Class 12 Chemistry Notes Chapter 12 (Aldehydes, Ketones and Carboxylic Acids) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 12: Aldehydes, Ketones, and Carboxylic Acids. This is a very important chapter for your exams, rich in reactions and concepts frequently tested. We'll break it down systematically, keeping the NCERT Exemplar perspective in mind.

Chapter 12: Aldehydes, Ketones and Carboxylic Acids - Detailed Notes

1. Introduction:

• These are organic compounds containing the carbonyl group (>C=O) as the functional group.
• 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').
• Carboxylic acids contain the carboxyl group (-COOH), which combines a carbonyl (>C=O) and a hydroxyl (-OH) group on the same carbon.
• The carbonyl carbon is sp2 hybridized, trigonal planar, with bond angles approx 120°.
• The C=O bond is polar due to oxygen's higher electronegativity, making the carbon atom electrophilic and the oxygen atom nucleophilic.

2. Nomenclature:

• Aldehydes: IUPAC: Replace '-e' of parent alkane with '-al'. (e.g., Methanal, Ethanal, Benzaldehyde). Common names often used (Formaldehyde, Acetaldehyde).
• Ketones: IUPAC: Replace '-e' of parent alkane with '-one'. Numbering gives the carbonyl carbon the lowest possible number. (e.g., Propanone, Butan-2-one, Acetophenone). Common names based on alkyl groups attached (Dimethyl ketone = Acetone, Ethyl methyl ketone).
• Carboxylic Acids: IUPAC: Replace '-e' of parent alkane with '-oic acid'. (e.g., Methanoic acid, Ethanoic acid, Benzoic acid). Common names widely used (Formic acid, Acetic acid).

3. Preparation Methods:

• Preparation of Aldehydes and Ketones:

  • Oxidation of Alcohols:
    • Primary alcohols → Aldehydes (using mild oxidizing agents like PCC - Pyridinium Chlorochromate). Strong agents yield carboxylic acids.
    • Secondary alcohols → Ketones (using CrO3, PCC, KMnO4, K2Cr2O7).
  • Dehydrogenation of Alcohols: Vapours of primary/secondary alcohols passed over heated Cu at 573 K give aldehydes/ketones respectively. Tertiary alcohols undergo dehydration.
  • Ozonolysis of Alkenes: Reductive ozonolysis (O3 followed by Zn/H2O) cleaves the C=C bond to give aldehydes and/or ketones depending on the alkene structure.
  • Hydration of Alkynes:
    • Ethyne + H2O (in presence of H2SO4/HgSO4) → Ethanal.
    • Other alkynes + H2O (H2SO4/HgSO4 - Kucherov's reaction) → Ketones (Markovnikov addition).

• Preparation of Aldehydes ONLY:

  • Rosenmund Reduction: Acyl chloride (RCOCl) reduced to aldehyde (RCHO) using H2/Pd-BaSO4. (BaSO4 poisons the catalyst to prevent further reduction to alcohol). Formaldehyde cannot be prepared.
  • Stephen Reaction: Nitriles (RCN) reduced to imine hydrochloride using SnCl2/HCl, followed by hydrolysis to yield aldehydes (RCHO).
  • DIBAL-H: Diisobutylaluminium hydride selectively reduces nitriles and esters to aldehydes at low temperatures.
  • From Hydrocarbons (Aromatic Aldehydes):
    • Etard Reaction: Toluene oxidized to Benzaldehyde using Chromyl chloride (CrO2Cl2) in CS2/CCl4.
    • Gattermann-Koch Reaction: Benzene or its derivatives treated with CO and HCl in presence of anhydrous AlCl3/CuCl gives Benzaldehyde or substituted benzaldehydes.

• Preparation of Ketones ONLY:

  • Friedel-Crafts Acylation: Benzene or substituted benzene reacts with acyl chloride (RCOCl) or acid anhydride ((RCO)2O) in presence of anhydrous AlCl3 (Lewis acid) to form aromatic ketones.
  • From Nitriles: Treating nitrile with Grignard reagent (RMgX) followed by hydrolysis yields a ketone.
  • From Acyl Chlorides: Reacting acyl chlorides with dialkylcadmium (R2Cd - prepared from Grignard reagent and CdCl2) gives ketones.

