSO₄/dil. H₂SO₄ gives acetaldehyde; higher terminal alkynes give methyl ketones (Markovnikov). (iii) Friedel-Crafts acylation of benzene with RCOCl/AlCl₃ gives aryl alkyl ketones; with CO+HCl (Gattermann-Koch) gives benzaldehyde. (iv) Etard reaction: toluene + CrO₂Cl₂ in CS₂ gives benzaldehyde via a chromium complex hydrolysed to PhCHO.
(c) From acid chlorides: Rosenmund reduction — RCOCl + H₂ over Pd-BaSO₄ (poisoned with sulphur/quinoline) selectively gives RCHO without further reduction. Reaction with dialkylcadmium (R₂Cd) prepared from RMgX gives ketones: 2 R′COCl + R₂Cd → 2 R′COR + CdCl₂. This is gentler than Grignard, which would over-add.
Q18. Explain in detail the nucleophilic addition reactions of aldehydes and ketones with (i) HCN, (ii) Grignard reagents, (iii) alcohols, (iv) ammonia derivatives.
Answer: (i) HCN: In presence of a small amount of base (CN⁻ is the actual nucleophile), CN⁻ attacks the carbonyl carbon, then protonation gives a cyanohydrin R₂C(OH)CN. These are useful intermediates — hydrolysis gives α-hydroxy acids, reduction gives β-amino alcohols.
(ii) Grignard reagent (RMgX): The carbanionic R⁻ attacks the carbonyl carbon, forming a magnesium alkoxide that is hydrolysed to an alcohol. HCHO + RMgX gives a 1° alcohol; RCHO gives a 2° alcohol; ketone gives a 3° alcohol. Example: CH₃MgI + CH₃CHO → CH₃CH(OMgI)CH₃ → propan-2-ol on H₃O⁺.
(iii) Alcohols: With dry HCl, one molecule of alcohol adds to the C=O to give a hemiacetal R–CH(OH)(OR′); a second alcohol replaces the OH to give the acetal R–CH(OR′)₂. Ketones similarly form ketals (slower, often using ethylene glycol with H⁺ to give cyclic ketals). Acetals/ketals are stable to base but hydrolysed by acid — used as protecting groups.
(iv) Ammonia derivatives (H₂N–Z): All proceed through a hemiaminal R₂C(OH)(NHZ) which loses water to give R₂C=N–Z. Specific products: NH₂–OH gives oximes; NH₂–NH₂ gives hydrazones; C₆H₅NH–NH₂ gives phenylhydrazones; NH₂–NH–CONH₂ gives semicarbazones; primary amine R′NH₂ gives imines (Schiff bases). These crystalline derivatives have sharp melting points and are used to identify carbonyl compounds.
Q19. Describe the methods of preparation of carboxylic acids and discuss factors affecting their acid strength.
Answer: Preparation methods: (1) Oxidation of primary alcohols and aldehydes by KMnO₄ or K₂Cr₂O₇/H⁺. (2) Oxidation of alkylbenzenes — toluene with KMnO₄ gives benzoic acid. (3) Hydrolysis of nitriles: R-CN + H₂O/H⁺ → R-COOH. (4) Hydrolysis of esters/amides under acid or base catalysis. (5) Carbonation of Grignard reagent: R-MgX + CO₂ → R-COO-MgX → R-COOH after H₃O⁺. (6) Industrially, formic acid is made from CO + NaOH, acetic acid from methanol + CO (Monsanto process).
Factors affecting acid strength: (i) Inductive effect — electron-withdrawing groups (–F, –Cl, –Br, –NO₂, –CN) stabilise the carboxylate by dispersing negative charge, increasing acidity. The order in halogenated acetic acids: F > Cl > Br > I (matches electronegativity). (ii) Number and position of EWGs — Cl₃CCOOH ≫ Cl₂CHCOOH > ClCH₂COOH; the effect falls sharply with distance (α > β > γ). (iii) Electron-donating groups (alkyl, –OCH₃ in some positions) destabilise the anion and reduce acidity. (iv) Resonance — aromatic acids: substituent at o/p position affects pKa via combined inductive and mesomeric effects (–NO₂ at p increases acidity strongly). (v) Hydrogen bonding (intramolecular) in ortho-substituted benzoic acids (e.g., salicylic acid) stabilises the anion and increases acidity.
