Aldehydes semicarbazide with

Experiments.—Being a primary hydrazide (of carbamic acid), semicarbazide reduces ammoniacal silver solutions and Fehling s solution. It reacts readily with aldehydes and ketones with the elimination of water and formation of semicarbazones, which, since they are more easily hydrolysed than are phenylhydrazones and oximes, are to be preferred to the latter for purposes of separation and purification of carbonyl compounds. Shake an aqueous solution of the hydrochloride (prepared as described above) with a few drops of benzaldehyde, isolate the semicarbazone and purify it by recrystallisation from alcohol. Melting point 214° decomp. Benzaldehyde semicarbazone is decomposed into its constituents by gentle warming with concentrated hydrochloric acid. 

Reaction CXXXl. Formation of Semioxamazones.—Semioxamazide, NH2.CO.CO.NH.NH2, possesses similar properties to semicarbazide, and reacts well with aldehydes, but with ketones the reaction does not seem to be generally applicable. 

Imines of aldehydes are relatively stable while those of ketones are unstable. Derivatives of imines that form stable compounds with aldehydes and ketones include phenylhydrazine, 2,4-dinitrophenyl-hydrazine, hydroxylamine, and semicarbazide. 

Semicarbazone. Semicarbazide, as its hydrochloride salt, reacts with aldehydes and ketones to form semicarbazones. 

In the reaction of guanidine derivatives 11 with base, the aminocarbodiimides 12 are generated, which undergo dimerization to form triaminotriazoles (R = H), or are intercepted with aldehydes or ketones to give the corresponding semicarbazides 13. ... 

A number of reactions of nitrogen-containing nucleophiles with aldehydes and ketones involve addition of the nitrogen to the carbon of the carbonyl group, followed by elimination of water to produce a double bond. Common examples are reactions of primary amines to produce substituted imines, reactions of secondary amines to produce enamines, reactions of hydrazine or substituted hydrazines to produce hydrazones, reactions of semicarbazides to give semicarbazones, and reactions of hydroxylamine to produce oximes. Usually these reactions are run with an acid catalyst. 

In addition, this synthetic scheme offers a mild reaction route for the degradation of a-hydroxy acids to the corresponding aldehydes containing one less carbon atom or for their conversion to aldehydes with the same number of carbon atoms. For example, reaction of 1,4-diphenylthio-semicarbazide with a-acetoxyacyl chlorides yields 5-(a-hydroxyalkyl)-1,2,4-triazolium salts, which, upon sodium hydride reduction and acid treatment, give benzaldehyde along with 138 (R = H) (Scheme 58) (75TL1889). Moreover, the 5-(a-hydroxyalkyl)-l,2,4-triazolium salts can be easily converted to the 5-alkyl compounds (Scheme 58) and borohydride reduction of the latter followed by acid hydrolysis provides a method for the transformation of a-hydroxy acids to aldehydes with the... 

Hydrazine reacts with aldehydes and ketones to give hydra-zones. Hydrazone derivatives react with aldehydes and ketone to give AT-substituted hydrazones. Hydroxylamine reacts with aldehydes and ketones to give oximes. Semicarbazone reacts with aldehydes and ketones to give semicarbazides. 

D Reactions of Semicarbazide with Cyclohexanone and 2-Fur aldehyde in Bicarbonate Buffer Solution... 

Semicarbazide reacts with aldehydes and ketones to produce a derivative that is called a semicarbazone (Eq. 25.23), a reaction that is discussed in Chapter 13. Because semicarbazide is unstable as the free base, it is usually stored in the form of its hydrochloric acid salt. In the procedure that follows, the free base is liberated from the salt by addition of sodium acetate. 

The heights of waves of semicarbazones are dependent on the equilibrium constant of the condensation reaction. This enables the analysis of some mixtures to be made. Generally speaking, the equilibrium is shifted in favour of semicarbazones more with aldehydes than ketones, and this permits the determination of aldehydes in the presence of smaller excesses of ketones. Thus butyraldehyde can be determined in the presence of a fourfold excess of acetone and a twofold excess of acetophenone, by recording the curves in OT M HCl with 0-02 m semicarbazide. 

Certain primary amine derivatives condense with aldehydes and ketones to form products that are less hydrolytically unstable than ordinary imines. Among these are hydroxylamine hydrazine, H2NNH2 (see also Exercise 3-2), and its derivatives, such as 2,4-dinitrophenyIhy-drazine and semicarbazide (all common names). They form so-called oximes, hydrazones. 

In Summary Primary amines attack aldehydes and ketones to form imines by condensation. Hydroxylamine gives oximes, hydrazines lead to hydrazones, and semicarbazide results in semicarbazones. Secondary amines react with aldehydes and ketones to give enamines. 

