Hydroxyimines

Hydroxyimines, commonly referred to in the literature as oximes, are organic compounds that feature the hydroxyimino functional group, R1R2C=NOH. The defining feature is a carbon–nitrogen double bond with a hydroxyl substituent on nitrogen (C=N–OH). Oximes are typically prepared by the condensation of aldehydes or ketones with hydroxylamine (NH2OH), a reaction that adds hydroxylamino groups across the carbonyl carbon and oxygen.

Preparation details: under mild basic or neutral conditions in aqueous or alcoholic solvent, aldehydes or ketones react with hydroxylamine hydrochloride to form oximes; the process is usually reversible in presence of water; E and Z isomers describe the configuration around C=N; for acyclic oximes E is usually more stable; substitution influences isomer distribution. Acetaldoxime is a common example.

Reactions and applications: Oximes can be reduced to primary amines by catalytic hydrogenation or other reducing methods; Beckmann rearrangement converts oximes to amides under acid; because the carbonyl group is masked as an oxime, oximes are used as protecting groups for carbonyl compounds in multi-step syntheses; O-alkyl oximes (oxime ethers) arise from O-alkylation and have distinct reactivity; cyclic oximes arise from cyclic ketones and can serve as intermediates.

Examples and properties: common oximes include acetaldoxime from acetaldehyde, benzaldoxime from benzaldehyde; physical properties vary with substituents; oximes can be isolated as solids or liquids; safety: handle with typical organic chemical precautions.