Aldehydes, Ketones and Carboxylic Acids
These classes share the carbonyl group (>C=O). In aldehydes the carbonyl C is bonded to H (and C); in ketones to two C; in carboxylic acids the carbonyl C also bears an -OH (the -COOH carboxyl group).
Structure and nomenclature
- Carbonyl C is sp2 hybridised, trigonal planar, bond angles about 120 degrees; C=O is polarised (C is electrophilic / Lewis acid, O is nucleophilic / Lewis base).
- IUPAC: alkane -e becomes -al (aldehyde) and -one (ketone); -CHO on a ring uses the suffix carbaldehyde; carboxylic acids use -oic acid. Benzaldehyde = benzenecarbaldehyde.
Preparation
- Aldehydes/ketones: oxidation or dehydrogenation of 1deg/2deg alcohols; ozonolysis of alkenes; hydration of alkynes (ethyne gives ethanal, others give ketones).
- Aldehydes: Rosenmund (RCOCl + H2/Pd-BaSO4), Stephen (RCN + SnCl2/HCl), DIBAL-H on nitriles/esters; aromatic aldehydes by Etard (CrO2Cl2), CrO3/acetic anhydride, side-chain chlorination + hydrolysis, Gatterman-Koch (CO + HCl/AlCl3).
- Ketones: acyl chloride + dialkylcadmium; nitrile + Grignard; Friedel-Crafts acylation.
Physical properties
Higher b.p. than hydrocarbons/ethers (dipole-dipole) but lower than alcohols (no intermolecular H-bonding among themselves). Lower members miscible with water (H-bond to water). Carboxylic acids boil even higher and exist as dimers (extensive H-bonding).Chemical reactions of carbonyl compounds
- Nucleophilic addition (Nu attacks sp2 carbon, sp2 to sp3, tetrahedral alkoxide). Aldehydes more reactive than ketones (steric + electronic). Examples: HCN -> cyanohydrin; NaHSO3 adduct (aldehydes favoured); alcohols -> hemiacetal/acetal; NH2-Z -> imine/oxime/hydrazone/2,4-DNP/semicarbazone.
- Reduction: NaBH4/LiAlH4 give alcohols; Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (NH2NH2/KOH) give -CH2-.
- Oxidation / tests: Tollens' (silver mirror), Fehling's (red-brown Cu2O; aromatic aldehydes negative), iodoform/haloform (methyl ketones, CH3CH(OH)-).
- alpha-H reactions: aldol and cross-aldol condensation give alpha,beta-unsaturated carbonyls. Aldehydes with no alpha-H undergo Cannizzaro (disproportionation in conc. alkali).
Carboxylic acids
- Prepared by oxidation of 1deg alcohols/aldehydes/alkenes, side-chain oxidation of alkylbenzenes, hydrolysis of nitriles/amides, Grignard + CO2 (adds one C).
- Acidity: stronger than alcohols and phenols (carboxylate stabilised by two equivalent resonance forms). EWG increases, EDG decreases acidity. Order of group effect: Ph < I < Br < Cl < F < CN < NO2 < CF3.
- Reactions: anhydride formation, esterification (nucleophilic acyl substitution), PCl5/PCl3/SOCl2 -> acyl chloride, NH3 -> amide; reduction by LiAlH4/diborane (NOT NaBH4); decarboxylation (sodalime), HVZ alpha-halogenation; ring substitution is meta-directing/deactivating (no Friedel-Crafts).