Class 12 Chemistry Notes Chapter 3 (Aldehydes, Ketones and Carboxylic Acids) – Chemistry-II Book

Detailed Notes with MCQs of one of the most important chapters for your exams: Chapter 3 - Aldehydes, Ketones, and Carboxylic Acids. These compounds are characterised by the presence of the carbonyl group (>C=O) in aldehydes and ketones, and the carboxyl group (-COOH) in carboxylic acids. Pay close attention, as many named reactions and conceptual questions arise from this chapter.

Chapter 3: Aldehydes, Ketones and Carboxylic Acids - Detailed Notes

1. Introduction

• Carbonyl Group (>C=O): The functional group present in both aldehydes and ketones.
  • In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom (R-CHO or HCHO).
  • In ketones, the carbonyl carbon is bonded to two alkyl or aryl groups (R-CO-R').
• Carboxyl Group (-COOH): The functional group present in carboxylic acids. It consists of a carbonyl group attached to a hydroxyl group (-OH).
• These compounds are widespread in nature and industry (e.g., vanillin, acetone, acetic acid).

2. Nomenclature

• Aldehydes (Alkanals):
  • IUPAC: Replace the terminal '-e' of the corresponding alkane with '-al'. The carbonyl carbon is always C1.
  • Examples: Methanal (HCHO), Ethanal (CH₃CHO), Propanal (CH₃CH₂CHO), Benzaldehyde (C₆H₅CHO).
  • Common Names: Often derived from the common names of carboxylic acids. (e.g., Formaldehyde, Acetaldehyde, Propionaldehyde).
• Ketones (Alkanones):
  • IUPAC: Replace the terminal '-e' of the corresponding alkane with '-one'. Number the chain to give the carbonyl carbon the lowest possible number.
  • Examples: Propanone (CH₃COCH₃), Butan-2-one (CH₃COCH₂CH₃), Pentan-3-one (CH₃CH₂COCH₂CH₃), Acetophenone (C₆H₅COCH₃), Benzophenone (C₆H₅COC₆H₅).
  • Common Names: Name the two alkyl/aryl groups attached to the carbonyl group followed by 'ketone'. (e.g., Dimethyl ketone (Acetone), Ethyl methyl ketone, Diphenyl ketone).
• Carboxylic Acids (Alkanoic Acids):
  • IUPAC: Replace the terminal '-e' of the corresponding alkane with '-oic acid'. The carboxyl carbon is always C1.
  • Examples: Methanoic acid (HCOOH), Ethanoic acid (CH₃COOH), Propanoic acid (CH₃CH₂COOH), Benzoic acid (C₆H₅COOH).
  • Common Names: Often derived from their natural sources. (e.g., Formic acid, Acetic acid, Propionic acid).

3. Structure of the Carbonyl Group

• The carbonyl carbon is sp² hybridized, forming three sigma (σ) bonds. The structure is trigonal planar with bond angles approx 120°.
• The remaining p-orbital on carbon overlaps with a p-orbital on oxygen to form a pi (π) bond.
• Oxygen is more electronegative than carbon, so the C=O bond is polar. Carbon has a partial positive charge (δ+) and oxygen has a partial negative charge (δ-). This polarity governs the reactivity.