• Preparation of Carboxylic Acids:

  • From Primary Alcohols and Aldehydes: Oxidation using strong oxidizing agents (KMnO4, K2Cr2O7, Jones reagent - CrO3 in H2SO4).
  • From Alkylbenzenes: Side chain oxidation of alkylbenzenes (with at least one benzylic hydrogen) using alkaline KMnO4 followed by acidification yields Benzoic acid, irrespective of the alkyl chain length.
  • From Nitriles and Amides: Hydrolysis (acidic or basic) of nitriles (RCN) or amides (RCONH2) gives carboxylic acids.
  • From Grignard Reagents: Reaction of Grignard reagent (RMgX) with carbon dioxide (dry ice) followed by acid hydrolysis. (Useful for stepping up the carbon chain).
  • From Acyl Halides and Anhydrides: Hydrolysis yields corresponding carboxylic acids.
  • From Esters: Acidic or basic hydrolysis (saponification) of esters yields carboxylic acids (or their salts in basic medium).

4. Physical Properties:

• Boiling Points: Aldehydes/Ketones have higher B.P. than non-polar hydrocarbons of comparable mass due to dipole-dipole interactions, but lower B.P. than corresponding alcohols due to absence of intermolecular H-bonding. Carboxylic acids have significantly higher B.P. due to extensive intermolecular H-bonding (exist as dimers).
• Solubility: Lower aldehydes/ketones (up to C4) are soluble in water due to H-bonding with water molecules. Solubility decreases with increasing chain length. Carboxylic acids (up to C4) are miscible with water due to H-bonding. Aromatic acids are sparingly soluble.

5. Chemical Reactions:

• Reactions of Aldehydes and Ketones:

  • Nucleophilic Addition Reactions (Characteristic Reaction):
    • Mechanism: Nucleophile attacks the electrophilic carbonyl carbon. Reactivity: Aldehydes > Ketones (due to less steric hindrance and greater positive charge density on carbonyl carbon in aldehydes).
    • Addition of HCN → Cyanohydrins.
    • Addition of NaHSO3 → Bisulphite addition product (crystalline, useful for separation/purification of aldehydes). Ketones react only if methyl ketones.
    • Addition of Grignard Reagents (RMgX) → Alcohols (Formaldehyde → 1° alcohol, other aldehydes → 2° alcohol, Ketones → 3° alcohol).
    • Addition of Alcohols → Hemiacetals/Hemiketals (unstable) → Acetals/Ketals (stable, formed in presence of dry HCl gas). Used as protecting groups for carbonyls.
    • Addition of Ammonia and its Derivatives (Z-NH2 where Z= R, Ar, OH, NH2, NHC6H5, NHCONH2, etc.) → C=N-Z + H2O. (e.g., Schiff's bases, oximes, hydrazones, phenylhydrazones, 2,4-dinitrophenylhydrazones (Brady's reagent - orange/yellow ppt, identification test), semicarbazones). Reaction is acid-catalyzed.
  • Reduction:
    • Reduction to Alcohols: Using NaBH4, LiAlH4, or catalytic hydrogenation (H2/Ni, Pt, Pd).
    • Reduction to Hydrocarbons:
      • Clemmensen Reduction: Carbonyl group reduced to -CH2- using Zn-Hg and conc. HCl. (Acidic medium).
      • Wolff-Kishner Reduction: Carbonyl group reduced to -CH2- using hydrazine (NH2NH2) followed by heating with strong base (KOH/NaOH) in high boiling solvent like ethylene glycol. (Basic medium).
  • Oxidation:
    • Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents. Ketones are generally resistant (require strong conditions, cleave C-C bond).
    • Tollens' Test (Silver Mirror Test): Aldehydes react with Tollens' reagent (ammoniacal silver nitrate) to give a silver mirror. Used to distinguish aldehydes from ketones. (RCHO + 2[Ag(NH3)2]+ + 3OH- → RCOO- + 2Ag↓ + 4NH3 + 2H2O).
    • Fehling's Test: Aliphatic aldehydes react with Fehling's solution (A: aq. CuSO4, B: alkaline sodium potassium tartrate) on heating to give a red-brown precipitate of Cu2O. Aromatic aldehydes do not give this test. (RCHO + 2Cu2+ + 5OH- → RCOO- + Cu2O↓ + 3H2O).
    • Haloform Reaction (Iodoform Test): Given by compounds having CH3CO- group (methyl ketones) or CH3CH(OH)- group (which can be oxidized to methyl ketone). React with NaOH/I2 (or NaOI) to give a yellow precipitate of iodoform (CHI3). Acetaldehyde is the only aldehyde that gives this test.
  • Reactions due to α-Hydrogen (Acidity): α-hydrogens are acidic due to resonance stabilization of the conjugate base (enolate ion).
    • Aldol Condensation: Aldehydes/ketones with at least one α-hydrogen react in presence of dilute alkali (e.g., NaOH, Ba(OH)2) to form β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols). On heating, these lose water to form α,β-unsaturated aldehydes/ketones.
    • Cross Aldol Condensation: Aldol condensation between two different aldehydes and/or ketones (at least one must have α-H). Can give a mixture of four products if both reactants have α-hydrogens.
    • Cannizzaro Reaction: Aldehydes without α-hydrogen (e.g., Formaldehyde, Benzaldehyde) undergo self-oxidation-reduction (disproportionation) on treatment with concentrated alkali (e.g., 50% NaOH/KOH) to yield one molecule of alcohol and one molecule of salt of carboxylic acid.
    • Cross Cannizzaro Reaction: Between two different aldehydes lacking α-hydrogen. Generally, the more reactive aldehyde (e.g., formaldehyde) gets oxidized.
  • Electrophilic Substitution Reaction (in Aromatic Aldehydes/Ketones): The -CHO and >C=O groups are deactivating and meta-directing towards electrophilic substitution (e.g., nitration, halogenation).

