Chapter: Aldehydes, Ketones and Carboxylic Acids – Detailed Notes for NEET/JEE Mains

1. Introduction and Nomenclature

A. Aldehydes and Ketones (Carbonyl Compounds)

• Carbonyl Group: The functional group in aldehydes and ketones is the carbonyl group (>C=O).
  • It is a polar group due to the difference in electronegativity between carbon and oxygen, with carbon having a partial positive charge (δ+) and oxygen a partial negative charge (δ−).
  • The carbon atom is sp2 hybridized, and the carbonyl group is planar.
• Aldehydes: Carbonyl group is bonded to at least one hydrogen atom.
  • General formula: R-CHO or Ar-CHO (where R is alkyl or aryl, H for formaldehyde).
• Ketones: Carbonyl group is bonded to two alkyl or aryl groups.
  • General formula: R-CO-R′ or Ar-CO-Ar′ or R-CO-Ar

Nomenclature:

• Aldehydes:
  • Common names: Derived from the corresponding carboxylic acid by replacing -ic acid with -aldehyde (e.g., Formaldehyde (HCHO), Acetaldehyde (CH3​CHO), Benzaldehyde (C6​H5​CHO)).
  • IUPAC names: “Alkanal.” The -e of alkane is replaced by -al. The carbonyl carbon is always numbered as 1. (e.g., Methanal, Ethanal, Benzaldehyde).
• Ketones:
  • Common names: Alkyl/aryl groups attached to the carbonyl group are named alphabetically, followed by “ketone” (e.g., Dimethyl ketone (Acetone, CH3​COCH3​), Ethyl methyl ketone).
  • IUPAC names: “Alkanone.” The -e of alkane is replaced by -one. The position of the carbonyl carbon is indicated by a number. (e.g., Propanone, Butan-2-one).

B. Carboxylic Acids

• Carboxyl Group: The functional group in carboxylic acids (-COOH), which is a combination of a carbonyl group (>C=O) and a hydroxyl group (-OH).
  • They are acidic due to the resonance stabilization of the carboxylate anion formed after losing a proton.
• General Formula: R-COOH or Ar-COOH

Nomenclature:

• Common names: Derived from the traditional names of the corresponding alkyl/aryl compounds or natural sources (e.g., Formic acid (HCOOH), Acetic acid (CH3​COOH), Benzoic acid (C6​H5​COOH)).
• IUPAC names: “Alkanoic acid.” The -e of alkane is replaced by -oic acid. The carboxyl carbon is always numbered as 1. (e.g., Methanoic acid, Ethanoic acid, Benzoic acid).

2. Methods of Preparation

A. Preparation of Aldehydes and Ketones

1. From Alcohols (Oxidation):
   • Primary alcohols oxidize to aldehydes:
     • RCH2​OHPCC (Pyridinium Chlorochromate)​RCHO (mild oxidant, prevents further oxidation to carboxylic acid)
     • RCH2​OHCrO3​ in anhydrous medium​RCHO
   • Secondary alcohols oxidize to ketones:
     • RCH(OH)R′PCC or K2​Cr2​O7​/H+​RCOR′
   • Dehydrogenation using Cu at 573 K:
     • 1∘ alcohol Cu, 573K​Aldehyde
     • 2∘ alcohol Cu, 573K​Ketone
2. From Hydrocarbons:
   • Ozonolysis of Alkenes:
     • R2​C=CR2​(i)O3​(ii)Zn/H2​O​R2​C=O+O=CR2​ (If one of the carbonyl carbons has an H, it forms aldehyde; if both R groups, then ketone).
   • Hydration of Alkynes: (For ketones, or acetaldehyde)
     • CH≡CHHgSO4​/H2​SO4​​CH3​CHO (Acetaldehyde)
     • CH3​C≡CHHgSO4​/H2​SO4​​CH3​COCH3​ (Acetone, Markovnikov’s addition of water to form enol, which tautomerizes to ketone).
   • Friedel-Crafts Acylation (for aromatic ketones):
     • Benzene + Acyl chloride/Acid anhydride Anhy. AlCl3​​Aryl Ketone
       • C6​H6​+CH3​COClAnhy. AlCl3​​C6​H5​COCH3​ (Acetophenone)+HCl
3. From Nitriles and Esters (for aldehydes and ketones):
   • Reduction of nitriles:
     • Stephen reaction: R-CN(i)SnCl2​/HCl(ii)H2​O​RCHO (for aldehydes)
     • Using DIBAL-H (Diisobutylaluminium hydride): R-CN(i)DIBAL-H(ii)H2​O​RCHO
     • Nitriles with Grignard reagents followed by hydrolysis (for ketones): R’CN+RMgXH2​O​RCOR′
   • Reduction of esters:
     • Using DIBAL-H: RCOOR′(i)DIBAL-H(ii)H2​O​RCHO (at low temperature)
4. Special preparations for Aldehydes:
   • Rosenmund Reduction: From acyl chlorides.
     • RCOCl+H2​Pd/BaSO4​,S or Quinoline​RCHO+HCl (BaSO$_4$ acts as a poison, preventing further reduction to alcohol).
   • Etard Reaction: From toluene to benzaldehyde.
     • Toluene (i)CrO2​Cl2​/CS2​(ii)H3​O+​Benzaldehyde (Chromyl chloride forms a chromium complex, which is then hydrolyzed).
   • Gattermann-Koch Reaction: From benzene to benzaldehyde.
     • Benzene CO+HCl/Anhy. AlCl3​/CuCl​Benzaldehyde