Aldehydes and ketones may frequently be identified by their semicarbazones, obtained by direct condensation with semicarbazide (or amino-urea), NH,NHCONH a compound which is a monacidic base and usually available as its monohydrochloride, NHjCONHNH, HCl. Semicarbazones are particularly useful for identification of con jounds (such as acetophenone) of which the oxime is too soluble to be readily isolated and the phenylhydrazone is unstable moreover, the high nitrogen content of semicarbazones enables very small quantities to be accurately analysed and so identified. The general conditions for the formation of semicarbazones are very similar to those for oximes and phenylhydrazones (pp. 93, 229) the free base must of course be liberated from its salts by the addition of sodium acetate. 

Oximes, hydrazines and semicarbazones. The hydrolysis products of these compounds, t.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. 

Acetaldehyde can be isolated and identified by the characteristic melting points of the crystalline compounds formed with hydrazines, semicarbazides, etc these derivatives of aldehydes can be separated by paper and column chromatography (104,113). Acetaldehyde has been separated quantitatively from other carbonyl compounds on an ion-exchange resin in the bisulfite form the aldehyde is then eluted from the column with a solution of sodium chloride (114). In larger quantities, acetaldehyde may be isolated by passing the vapor into ether, then saturating with dry ammonia acetaldehyde—ammonia crystallizes from the solution. Reactions with bisulfite, hydrazines, oximes, semicarb azides, and 5,5-dimethyl-1,3-cyclohexanedione [126-81 -8] (dimedone) have also been used to isolate acetaldehyde from various solutions. 

Pyridazine aldehydes and ketones with the carbonyl group at the ring or in a side chain react in the usual manner. They form hydrazones, semicarbazides, oximes, etc. Side-chain aldehydes can be easily oxidized to pyridazinecarboxylic acids with silver nitrate and side-chain ketones are oxidized to carboxylic acids by treatment with potassium permanganate or hydrogen peroxide. 

Catalytic reduction of the nitrile 79 in the presence of semicarbazide affords initially the semicarbazone of 80. Hydrolysis-interchange, for example in the presence of pyruvic acid, gives the aldehyde 80. Condensation with the half ester of malonic acid leads to the acrylic ester 81 the double bond is then removed by means of catalytic reduction (82). Base catalyzed reaction of the... 

C ( propyl) N phenylmtrone to N phenylmaleimide, 46, 96 semicarbazide hydrochloride to ami noacetone hydiochlonde, 46,1 tetraphenylcyclopentadienone to diphenyl acetylene, 46, 44 Alcohols, synthesis of equatorial, 47, 19 Aldehydes, aromatic, synthesis of, 47, 1 /3-chloro a,0 unsaturated, from ke tones and dimethylformamide-phosphorus oxy chloride, 46, 20 from alky 1 halides, 47, 97 from oxidation of alcohols with dimethyl sulfoxide, dicyclohexyl carbodumide, and pyndimum tnfluoroacetate, 47, 27 Alkylation, of 2 carbomethoxycyclo pentanone with benzyl chloride 45,7... 

The basic amino group of the 1-position in semicarbazide or thiosemi-carbazide may be used to react by a substitution reaction with activated halides [52], ethers [51], hydroxy [53], phenoxy [54], and amino groups [55] to yield substituted 1-semicarbazides or thiosemicarbazides. In addition, the amino group of the 1-position may add to electron-deficient double bonds [56]. Formaldehyde and other aldehydes may add to all the available free NH groups to give methylol, alkylol, or polymeric products under basic conditions [57]. Aldehydes or ketenes usually give semicarbazone derivatives, and these in turn are used analytically to identify the purity or structure of a known aldehyde [3]. 

Semicarbazones are prepared by dissolving semicarbazide hydrochloride (ca Ig) and sodium acetate (ca 1.5g) in water (8-lOml) and adding the aldehyde or ketone (0.5-lg) and stirring. The semicarbazone crystallises out and is recrystallised from ethanol or aqueous ethanol. These are hydrolysed by steam distillation in the presence of oxalic acid or better by exchange with pyruvic acid (Hershberg dOC 13 542 1948). 

Isolation of aldehydes and ketonesAldehydes and ketones are readily separated from other organic compounds by heating a solution of the mixture in hexane or toluene with semicarbazide supported on silica gel (1 4 or 1 9) at about 70° for 12-18 hours. After filtration the carbonyl compound is regenerated from the solid phase by treatment with oxalic acid in a two-phase system, water and toluene or heptane. Girard T reagent (1, 410-411) supported on silica gel can also be used, but offers no advantages over supported semicarbazide. 

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