4. Preparation of Aldehydes and Ketones

• (a) By Oxidation of Alcohols:
  • Primary alcohols → Aldehydes (using mild oxidizing agents like PCC - Pyridinium Chlorochromate). Strong agents (KMnO₄, K₂Cr₂O₇) oxidize them further to carboxylic acids.
  • Secondary alcohols → Ketones (using CrO₃, PCC, KMnO₄, K₂Cr₂O₇).
• (b) By Dehydrogenation of Alcohols:
  • Vapours of primary or secondary alcohols passed over heated copper (Cu) at 573 K.
  • Primary alcohols → Aldehydes
  • Secondary alcohols → Ketones
• (c) From Hydrocarbons:
  • Ozonolysis of Alkenes: Alkenes react with ozone (O₃) followed by reductive cleavage (Zn/H₂O) to give aldehydes, ketones, or a mixture depending on the alkene structure.
  • Hydration of Alkynes: Addition of water to alkynes in the presence of H₂SO₄ and HgSO₄. Ethyne gives ethanal. Other alkynes give ketones (Markovnikov's rule).
• (d) Preparation of Aldehydes (Specific Methods):
  • Rosenmund Reduction: Acid chlorides (RCOCl) are hydrogenated over catalyst Pd/BaSO₄ (partially deactivated/poisoned). RCOCl + H₂ → RCHO + HCl.
  • Stephen Reaction: Nitriles (RCN) are reduced to imine hydrochloride by SnCl₂/HCl, which on hydrolysis gives the corresponding aldehyde. RCN → [RCH=NH₂⁺Cl⁻] → RCHO.
  • DIBAL-H: Nitriles or Esters can be selectively reduced to aldehydes using Diisobutylaluminium hydride (DIBAL-H) followed by hydrolysis.
  • From Aromatic Hydrocarbons:
    • Etard Reaction: Oxidation of toluene (or derivatives) with chromyl chloride (CrO₂Cl₂) in CS₂ or CCl₄ gives a chromium complex, which on hydrolysis yields benzaldehyde.
    • Gattermann-Koch Reaction: Benzene or its derivatives treated with CO and HCl in the presence of anhydrous AlCl₃ or CuCl gives benzaldehyde.
• (e) Preparation of Ketones (Specific Methods):
  • Friedel-Crafts Acylation: Treating arenes with acid chlorides (RCOCl) or acid anhydrides ((RCO)₂O) in the presence of anhydrous AlCl₃ (Lewis acid catalyst). ArH + RCOCl → ArCOR + HCl.
  • From Nitriles: Treating nitriles with Grignard reagent (RMgX) followed by hydrolysis. R'CN + RMgX → [R'C(R)=NMgX] → R'COR.
  • From Acid Chlorides: Treating acid chlorides with dialkyl cadmium (R₂Cd), prepared from Grignard reagent and CdCl₂. 2R'COCl + R₂Cd → 2R'COR + CdCl₂.

5. Physical Properties

• Boiling Points: Aldehydes and ketones have higher boiling points than non-polar hydrocarbons of comparable molecular mass due to dipole-dipole interactions. However, they have lower boiling points than corresponding alcohols due to the absence of intermolecular hydrogen bonding.
• Solubility: Lower aldehydes and ketones (like methanal, ethanal, propanone) are miscible with water due to H-bonding between the carbonyl oxygen and water molecules. Solubility decreases as the alkyl chain length increases.
• Odour: Lower aldehydes have pungent odours, while higher ones and ketones are generally pleasant smelling.