B. Preparation of Carboxylic Acids

1. From Primary Alcohols and Aldehydes (Oxidation):
   • RCH2​OHKMnO4​ or K2​Cr2​O7​/H+​RCOOH
   • RCHOKMnO4​ or K2​Cr2​O7​/H+​RCOOH (or mild oxidizing agents like Tollens’ or Fehling’s reagent for aldehydes).
2. From Alkylbenzenes (Oxidation):
   • Alkylbenzenes with at least one benzylic hydrogen are oxidized by strong oxidizing agents (KMnO4​) to benzoic acid.
     • Toluene KMnO4​/KOH, heat​Potassium benzoateH+​Benzoic acid
     • Even long alkyl chains are completely oxidized to -COOH.
3. From Nitriles and Amides (Hydrolysis):
   • Nitriles: R-CNH2​O/H+ or OH−​RCOOH
   • Amides: RCONH2​H2​O/H+ or OH−​RCOOH
4. From Grignard Reagents:
   • Reaction with carbon dioxide (dry ice) followed by hydrolysis.
     • RMgX+CO2​→RCOOMgXH3​O+​RCOOH
5. From Acyl Halides and Anhydrides (Hydrolysis):
   • RCOClH2​O​RCOOH
   • (RCO)2​OH2​O​2RCOOH

3. Physical Properties

A. Aldehydes and Ketones

• Boiling Points: Higher than hydrocarbons of comparable molecular mass due to dipole-dipole interactions between polar carbonyl groups. However, lower than alcohols of comparable molecular mass due to the absence of intermolecular hydrogen bonding.
• Solubility: Lower aldehydes and ketones (up to 4-5 carbon atoms) are soluble in water due to their ability to form hydrogen bonds with water molecules. Solubility decreases with increasing alkyl chain length.

B. Carboxylic Acids

• Boiling Points: Significantly higher than aldehydes, ketones, and even alcohols of comparable molecular mass. This is due to extensive intermolecular hydrogen bonding, where two carboxylic acid molecules form a dimer.
• Solubility: Lower carboxylic acids (up to 4-5 carbons) are miscible with water due to hydrogen bonding. Solubility decreases rapidly with increasing chain length.

4. Chemical Reactions

A. Reactions of Aldehydes and Ketones

Aldehydes and ketones undergo nucleophilic addition reactions due to the polar nature of the carbonyl group (δ+ C and δ− O). Aldehydes are generally more reactive than ketones towards nucleophilic addition due to:

1. Steric effects: Ketones have two bulky alkyl/aryl groups, whereas aldehydes have one alkyl/aryl group and one hydrogen atom (or two hydrogens in formaldehyde), leading to less steric hindrance around the carbonyl carbon in aldehydes.
2. Electronic effects: The two alkyl groups in ketones (electron-donating +I effect) reduce the partial positive charge on the carbonyl carbon more effectively than in aldehydes, making the carbonyl carbon less electrophilic.
3. Nucleophilic Addition Reactions:
   • Addition of HCN (Hydrogen Cyanide): Forms cyanohydrins.
     • >C=O+HCN→>C(OH)CN
   • Addition of NaHSO3​ (Sodium Hydrogensulphite): Forms crystalline bisulphite addition compounds (useful for separation and purification).
     • >C=O+NaHSO3​→>C(OH)SO3​Na
   • Addition of Grignard Reagents (RMgX): (already covered in alcohol preparation)
   • Addition of Alcohols:
     • Aldehydes with 1∘ alcohol (in presence of dry HCl gas) form acetals via hemiacetals.
     • Ketones with 1∘ alcohol (in presence of dry HCl gas) form ketals via hemiketals.
     • Cyclic acetals/ketals are formed with ethylene glycol (used as protecting groups for carbonyl).
       • >C=O+HOCH2​CH2​OHHCl gas​cyclic ketal/acetal+H2​O
4. Addition-Elimination Reactions (Condensation reactions with Ammonia Derivatives):
   • Products contain >C=N-Z linkage (where Z is from the ammonia derivative H2​N-Z).
   • Examples:
     • Reaction with Hydroxylamine (NH2​OH): Forms Oximes (>C=N-OH).
     • Reaction with Hydrazine (NH2​NH2​): Forms Hydrazones (>C=N-NH2​).
     • Reaction with Phenylhydrazine (C6​H5​NHNH2​): Forms Phenylhydrazones (>C=N-NHC6​H5​).
     • Reaction with 2,4-Dinitrophenylhydrazine (2,4-DNP): Forms 2,4-Dinitrophenylhydrazones (yellow/orange/red precipitates, used as a test for carbonyl compounds).
     • Reaction with Semicarbazide (NH2​CONHNH2​): Forms Semicarbazones.
5. Reduction Reactions:
   • Reduction to Alcohols: (already covered in alcohol preparation)
     • Aldehydes to 1∘ alcohols; Ketones to 2∘ alcohols (using LiAlH4​, NaBH4​, or catalytic hydrogenation).
   • Reduction to Hydrocarbons:
     • Clemmensen Reduction: >C=OZn-Hg/Conc. HCl​>CH2​ (Converts carbonyl group to methylene group).
     • Wolff-Kishner Reduction: >C=O(i)NH2​NH2​,KOH/Ethylene Glycol, heat​>CH2​
6. Oxidation Reactions:
   • Aldehydes: Easily oxidized to carboxylic acids (even by mild oxidizing agents) due to the presence of an oxidizable H atom on the carbonyl carbon.
     • Tollens’ Test: RCHO+2[Ag(NH3​)2​]++3OH−→RCOO−+2Ag↓(Silver mirror)+2H2​O+4NH3​ (Test for aldehydes).
     • Fehling’s Test: RCHO+2Cu2+ (from Fehling’s solution)+5OH−→RCOO−+Cu2​O↓(Red ppt)+3H2​O (Test for aliphatic aldehydes).
     • Benedict’s Test: Similar to Fehling’s.
   • Ketones: Are not easily oxidized. Require strong oxidizing agents and drastic conditions (high temperature, strong acid) to oxidize, leading to C-C bond cleavage and a mixture of carboxylic acids with fewer carbon atoms. (Popoff’s Rule: C-C bond breaking occurs such that the carbonyl group stays with the smaller alkyl group, if unsymmetrical).
7. Reactions due to α-hydrogen: Aldehydes and ketones having at least one α-hydrogen atom exhibit various reactions.
   • Aldol Condensation: Carbonyl compounds with α-hydrogens react in the presence of dilute alkali to form β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols), which upon heating lose water to form α,β-unsaturated carbonyl compounds.
     • CH3​CHO+CH3​CHODil. NaOH​CH3​CH(OH)CH2​CHOheat​CH3​CH=CHCHO
     • Cross Aldol Condensation: Between two different carbonyl compounds (at least one with α-H). Can lead to a mixture of up to four products.
     • If one carbonyl compound lacks α-H (e.g., HCHO, C$_6H_5$CHO), it acts as the acceptor.
   • Cannizzaro Reaction: Aldehydes which do not have an α-hydrogen atom (e.g., formaldehyde, benzaldehyde) undergo disproportionation (self-oxidation and reduction) in the presence of concentrated alkali to form an alcohol and a carboxylic acid salt.
     • 2HCHOConc. NaOH​CH3​OH+HCOONa
     • Cross Cannizzaro Reaction: Between two different aldehydes, both lacking α-H.
8. Iodoform Reaction: (Test for CH3​CO group or CH3​CH(OH) group)
   • Compounds containing a methyl ketone (CH3​CO-) group or a methyl secondary alcohol (CH3​CH(OH)-) group (which can be oxidized to methyl ketone) react with I2​ and NaOH (or NaOI) to give a yellow precipitate of iodoform (CHI3​).
     • CH3​COCH3​I2​/NaOH​CHI3​↓ (yellow ppt)+CH3​COONa+NaI+H2​O