• Reactions of Carboxylic Acids:

  • Reactions involving Cleavage of O–H Bond (Acidity):
    • React with active metals (Na, K), alkalis (NaOH, KOH), carbonates (Na2CO3), and bicarbonates (NaHCO3) to produce H2 gas or salt + H2O or salt + H2O + CO2 respectively. Reaction with NaHCO3 producing CO2 effervescence is a test for carboxylic acids.
    • Acidity: Stronger acids than alcohols and phenols (due to resonance stabilization of carboxylate anion). Electron-withdrawing groups (EWG like -Cl, -NO2) increase acidity; Electron-donating groups (EDG like -CH3, -OCH3) decrease acidity. Acidity order: Halogen acids > Carboxylic acids > Phenols > Water > Alcohols.
  • Reactions involving Cleavage of C–OH Bond:
    • Formation of Anhydrides: Dehydration on heating with dehydrating agent (P2O5) or reaction of acid salt with acyl chloride.
    • Esterification (Fischer Esterification): Reaction with alcohols in presence of conc. H2SO4 (acid catalyst) or HCl gas to form esters (reversible reaction).
    • Reaction with PCl5, PCl3, SOCl2: Forms acyl chlorides (RCOCl). SOCl2 is preferred as byproducts (SO2, HCl) are gases.
    • Reaction with Ammonia (NH3): Forms ammonium salt, which on heating gives amides (RCONH2).
  • Reactions involving –COOH Group:
    • Reduction: Reduced to primary alcohols using strong reducing agents like LiAlH4 or B2H6 (diborane). Note: NaBH4 does not reduce -COOH group.
    • Decarboxylation: Sodium salts of carboxylic acids heated with soda-lime (NaOH + CaO) lose CO2 to form hydrocarbons with one carbon less.
  • Substitution Reactions in the Hydrocarbon Part:
    • Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids having an α-hydrogen react with Cl2 or Br2 in the presence of small amount of red phosphorus to give α-halo carboxylic acids.

Key Distinguishing Tests:

• Aldehydes vs Ketones: Tollens' test, Fehling's test (Aldehydes positive, Ketones negative).
• Methyl Ketones vs Other Ketones: Iodoform test (Methyl ketones positive).
• Carboxylic Acids vs Phenols: NaHCO3 test (Carboxylic acids give CO2 effervescence, Phenols generally do not).
• Aliphatic vs Aromatic Aldehydes: Fehling's test (Aliphatic positive, Aromatic negative).