6. Chemical Reactions of Aldehydes and Ketones

• Characteristic Reaction: Nucleophilic Addition (due to the polarity of >C=O group). The nucleophile attacks the electrophilic carbon (δ+).
  • Mechanism: Nucleophile attacks the carbonyl carbon; π-electrons shift to oxygen; tetrahedral intermediate forms; protonation of oxygen (usually by solvent or acid) gives the addition product. Reactivity: Aldehydes > Ketones (due to steric hindrance and electronic factors in ketones).
  • (i) Addition of HCN: Forms cyanohydrins. Base catalysed.
  • (ii) Addition of NaHSO₃: Forms bisulphite addition products (crystalline solids). Useful for separation/purification of aldehydes. Reaction is reversible.
  • (iii) Addition of Grignard Reagents (RMgX): Forms alcohols after hydrolysis. HCHO → Primary alcohol; Other aldehydes → Secondary alcohol; Ketones → Tertiary alcohol.
  • (iv) Addition of Alcohols: Forms hemiacetals/hemiketals (unstable) and then acetals/ketals (stable) in the presence of dry HCl gas. Acetal/Ketal formation is reversible.
  • (v) Addition of Ammonia and its Derivatives (Z-NH₂ where Z = Alkyl, Aryl, OH, NH₂, NHC₆H₅, NHCONH₂ etc.): Forms compounds containing >C=N-Z bond via elimination of water. Reaction is pH dependent (weakly acidic medium favoured).
    • NH₂OH (Hydroxylamine) → Oximes (>C=NOH)
    • NH₂NH₂ (Hydrazine) → Hydrazones (>C=NNH₂)
    • C₆H₅NHNH₂ (Phenylhydrazine) → Phenylhydrazones (>C=NNHC₆H₅)
    • 2,4-Dinitrophenylhydrazine (2,4-DNP or Brady's reagent) → 2,4-Dinitrophenylhydrazones (Orange/Yellow/Red precipitates - Test for carbonyl group).
    • NH₂CONHNH₂ (Semicarbazide) → Semicarbazones (>C=NNHCONH₂)
• Reduction:
  • (i) Reduction to Alcohols: Using reducing agents like NaBH₄ (Sodium borohydride) or LiAlH₄ (Lithium aluminium hydride). Aldehydes → Primary alcohols; Ketones → Secondary alcohols.
  • (ii) Reduction to Hydrocarbons: Carbonyl group (>C=O) reduced to methylene group (-CH₂-).
    • Clemmensen Reduction: Using Zinc amalgam (Zn-Hg) and concentrated HCl. Used for compounds stable in acid.
    • Wolff-Kishner Reduction: Using hydrazine (NH₂NH₂) followed by heating with a strong base (KOH or NaOH) in a high boiling solvent like ethylene glycol. Used for compounds stable in base.
• Oxidation:
  • Aldehydes are easily oxidized to carboxylic acids with the same number of carbon atoms, even by mild oxidizing agents.
  • Ketones are generally resistant to oxidation. They are oxidized only by strong oxidizing agents (KMnO₄/H⁺, K₂Cr₂O₇/H⁺) under vigorous conditions (high temp), leading to cleavage of C-C bonds and formation of a mixture of carboxylic acids with fewer carbon atoms (Popov's rule: during oxidation of unsymmetrical ketones, the >C=O group stays preferentially with the smaller alkyl group).
  • Distinguishing Tests for Aldehydes:
    • Tollen's Test (Silver Mirror Test): Aldehydes (both aliphatic and aromatic) reduce Tollen's reagent (ammoniacal silver nitrate solution) to metallic silver. RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + 2H₂O. Ketones do not give this test.
    • Fehling's Test: Aldehydes (only aliphatic) reduce Fehling's solution (Solution A: aq. CuSO₄; Solution B: alkaline sodium potassium tartrate - Rochelle salt) to red-brown precipitate of cuprous oxide (Cu₂O). RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O↓ + 3H₂O. Aromatic aldehydes do not give this test.
• Reactions due to α-Hydrogen:
  • Acidity of α-Hydrogens: The α-hydrogens (hydrogens attached to the α-carbon, which is adjacent to the carbonyl carbon) are acidic due to the electron-withdrawing effect of the carbonyl group and resonance stabilization of the resulting carbanion (enolate ion).
  • Aldol Condensation: Aldehydes and ketones having at least one α-hydrogen react in the presence of dilute alkali (like NaOH, Ba(OH)₂) to form β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols). On heating, aldols/ketols readily lose water to form α,β-unsaturated aldehydes or ketones.
  • Cross Aldol Condensation: Aldol condensation between two different aldehydes and/or ketones (at least one must have α-hydrogen). Gives a mixture of four products if both reactants have α-hydrogens. Can be synthetically useful if one reactant has no α-hydrogen.
  • Cannizzaro Reaction: Aldehydes which do not have any α-hydrogen (e.g., HCHO, C₆H₅CHO) undergo self-oxidation and reduction (disproportionation) on treatment with concentrated alkali (like 50% NaOH or KOH). One molecule is oxidized to the salt of carboxylic acid, and another is reduced to the alcohol. 2HCHO + Conc. NaOH → HCOONa + CH₃OH.
• Electrophilic Substitution Reaction (in Aromatic Aldehydes/Ketones):
  • The -CHO and >C=O groups are electron-withdrawing and deactivating towards electrophilic substitution. They direct the incoming electrophile to the meta position in the benzene ring (e.g., nitration of benzaldehyde gives m-nitrobenzaldehyde).

7. Uses of Aldehydes and Ketones

• Formaldehyde (Methanal): Used as formalin (40% aq. solution) for preserving biological specimens, preparation of Bakelite, urea-formaldehyde resins.
• Acetaldehyde (Ethanal): Used in making acetic acid, ethyl acetate, polymers.
• Benzaldehyde: Used in perfumery and dye industries.
• Acetone (Propanone): Common industrial solvent (for paints, varnishes, nail polish remover), synthesis of other chemicals.