B. Reactions of Carboxylic Acids

1. Acidity of Carboxylic Acids:
   • Stronger acids than alcohols and phenols.
   • React with metals, metal carbonates, bicarbonates, and hydroxides (unlike phenols/alcohols which generally don’t react with bicarbonates).
   • Effect of substituents on acidity:
     • Electron-withdrawing groups (EWG): Increase acidity by stabilizing the carboxylate anion (e.g., −NO2​, halogens, −CN). The effect decreases with distance.
       • Example: FCH2​COOH>ClCH2​COOH>BrCH2​COOH>ICH2​COOH
       • Example: Cl3​CCOOH>Cl2​CHCOOH>ClCH2​COOH>CH3​COOH
     • Electron-donating groups (EDG): Decrease acidity by destabilizing the carboxylate anion (e.g., alkyl groups like −CH3​).
       • Example: HCOOH>CH3​COOH>CH3​CH2​COOH
2. Reactions involving cleavage of O-H bond: (Acidic nature)
   • Forms salts with active metals, alkalies, carbonates, bicarbonates.
3. Reactions involving cleavage of C-OH bond:
   • Formation of Acid Anhydrides:
     • 2RCOOHP2​O5​/heat​(RCO)2​O+H2​O
   • Esterification: Reaction with alcohols in the presence of acid catalyst.
     • RCOOH+R’OHH+​RCOOR′+H2​O
   • Formation of Acyl Chlorides:
     • RCOOH+PCl5​→RCOCl+POCl3​+HCl
     • RCOOH+SOCl2​→RCOCl+SO2​+HCl (Best method)
   • Formation of Amides:
     • RCOOH+NH3​heat​RCONH2​+H2​O
4. Reduction Reactions:
   • Reduction to Alcohols: (already covered in alcohol preparation)
     • RCOOHLiAlH4​​RCH2​OH (Strong reducing agent needed).
5. Reactions involving the -COOH group and the alkyl chain:
   • Hell-Volhard-Zelinsky (HVZ) Reaction: Halogenation at the α-carbon.
     • Carboxylic acids with an α-hydrogen react with chlorine or bromine in the presence of red phosphorus to form α-halo carboxylic acids.
     • R-CH2​-COOH(i)X2​/Red P(ii)H2​O​R-CH(X)-COOH
6. Ring Substitution (for aromatic carboxylic acids):
   • The −COOH group is a deactivating and meta-directing group.
   • Nitration: Benzoic acid Conc. HNO3​/Conc. H2​SO4​​m-nitrobenzoic acid (major)

5. Important Commercial Compounds

A. Formaldehyde (Methanal, HCHO)

• Preparation: Catalytic oxidation of methanol.
• Uses: Manufacture of bakelite (phenol-formaldehyde resin), urea-formaldehyde glues, formalin (40% aqueous solution, disinfectant, preservative for biological specimens).

B. Acetaldehyde (Ethanal, CH3​CHO)

• Preparation: Oxidation of ethanol, Wacker process (oxidation of ethene).
• Uses: Manufacture of acetic acid, acetic anhydride, ethyl acetate, polymers.

C. Acetone (Propanone, CH3​COCH3​)

• Preparation: From cumene (along with phenol), fermentation of molasses.
• Uses: Solvent for paints, varnishes, nail polish remover, manufacture of polymers, chloroform.

D. Acetic Acid (Ethanoic Acid, CH3​COOH)

• Preparation: Oxidation of ethanol, carbonylation of methanol (Monsanto process), fermentation of molasses (vinegar).
• Uses: Solvent, in food (vinegar), manufacture of plastics, dyes, pharmaceuticals, acetic anhydride, esters.

E. Benzoic Acid (C6​H5​COOH)

• Preparation: Oxidation of toluene.
• Uses: Food preservative (sodium benzoate), in pharmaceuticals (e.g., anti-fungal), synthesis of other organic compounds.