Chapter 8: Aldehydes, Ketones and Carboxylic Acids | Summary

The carbonyl carbon is sp²-hybridised, forming three sigma bonds in a trigonal planar arrangement with bond angles of approximately 120°. The fourth valence electron of carbon forms a π-bond with oxygen through p-orbital overlap. The oxygen atom retains two lone pairs of electrons.

Due to the higher electronegativity of oxygen, the C=O bond is polarised, making the carbonyl carbon electrophilic (Lewis acid) and the oxygen nucleophilic (Lewis base). The polarity is explained through resonance between a neutral structure (A) and a dipolar structure (B), resulting in substantial dipole moments and greater polarity than ethers.

Aldehydes are generally prepared by: (1) oxidation of primary alcohols, (2) dehydrogenation of primary alcohols over Ag or Cu catalysts, (3) ozonolysis of alkenes, (4) hydration of alkynes (ethyne gives acetaldehyde), (5) Rosenmund reduction of acyl chlorides using H₂/Pd-BaSO₄, (6) Stephen reaction — reduction of nitriles with SnCl₂/HCl, (7) DIBAL-H reduction of nitriles or esters. Aromatic aldehydes are additionally prepared by Etard reaction (CrO₂Cl₂), oxidation with CrO₃/acetic anhydride, side-chain chlorination followed by hydrolysis, and Gatterman-Koch reaction (CO/HCl/AlCl₃).

Ketones are prepared by: (1) oxidation of secondary alcohols, (2) hydration of alkynes (all alkynes except ethyne), (3) reaction of acyl chlorides with dialkylcadmium (R₂Cd), (4) treating nitriles with Grignard reagents followed by hydrolysis, and (5) Friedel-Crafts acylation — treatment of benzene or substituted benzene with acyl chloride in the presence of anhydrous AlCl₃.

Methanal is a gas at room temperature; ethanal is a volatile liquid; higher aldehydes and ketones are liquid or solid. Their boiling points are higher than hydrocarbons and ethers of comparable molecular mass due to dipole-dipole interactions, but lower than alcohols of similar mass because they lack intermolecular hydrogen bonding.

Lower members (methanal, ethanal, propanone) are miscible with water due to hydrogen bond formation with water molecules. Solubility decreases with increasing chain length. Lower aldehydes have sharp, pungent odours; higher members have more fragrant, pleasant smells, making them valuable in perfumery and flavouring.

The primary reaction type of aldehydes and ketones is nucleophilic addition. A nucleophile attacks the electrophilic carbonyl carbon perpendicularly, changing hybridisation from sp² to sp³ and forming a tetrahedral alkoxide intermediate, which then captures a proton. Aldehydes are more reactive than ketones due to both steric (one vs. two bulky groups) and electronic (reduced electrophilicity from two electron-donating alkyl groups in ketones) reasons. Key nucleophilic addition reactions include: addition of HCN (forms cyanohydrins), NaHSO₃ (forms bisulphite adducts, useful for purification), alcohols (forms hemiacetals and acetals), and ammonia derivatives (forms imines, oximes, hydrazones, semicarbazones, 2,4-DNP derivatives).

Reduction reactions: NaBH₄ or LiAlH₄ or catalytic hydrogenation reduces C=O to alcohol. Clemmensen reduction (Zn-Hg/HCl) and Wolff-Kishner reduction (N₂H₄/KOH/ethylene glycol) reduce C=O to CH₂. Oxidation: Aldehydes are readily oxidised to carboxylic acids; Tollens' test (ammoniacal AgNO₃, silver mirror) and Fehling's test (reddish-brown Cu₂O precipitate) distinguish aldehydes from ketones. Ketones require vigorous oxidation with C–C bond cleavage. Haloform reaction: methyl ketones are oxidised by NaOX to give haloforms and sodium carboxylate. Reactions due to α-hydrogen: aldol condensation in dilute alkali gives β-hydroxy aldehydes/ketones, which dehydrate to α,β-unsaturated carbonyl compounds. Cross aldol condensation involves two different carbonyl compounds. Cannizzaro reaction: aldehydes without α-hydrogen undergo disproportionation in concentrated alkali. Electrophilic substitution: aromatic carbonyl acts as deactivating, meta-directing group.

Carboxylic acids contain the –COOH (carboxyl) group, which consists of a carbonyl group attached to a hydroxyl group. Common names are derived from Latin/Greek names of natural sources (e.g., formic acid from ants, acetic acid from vinegar). IUPAC names replace '-e' of the parent alkane with '-oic acid', with the carboxyl carbon numbered one. Dicarboxylic acids use prefixes like 'dicarboxylic acid' with position numbers.

The carboxyl carbon is sp²-hybridised with bond angles of ~120°. The carboxyl carbon is less electrophilic than a simple carbonyl carbon due to resonance donation from the hydroxyl oxygen. Higher carboxylic acids (C₁₂–C₁₈), called fatty acids, occur in natural fats as glycerol esters. They serve as starting materials for anhydrides, esters, amides, and acyl halides.