8. Carboxylic Acids

• Characterized by the carboxyl functional group (-COOH).

9. Nomenclature and Structure of Carboxyl Group

• Already covered in Section 2.
• Structure: The carbon atom in -COOH is sp² hybridized. The bonds are C=O, C-O, and O-H. The bond lengths in the carboxylate anion (RCOO⁻) are intermediate between C=O and C-O due to resonance.

10. Methods of Preparation of Carboxylic Acids

• (a) From Primary Alcohols and Aldehydes: Oxidation using strong oxidizing agents (KMnO₄/H⁺, K₂Cr₂O₇/H⁺, CrO₃/H₂SO₄ - Jones reagent). RCH₂OH or RCHO → RCOOH.
• (b) From Alkylbenzenes: Oxidation of alkylbenzenes (with at least one benzylic hydrogen) with KMnO₄/KOH (followed by acidification) gives benzoic acid, irrespective of the length of the side chain.
• (c) From Nitriles and Amides: Hydrolysis of nitriles (RCN) or amides (RCONH₂) with acid or alkali yields carboxylic acids. RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺. RCONH₂ + H₂O + H⁺ → RCOOH + NH₄⁺.
• (d) From Grignard Reagents: Grignard reagents react with carbon dioxide (dry ice) followed by acid hydrolysis. RMgX + O=C=O → [RCOOMgX] → RCOOH. Useful for stepping up the carbon chain.
• (e) From Acyl Halides and Anhydrides: Hydrolysis with water. RCOCl + H₂O → RCOOH + HCl. (RCO)₂O + H₂O → 2RCOOH.
• (f) From Esters: Acidic or basic hydrolysis of esters. RCOOR' + H₂O ⇌ RCOOH + R'OH (Acidic hydrolysis - reversible). RCOOR' + NaOH → RCOONa + R'OH (Basic hydrolysis - saponification - irreversible).

11. Physical Properties

• Boiling Points: Carboxylic acids have significantly higher boiling points than aldehydes, ketones, and even alcohols of comparable molecular masses. This is due to extensive intermolecular hydrogen bonding, often existing as dimers in the vapour phase or aprotic solvents.
• Solubility: Simple aliphatic carboxylic acids (up to 4 carbons) are miscible with water due to H-bonding. Solubility decreases with increasing alkyl chain length. Aromatic acids are nearly insoluble in cold water.

12. Chemical Reactions of Carboxylic Acids

• Reactions Involving Cleavage of O-H Bond (Acidity):
  • Carboxylic acids are acidic (Bronsted acids), dissociating in water: RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺.
  • They react with active metals (Na, K, Mg) to liberate H₂.
  • They react with alkalis (NaOH, KOH) to form salts and water.
  • They react with weaker bases like carbonates (Na₂CO₃) and bicarbonates (NaHCO₃) to liberate CO₂ gas (Test for -COOH group).
  • Acidity Comparison: Generally stronger acids than alcohols and phenols. Electron-withdrawing groups (like -Cl, -NO₂) increase acidity, while electron-donating groups (like -CH₃, -OCH₃) decrease acidity. Acidity order: Formic acid > Acetic acid. Halogenated acids: FCH₂COOH > ClCH₂COOH > BrCH₂COOH > ICH₂COOH.
• Reactions Involving Cleavage of C-OH Bond:
  • (i) Formation of Anhydrides: Dehydration by heating with a dehydrating agent like P₂O₅ or conc. H₂SO₄. 2RCOOH → (RCO)₂O + H₂O.
  • (ii) Esterification: Reaction with alcohols in the presence of an acid catalyst (conc. H₂SO₄ or dry HCl gas). Reversible reaction. RCOOH + R'OH ⇌ RCOOR' + H₂O.
  • (iii) Reaction with PCl₅, PCl₃, SOCl₂: Forms acid chlorides (RCOCl). Thionyl chloride (SOCl₂) is preferred as byproducts (SO₂, HCl) are gaseous. RCOOH + SOCl₂ → RCOCl + SO₂ + HCl.
  • (iv) Reaction with Ammonia (NH₃): Forms ammonium salt, which on heating gives amides (RCONH₂). RCOOH + NH₃ ⇌ [RCOO⁻NH₄⁺] → RCONH₂ + H₂O.
• Reactions Involving the -COOH Group:
  • (i) Reduction: Reduced to primary alcohols by strong reducing agents like LiAlH₄ or B₂H₆ (diborane). NaBH₄ does not reduce -COOH group. RCOOH → RCH₂OH.
  • (ii) Decarboxylation: Removal of CO₂. Sodium salts of carboxylic acids heated with soda lime (NaOH + CaO) give hydrocarbons with one carbon less. RCOONa + NaOH(CaO) → RH + Na₂CO₃. Kolbe's electrolysis is another decarboxylation method.
• Substitution Reactions in the Hydrocarbon Part:
  • (i) Halogenation (Hell-Volhard-Zelinsky - HVZ Reaction): Carboxylic acids having an α-hydrogen react with Cl₂ or Br₂ in the presence of red phosphorus to give α-halo carboxylic acids. RCH₂COOH + X₂/Red P → RCH(X)COOH.
  • (ii) Ring Substitution (Aromatic Acids): The -COOH group is deactivating and meta-directing towards electrophilic substitution (e.g., nitration, bromination). Friedel-Crafts reactions do not occur because the catalyst (AlCl₃) gets bonded to the carboxyl group.