Multiple Choice Questions (MCQs)

1. Which of the following reagents is suitable for converting propan-2-ol to propanone?
   (a) PCC (Pyridinium Chlorochromate)
   (b) Alkaline KMnO4
   (c) CrO3 in anhydrous medium
   (d) Both (a) and (c)

2. Rosenmund reduction of benzoyl chloride gives:
   (a) Benzyl alcohol
   (b) Benzoic acid
   (c) Benzaldehyde
   (d) Benzene

3. Which of the following compounds will give a positive Tollens' test?
   (a) Acetone
   (b) Acetophenone
   (c) Benzaldehyde
   (d) Diethyl ketone

4. Cannizzaro reaction is NOT given by:
   (a) Formaldehyde
   (b) Acetaldehyde
   (c) Benzaldehyde
   (d) 2,2-Dimethylpropanal

5. The correct order of reactivity towards nucleophilic addition reaction is:
   (a) CH3CHO > CH3COCH3 > HCHO
   (b) HCHO > CH3CHO > CH3COCH3
   (c) CH3COCH3 > CH3CHO > HCHO
   (d) HCHO > CH3COCH3 > CH3CHO

6. The reagent used in Gattermann-Koch reaction is:
   (a) CO + HCl + Anhydrous AlCl3/CuCl
   (b) CrO2Cl2 in CS2
   (c) H2 / Pd-BaSO4
   (d) SnCl2 / HCl followed by H3O+

7. Which of the following carboxylic acids is the strongest?
   (a) CH3COOH
   (b) ClCH2COOH
   (c) FCH2COOH
   (d) HCOOH

8. Iodoform test is given by:
   (a) Methanol
   (b) Ethanol
   (c) Propan-1-ol
   (d) Benzaldehyde

9. The product formed when ethanal undergoes Aldol condensation followed by heating is:
   (a) Butan-2-one
   (b) But-2-enal
   (c) Butan-1-ol
   (d) Butanoic acid

10. Hell-Volhard-Zelinsky (HVZ) reaction involves the formation of:
    (a) β-halo carboxylic acid
    (b) α,β-unsaturated carboxylic acid
    (c) α-halo carboxylic acid
    (d) Acid halide

Answer Key for MCQs:

1. (d) - Both PCC and CrO3 are suitable for oxidizing secondary alcohols to ketones. Alkaline KMnO4 is too strong and might cleave the molecule.
2. (c) - Rosenmund reduction converts acyl chlorides to aldehydes.
3. (c) - Benzaldehyde is an aldehyde and gives Tollens' test. Ketones (a, d) and Acetophenone (aromatic ketone) do not.
4. (b) - Acetaldehyde has α-hydrogens, so it undergoes Aldol condensation, not Cannizzaro reaction.
5. (b) - Reactivity decreases with increasing steric hindrance and decreasing positive charge on carbonyl carbon. HCHO (least hindered, most reactive) > Aldehydes > Ketones.
6. (a) - This is the specific reagent combination for Gattermann-Koch reaction.
7. (c) - Acidity increases with the electron-withdrawing effect of the substituent. Fluorine is the most electronegative, hence FCH2COOH is the strongest acid among the options.
8. (b) - Ethanol (CH3CH2OH) can be oxidized to Acetaldehyde (CH3CHO), which has the required CH3CO- structure after oxidation (or can be considered as having CH3CH(OH)- group). Methanol, Propan-1-ol, and Benzaldehyde do not have the required structure.
9. (b) - Ethanal (CH3CHO) undergoes Aldol condensation to form 3-hydroxybutanal, which upon heating dehydrates to form But-2-enal (α,β-unsaturated aldehyde).
10. (c) - HVZ reaction introduces a halogen (Cl or Br) at the α-carbon position of a carboxylic acid having α-hydrogen.

Study these notes thoroughly, focusing on the reaction conditions, reagents, and specific applications like distinguishing tests. Remember to practice conversions and reaction mechanisms where relevant. Good luck!