Oximes, Hydrazones, and Semicarbazones

Oximes, Hydrazones, and Semicarbazones.—Hydrazones and semicarbazones of 3-oxo-A - and -A -steroids may be smoothly converted into the parent ketones by treatment with benzene seleninic anhydride.  

Beckmann fragmentation of the hydroxy-oxime (119) gave the 16,17-seco-compound (120) which can be converted, by NaBH4 reduction and acid hydrolysis, into the lactone (121). The lactone (121) may also be obtained directly upon acidification of the reaction mixture from heating the hydroxy-oxime (119) with 

KOH-ethylene glycol. This latter reaction apparently involves an unusual base-catalysed Beckmann fragmentation and reduction of the resultant aldehyde by KOH-ethylene glycol. 

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Oximes, hydrazones and semicarbazones are readily prepared from the aldehydes (37, 106). 

Functional modifiers are used in binary nomenclature to name anhydrides, e.g. acetic anhydride for (CH3-C0)20, and derivatives of ketones and aldehydes such as oximes, hydrazones and semicarbazones, e.g. acetone oxime for (CH3)2C=NOH. 

Chromanones form oximes, hydrazones and semicarbazones normally but flavanone (639) can give one of three products with hydrazine according to the conditions (Scheme 35)... 

Oximes, hydrazones and semicarbazones. The hydrolysis products of these compounds, i.e. aldehydes and ketones, may be sensitive to alkali (this is particularly so for aldehydes) it is best, therefore, to conduct the hydrolysis with strong mineral acid. After hydrolysis the aldehyde or ketone may be isolated by distillation with steam, extraction with ether or, if a solid, by filtration, and then identified. The acid solution may be examined for hydroxylamine or hydrazine or semicarbazide substituted hydrazines of the aromatic series are precipitated as oils or solids upon the addition of alkali. 

Imines in which the nitrogen atom carries an electronegative group are usually stable examples include oximes, hydrazones, and semicarbazones. 

Oximes, hydrazones, and semicarbazones require acid or base catalysis to be hydrolysed. 

Oximes, hydrazones and semicarbazones have been hydrolyzed in very good yields by heating in an aqueous suspension of Dowex-50 or an aqueous acetone suspension of Amberlyst-15. 2... 

These compounds are more stable than imines because the electronegative substituent can participate in delocalization of the imine double bond. Delocalization decreases the small positive charge on the carbon atom of the imine double bond and raises the energy of the LUMO, making it less susceptible to nucleophilic attack. Oximes, hydrazones, and semicarbazones require acid or base catalysis to be hydrolysed. 

The addition of nucleophilic reagents to the carbonyl group is of very common occurrence in organic chemistry. In many instances it is followed by loss of water, for example in the formation of oximes, hydrazones, and semicarbazones, frequently leading to rather complex kinetics. Similarly, if the carbon atom of the carbonyl group bears a group which can be readily lost, the formation of a tetrahedral intermediate often represents an intermediate stage in a replacement reaction of the type... 

Another property of oximes, hydrazones, and semicarbazones is that they are highly crystalline and therefore exhibit sharp melting points. These characteristics aid in isolation, purification, and characterization of aldehydes and ketones. 

The other classes of compounds containing sp nitrogen atoms that we will meet are shown in Figure 4.5, imines, oximes, hydrazones, and semicarbazones. In terms of the types of orbitals used in the bonding, all the classes are the same as far as the carbon-nitrogen double bond is concerned, so we ll look at the oxime as an example. Some parts of this are straightforward and already known to us. The carbon of the double bond is sp hybridized and forms three a-bonds to... 

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