13. Uses of Carboxylic Acids

• Methanoic Acid (Formic Acid): Used in rubber, textile, dyeing, leather industries.
• Ethanoic Acid (Acetic Acid): Used as solvent, vinegar (3-5% solution), synthesis of cellulose acetate, vinyl acetate.
• Benzoic Acid: Used as food preservative (sodium benzoate), in medicines.
• Higher fatty acids are used in making soaps and detergents.

Practice MCQs

1. Which of the following reagents is used in the Rosenmund reduction?
   (a) H₂ / Ni
   (b) NaBH₄
   (c) H₂ / Pd-BaSO₄
   (d) Zn-Hg / Conc. HCl

2. The reaction of acetaldehyde with Tollen's reagent produces:
   (a) Silver mirror
   (b) Red precipitate
   (c) Acetic acid only
   (d) No reaction

3. Which compound undergoes Cannizzaro reaction?
   (a) Ethanal
   (b) Propanal
   (c) Benzaldehyde
   (d) Propanone

4. The product formed when propanone reacts with HCN followed by hydrolysis is:
   (a) Lactic acid
   (b) 2-Hydroxy-2-methylpropanoic acid
   (c) Propan-1-ol
   (d) Propan-2-ol

5. Identify the reagent used in the Etard reaction for converting toluene to benzaldehyde.
   (a) Alkaline KMnO₄
   (b) CrO₂Cl₂ in CS₂
   (c) SnCl₂ / HCl
   (d) CO / HCl / Anhyd. AlCl₃

6. Which of the following is the strongest acid?
   (a) CH₃COOH
   (b) HCOOH
   (c) ClCH₂COOH
   (d) FCH₂COOH

7. The reaction of carboxylic acids with alcohols in the presence of conc. H₂SO₄ is known as:
   (a) Saponification
   (b) Esterification
   (c) Decarboxylation
   (d) HVZ reaction

8. Which test is used to distinguish between ethanal and propanone?
   (a) 2,4-DNP test
   (b) Lucas test
   (c) Fehling's test
   (d) Iodoform test

9. Clemmensen reduction of a ketone is carried out using:
   (a) LiAlH₄
   (b) H₂ / Ni
   (c) Zn-Hg / Conc. HCl
   (d) NH₂NH₂ / KOH / Ethylene glycol

10. The Hell-Volhard-Zelinsky (HVZ) reaction involves the reaction of carboxylic acids (having α-hydrogen) with:
    (a) Red P / X₂
    (b) SOCl₂
    (c) LiAlH₄
    (d) Soda lime

Answer Key for MCQs:

1. (c)
2. (a)
3. (c)
4. (b)
5. (b)
6. (d)
7. (b)
8. (c) [Fehling's test is given by ethanal (aliphatic aldehyde) but not by propanone (ketone). Iodoform test is given by both.]
9. (c)
10. (a)

Make sure you thoroughly understand the reaction mechanisms, conditions, and applications, especially the named reactions and distinguishing tests. This chapter forms the basis for many conversion-based questions. Good luck with your preparation!