Q20. Explain the chemical reactions of carboxylic acids: esterification, with NaHCO₃, formation of acid halides/anhydrides/amides, reduction, HVZ, and decarboxylation.
Answer: (1) Esterification: R-COOH + R′-OH ⇌ R-COO-R′ + H₂O, catalysed by conc. H₂SO₄. The mechanism is acid-catalysed nucleophilic acyl substitution (PADPED — protonation, addition, deprotonation, protonation, elimination, deprotonation). The reaction is reversible; excess alcohol or removal of water shifts equilibrium to ester.
(2) With NaHCO₃: R-COOH + NaHCO₃ → R-COONa + H₂O + CO₂↑. Effervescence of CO₂ is a diagnostic test that distinguishes carboxylic acids from phenols (phenols do not react with NaHCO₃).
(3) Acid halides: R-COOH with PCl₅, PCl₃, or SOCl₂ gives R-COCl. SOCl₂ is preferred because by-products SO₂ and HCl are gases.
(4) Anhydrides: 2 R-COOH on heating with P₂O₅ (dehydrating agent) gives (RCO)₂O + H₂O; alternatively R-COOH + R-COCl gives mixed anhydride.
(5) Amides: R-COOH + NH₃ → R-COO⁻NH₄⁺, which on heating loses water to give R-CONH₂.
(6) Reduction: LiAlH₄ in dry ether reduces R-COOH to R-CH₂OH (primary alcohol). B₂H₆ also reduces selectively without affecting –NO₂, –CN, ester, or halide groups. NaBH₄ does not reduce –COOH.
(7) HVZ reaction: Carboxylic acids with α-hydrogen react with Cl₂/Br₂ in presence of red phosphorus (which forms PCl₃/PBr₃ in situ) to give α-halocarboxylic acids. Mechanism passes through acid halide → enol → α-halo acid halide → α-halo acid.
(8) Decarboxylation: Sodium salts of carboxylic acids on heating with soda lime (NaOH + CaO) lose CO₂ to give alkanes: R-COONa + NaOH → R-H + Na₂CO₃. Aromatic acids similarly give arenes (benzoic acid → benzene). Kolbe electrolysis of sodium salts of carboxylic acids also produces alkanes (and CO₂) at the anode.
Multiple Choice Questions
Q1. The carbonyl carbon is:
(a) sp hybridised
(b) sp² hybridised
(c) sp³ hybridised
(d) sp³d hybridised
Answer: (b) sp² hybridised.
Q2. Tollens’ reagent is used to test:
(a) ketones
(b) alcohols
(c) aldehydes
(d) ethers
Answer: (c) aldehydes.
Q3. Which compound gives a positive iodoform test?
(a) Methanol
(b) Propan-2-ol
(c) Diethyl ether
(d) Benzaldehyde
Answer: (b) Propan-2-ol — a methyl carbinol (CH₃CHOH–R).
Q4. Rosenmund reduction converts:
(a) RCOCl → RCH₂OH
(b) RCOCl → RCHO
(c) RCOCl → RCH₃
(d) RCOCl → RCOOH
Answer: (b) RCOCl → RCHO.
Q5. The aldol product is a:
(a) β-hydroxy ester
(b) β-hydroxy carbonyl compound
(c) α-hydroxy acid
(d) γ-hydroxy ketone
Answer: (b) β-hydroxy carbonyl compound.
Q6. Which of the following is the strongest acid?
(a) HCOOH
(b) CH₃COOH
(c) ClCH₂COOH
(d) Cl₃CCOOH
Answer: (d) Cl₃CCOOH.
Q7. Etard reaction converts toluene to:
(a) Benzoic acid
(b) Benzyl alcohol
(c) Benzaldehyde
(d) Acetophenone
Answer: (c) Benzaldehyde.
Q8. The reagent in the Wolff-Kishner reduction is:
(a) Zn-Hg/HCl
(b) NH₂NH₂/KOH/Δ
(c) LiAlH₄
(d) H₂/Ni
Answer: (b) NH₂NH₂/KOH/Δ.
Q9. Which acid does not have α-hydrogen?
(a) Acetic acid
(b) Propionic acid
(c) Formic acid
(d) Butyric acid
Answer: (c) Formic acid (HCOOH — the H is attached to the carbonyl carbon, not an α-carbon).
