Carbonyls, carboxylic acids and chirality
Aldehydes and ketones
Aldehydes (RCHO) and ketones (RCOR') contain the polar C=O group; the carbon is δ+ and attacked by nucleophiles. Nucleophilic addition: HCN (from KCN + acid) adds to give hydroxynitriles; NaBH₄ (source of H⁻) reduces aldehydes to primary alcohols and ketones to secondary alcohols. Tests: 2,4-DNPH gives an orange precipitate with any carbonyl. Only aldehydes reduce Tollens' reagent (silver mirror) and Fehling's (blue → brick-red), because aldehydes can be oxidised. The iodoform test (yellow CHI₃) detects CH₃CO–. Because C=O is planar, CN⁻ attacks from either side, so a chiral product forms as a racemic mixture.
≈ 50 minCarboxylic acids and esters
Carboxylic acids (RCOOH) are made by oxidising primary alcohols or aldehydes (reflux with acidified dichromate) or hydrolysing nitriles with acid. They are weak acids: react with metals (H₂), bases (salt + water) and carbonates (CO₂: the test that distinguishes them from phenols and alcohols). LiAlH₄ reduces them to primary alcohols. With alcohols and conc. H₂SO₄ they form esters, named alkyl-first (methyl ethanoate). Ester hydrolysis: acid (reversible, gives acid + alcohol) or alkaline (irreversible, gives carboxylate salt + alcohol: saponification). Esters are used as flavourings, solvents and plasticisers; fats are triesters of glycerol.
≈ 45 minOptical isomerism and chirality
A chiral centre is a carbon with four different groups attached. Molecules with one chiral centre exist as two enantiomers: non-superimposable mirror images. Enantiomers have identical physical properties except that they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture (50 : 50) is optically inactive. Reactions through planar intermediates (carbonyl addition, SN1 carbocations) give racemic products; SN2 gives a single enantiomer with inversion. Drugs are often made as single enantiomers because only one fits the biological target (thalidomide is the classic warning).
≈ 40 minAcyl chlorides and acylation
Acyl chlorides (RCOCl) are made from carboxylic acids with SOCl₂, PCl₅ or PCl₃. The carbonyl carbon is very δ+ (bonded to O and Cl), so they react vigorously at room temperature with nucleophiles by nucleophilic addition–elimination, giving off steamy HCl fumes: water → carboxylic acid; alcohol (or phenol) → ester; ammonia → primary amide; primary amine → N-substituted amide. Acid anhydrides ((RCO)₂O) react the same way but more slowly, releasing RCOOH instead of HCl; ethanoic anhydride is preferred industrially for aspirin (cheaper, safer, less corrosive). Ease of hydrolysis: acyl chloride ≫ alkyl chloride ≫ aryl chloride.
≈ 45 min