Oxime

In organic chemistry, an oxime is an organic compound belonging to the imines, with the general formula RR’C=N−OH, where R is an organic side-chain and R' may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds. Amidoximes are oximes of amides (R¹C(=O)NR²R³) with general structure R¹C(=NOH)NR²R³.

Oximes are usually generated by the reaction of hydroxylamine with aldehydes (R−CH=O) or ketones (RR’C=O). The term oxime dates back to the 19th century, a combination of the words oxygen and imine.

If the two side-chains on the central carbon are different from each other—either an aldoxime, or a ketoxime with two different "R" groups—the oxime can often have two different geometric stereoisomeric forms according to the E/Z configuration. An older terminology of syn and anti was used to identify especially aldoximes according to whether the R group was closer or further from the hydroxyl. Both forms are often stable enough to be separated from each other by standard techniques.

Oximes have three characteristic bands in the infrared spectrum, whose wavelengths corresponding to the stretching vibrations of its three types of bonds: 3600 cm⁻¹ (O−H), 1665 cm⁻¹ (C=N) and 945 cm⁻¹ (N−O).

In aqueous solution, aliphatic oximes are 10²- to 10³-fold more resistant to hydrolysis than analogous hydrazones.

Oximes can be synthesized by condensation of an aldehyde or a ketone with hydroxylamine. The condensation of aldehydes with hydroxylamine gives aldoximes, and ketoximes are produced from ketones and hydroxylamine. In general, oximes exist as colorless crystals or as thick liquids and are poorly soluble in water. Therefore, oxime formation can be used for the identification of ketone or aldehyde functional groups.

Certain metal salts reduce nitro compounds to oximes.

Oximes can also be obtained from rearrangement of unstable nitroso compounds. Thus alkyl nitrites react with carbon acids to give oximes: methyl ethyl ketone with ethyl nitrite, propiophenone with methyl nitrite, and phenacyl chloride with butyl nitrite, all in ethereal hydrochloric acid. Alternatively, sodium nitrite in glacial acetic acid nitrosates ethyl acetoacetate and malononitrile.