Methods include: (1) Oxidation of primary alcohols and aldehydes using KMnO₄, K₂Cr₂O₇, or Jones reagent. (2) Oxidation of alkylbenzenes — the entire side chain is oxidised to –COOH irrespective of chain length, using KMnO₄/KOH or CrO₃ (tertiary groups are not oxidised). (3) Hydrolysis of nitriles — with H⁺ or OH⁻ catalyst via amide intermediates. (4) From Grignard reagents — reaction with dry CO₂ followed by acidification, useful for ascending the carboxylic acid series by one carbon. (5) Hydrolysis of acyl halides and anhydrides with water or aqueous base. (6) Hydrolysis of esters — acidic hydrolysis gives the acid directly; basic hydrolysis gives carboxylate salt, which is then acidified.

Aliphatic carboxylic acids up to nine carbons are colourless liquids with unpleasant odours; higher members are wax-like, odourless solids. Their boiling points are higher than aldehydes, ketones, and even alcohols of comparable molecular mass due to extensive intermolecular hydrogen bonding. Carboxylic acids exist as hydrogen-bonded dimers even in the vapour phase or in aprotic solvents.

Lower carboxylic acids (up to four carbons) are miscible with water; solubility decreases with chain length due to increased hydrophobic interaction of the alkyl chain. Benzoic acid is nearly insoluble in cold water. Carboxylic acids are also soluble in less polar organic solvents such as benzene, ether, and chloroform.

Reactions involving O–H bond cleavage (acidity): Carboxylic acids react with electropositive metals (evolving H₂), alkalies (forming salts), and uniquely with carbonates and bicarbonates (evolving CO₂), which confirms the presence of –COOH. They ionise in water to give resonance-stabilised carboxylate anions. Acid strength is measured by pKa; smaller pKa = stronger acid. Carboxylic acids are stronger than alcohols (pKa ~16) and phenols (pKa ~10) but weaker than mineral acids. Electron-withdrawing groups (EWG) increase acidity by stabilising the carboxylate anion; electron-donating groups (EDG) decrease acidity.

Reactions involving C–OH bond cleavage: (1) Anhydride formation — heating with H₂SO₄ or P₂O₅. (2) Esterification — with alcohols in the presence of H⁺ catalyst (reversible, nucleophilic acyl substitution mechanism). (3) Acid chloride formation — with PCl₅, PCl₃, or SOCl₂ (SOCl₂ preferred for ease of purification). (4) Amide formation — reaction with NH₃ gives ammonium salt, which on heating gives amide. Reactions involving the –COOH group: (1) Reduction to primary alcohol using LiAlH₄ or diborane. (2) Decarboxylation — sodium salts heated with sodalime give hydrocarbons; Kolbe electrolysis also causes decarboxylation. Reactions in hydrocarbon part: (1) Hell-Volhard-Zelinsky reaction — α-halogenation with Cl₂ or Br₂ in presence of red phosphorus. (2) Ring substitution in aromatic acids — carboxyl group is meta-directing and deactivating; Friedel-Crafts reaction does not occur.

Formaldehyde (40% solution = formalin) is used to preserve biological specimens and to prepare bakelite and urea-formaldehyde glues. Acetaldehyde is a starting material for acetic acid, ethyl acetate, vinyl acetate, polymers, and drugs. Benzaldehyde is used in perfumery and dye industries. Acetone and ethyl methyl ketone are common industrial solvents. Methanoic acid is used in rubber, textile, dyeing, leather, and electroplating. Ethanoic acid is used as solvent and vinegar.

Hexanedioic acid (adipic acid) is used in manufacturing nylon-6,6. Esters of benzoic acid are used in perfumery. Sodium benzoate is used as a food preservative. Higher fatty acids are raw materials for soaps and detergents. Many naturally occurring aldehydes and ketones such as vanillin, cinnamaldehyde, camphor, and salicylaldehyde are used in food flavouring and perfumery.

This chapter establishes aldehydes, ketones, and carboxylic acids as three of the most significant functional-group families in organic chemistry. Their shared carbonyl group imparts high polarity, distinctive physical properties, and a rich reaction chemistry — most notably nucleophilic addition for aldehydes/ketones and acid-base as well as substitution reactions for carboxylic acids. Key named reactions such as Rosenmund reduction, Aldol condensation, Cannizzaro reaction, Tollens'/Fehling's tests, Clemmensen/Wolff-Kishner reduction, and Hell-Volhard-Zelinsky reaction form the conceptual backbone of the chapter.

For board examinations, students must be thorough with IUPAC nomenclature, all preparation methods (especially named reactions), mechanisms of nucleophilic addition and esterification, factors affecting acidity of carboxylic acids, and tests to distinguish aldehydes from ketones. The applied dimension — industrial uses of formaldehyde, acetaldehyde, adipic acid, and benzoic acid — also carries examination weight. Mastery of this chapter directly supports organic synthesis problems, identification exercises, and multi-step conversion questions that regularly appear in CBSE Class 12 papers.