Q10. Cannizzaro reaction is given by:
(a) Acetaldehyde
(b) Propanal
(c) Benzaldehyde
(d) Butanal
Answer: (c) Benzaldehyde — has no α-hydrogen.
Fill in the Blanks
Q1. The general formula of an aldehyde is __________.
Answer: R–CHO (or CₙH₂ₙO for saturated aliphatic aldehydes).
Q2. The reagent used in HVZ reaction is __________.
Answer: Cl₂ or Br₂ in the presence of red phosphorus.
Q3. Acetone reacts with HCN to give __________.
Answer: Acetone cyanohydrin, (CH₃)₂C(OH)CN — 2-hydroxy-2-methylpropanenitrile.
Q4. Carboxylic acids exist as __________ in vapour phase due to hydrogen bonding.
Answer: Cyclic dimers.
Q5. Decarboxylation of sodium acetate with soda lime gives __________.
Answer: Methane (CH₄).
True or False
Q1. Aldehydes are more reactive than ketones towards nucleophilic addition.
Answer: True. Aldehydes have less steric hindrance and only one electron-donating alkyl group on the carbonyl carbon.
Q2. Ketones give a silver mirror with Tollens’ reagent.
Answer: False. Only aldehydes (and some α-hydroxy ketones) give the silver mirror.
Q3. Carboxylic acids are stronger than phenols.
Answer: True. The carboxylate ion is more resonance-stabilised over two equivalent oxygens than the phenoxide ion.
Q4. The Cannizzaro reaction is a disproportionation reaction.
Answer: True. One aldehyde molecule is oxidised while another is reduced.
Q5. LiAlH₄ reduces a carboxylic acid to an aldehyde.
Answer: False. LiAlH₄ reduces –COOH all the way to a primary alcohol (–CH₂OH).
Glossary
| Term | Meaning |
|---|---|
| Carbonyl group | The C=O functional group present in aldehydes, ketones, acids, esters and amides. |
| Aldehyde | Carbonyl compound with at least one H on the carbonyl carbon (R–CHO). |
| Ketone | Carbonyl compound with two carbon groups bonded to the carbonyl carbon (R–CO–R′). |
| Cyanohydrin | Addition product of HCN with a carbonyl compound: R₂C(OH)CN. |
| Acetal/Ketal | R₂C(OR′)₂ — addition product of two alcohols with an aldehyde or ketone; protecting group. |
| Schiff base / Imine | R₂C=N–R′, formed from a carbonyl compound and primary amine with loss of water. |
| Oxime | R₂C=N–OH, formed from carbonyl compound + hydroxylamine. |
| Hydrazone | R₂C=N–NH₂ or R₂C=N–NHR′ from carbonyl + hydrazines. |
| Aldol reaction | Base-catalysed self-condensation of α-H carbonyl compounds → β-hydroxy carbonyl, then α,β-unsaturated carbonyl on dehydration. |
| Cannizzaro reaction | Disproportionation of α-H-free aldehyde to alcohol + carboxylate by conc. NaOH. |
| Tollens’ test | Silver mirror test for aldehydes: Ag(NH₃)₂⁺ → Ag. |
| Fehling’s test | Brick-red Cu₂O precipitate from aldehyde + alkaline cupric tartrate. |
| Iodoform test | Yellow CHI₃ precipitate from methyl ketones, CH₃CHO, and ethanol with I₂/NaOH. |
| Etard reaction | CrO₂Cl₂ in CS₂ converts toluene to benzaldehyde via a chromium complex. |
| Gattermann-Koch | Synthesis of benzaldehyde from benzene + CO + HCl with AlCl₃/CuCl. |
| Rosenmund reduction | RCOCl + H₂/Pd-BaSO₄ → RCHO; the catalyst is poisoned to prevent overreduction. |
| HVZ reaction | Hell-Volhard-Zelinsky α-halogenation of carboxylic acids by X₂/red P. |
| Esterification | Reversible acid-catalysed condensation of acid + alcohol → ester + water. |
| Decarboxylation | Loss of CO₂ from a carboxylate, e.g., RCOONa + NaOH/CaO → RH + Na₂CO₃. |
| Carboxylate dimer | Two RCOOH molecules linked by a pair of O–H···O=C hydrogen bonds, forming an 8-ring. |