Aldehydes, Ketones and Carboxylic Acids notes

This chapter studies organic compounds containing the carbonyl group, especially aldehydes, ketones and carboxylic acids. Aldehydes have the group -CHO, ketones have >C=O within the carbon chain, and carboxylic acids have -COOH. Their chemistry is governed by the polarity of the carbonyl bond: carbon becomes electrophilic and oxygen becomes electron rich. NEET questions commonly test nomenclature, preparation methods, nucleophilic addition, oxidation, reduction, aldol condensation, Cannizzaro reaction, acidity of carboxylic acids, named reactions and laboratory tests. Mastery comes from connecting structure with reactivity: aldehydes are generally more reactive than ketones, and carboxylic acids are acidic due to resonance-stabilised carboxylate ions.

Aldehydes and ketones are carbonyl compounds containing a C=O group. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen, so the functional group is -CHO. In ketones, the carbonyl carbon is bonded to two carbon groups, giving R-CO-R'. They are classified as aliphatic or aromatic and named using suffixes -al for aldehydes and -one for ketones. The carbonyl carbon is sp2 hybridised, trigonal planar and electrophilic because oxygen pulls electron density. Their physical properties depend on polarity, molecular mass and absence of intermolecular hydrogen bonding. Aldehydes are generally more reactive than ketones because ketones have two electron-donating alkyl groups and greater steric hindrance.

Preparation methods connect functional group interconversion across alcohols, hydrocarbons, acid derivatives, nitriles and Grignard reagents. Aldehydes are prepared by controlled oxidation of primary alcohols, dehydrogenation, Rosenmund reduction of acid chlorides, Stephen reduction of nitriles and ozonolysis of alkenes. Ketones are prepared by oxidation of secondary alcohols, hydration of alkynes, Friedel-Crafts acylation, reaction of acid chlorides with dialkyl cadmium or organocuprates, and ozonolysis. Carboxylic acids are prepared by oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, hydrolysis of acid derivatives, Grignard carboxylation and side-chain oxidation of alkyl benzenes. NEET often tests reagent selectivity and product prediction.

The properties of aldehydes and ketones arise from the polar carbonyl group. Physically, they show dipole-dipole interactions, moderate boiling points and water solubility for lower members due to hydrogen bonding with water. Chemically, the electrophilic carbonyl carbon undergoes nucleophilic addition with HCN, NaHSO3, alcohols, ammonia derivatives and Grignard reagents. Aldehydes oxidise readily to acids, while ketones need strong conditions. Reduction gives alcohols or hydrocarbons depending on reagent. Aldol condensation occurs when alpha hydrogen is present, forming beta-hydroxy aldehydes/ketones followed by dehydration. Cannizzaro reaction occurs in aldehydes without alpha hydrogen, giving one alcohol and one carboxylate salt by disproportionation.

Carboxylic acids contain the carboxyl group, -COOH, which combines a carbonyl group and a hydroxyl group on the same carbon. They are named using the suffix -oic acid, while aromatic acids such as benzoic acid retain common names. Their high boiling points arise from strong intermolecular hydrogen bonding and dimer formation. Their acidic character is due to resonance stabilisation of the carboxylate ion after loss of H+. Electron-withdrawing groups increase acidity, especially when closer to -COOH, while electron-donating groups decrease acidity. Carboxylic acids are prepared by oxidation, hydrolysis of nitriles and derivatives, and Grignard carboxylation. They form salts, esters, acid chlorides, amides, anhydrides and undergo HVZ and decarboxylation reactions.

This topic consolidates the most testable reactions, qualitative tests and applications of aldehydes, ketones and carboxylic acids. Named reactions such as Rosenmund reduction, Stephen reaction, Etard reaction, Clemmensen reduction, Wolff-Kishner reduction, aldol condensation, Cannizzaro reaction, haloform reaction and HVZ reaction are essential for NEET. Laboratory tests include 2,4-DNP for carbonyl compounds, Tollens' and Fehling's tests for aldehydes, Schiff's test, sodium bisulphite addition, iodoform test for methyl ketones and NaHCO3 test for carboxylic acids. Uses are also important: formalin as disinfectant and preservative, acetone as solvent, benzaldehyde in perfumes, acetic acid in vinegar, benzoic acid as preservative and carboxylic acids in metabolism.