Semicarbazides semicarbazones

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

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. 

A) Semicarbazones. Prepared according to the directions given for acetophenone semicarbazone (p. 258), but use twice the amount of semicarbazide hydrochloride and sodium acetate. (M.ps., p. 549.)... 

Semicarbazones. Dissolve 1 g. of semicarbazide hydrochloride and 1 5g. of crystallised sodium acetate in 8-10 ml. of water add 0 - 5-1 g. of the aldehyde or ketone and shake. If the mixture is turbid, add alcohol (acetone-free) or water until a clear solution is obtained shake the mixture for a few minutes and allow to stand. Usually the semicarbazone crystallises from the cold solution on standing, the time varying from a few minutes to several hours. The reaction may be accelerated,... 

When semicarbazide Ls heated in the absence of a carbonyl compound for long periods, condensation to blurea, NHjCONHNHCONHj, m.p. 247-250 (decomp.), may result occasionally this substance may be produced in the normal preparation of a semicarbazone that forms slowly. Biurea is sparingly soluble in alcohol and soluble in hot water, whereas semicarbazones with melting points in the same range are insoluble in water this enables it to be readily distinguished from a semicarbazone. 

Dissolve 7 g. of pure oleic acid in 30 ml. of dry ethyl chloride (chloroform may be used but is less satisfactory), and ozonise at about —30°. Remove the solvent under reduced pressure, dissolve the residue in 50 ml. of dry methyl alcohol and hydrogenate as for adipic dialdehyde in the presence of 0 5 g. of palladium - calcium carbonate. Warm the resulting solution for 30 minutes with a slight excess of semicarbazide acetate and pour into water. Collect the precipitated semicarbazones and dry the... 

To isolate the semicarbazide hydrochloride, the filtered reaction mixture Is treated with excess of acetone and the resulting acetone semicarbazone is decomposed with concentrated hydrochloric acid. 

To decompose the acetone semicarbazone, warm 58 g. with 50 ml. of concentrated hydrochloric acid until it just dissolves. Cool in ice the semicarbazide hydrochloride separates as a thick crystaUine mass. Filter at the pump through a sintered glass funnel, and wash with a small quantity of alcohol and then with ether dry in the air. The yield of pure semicarbazide hydrochloride, m.p. 173° (decomp.), is 35 g. A further quantity of product may be obtained either by saturating the mother liquor with hydrogen chloride or by treating it with twice its volume of alcohol and then with ether. 

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. 

Reactions with Amines and Amides. Hydroxybenzaldehydes undergo the normal reactions with aUphatic and aromatic primary amines to form imines and Schiff bases reaction with hydroxylamine gives an oxime, reaction with hydrazines gives hydrazones, and reactions with semicarbazide give semicarbazones. The reaction of 4-hydroxybenzaldehyde with hydroxylamine hydrochloride is a convenient method for the preparation of 4-cyanophenol (52,53). 

A-Homo-5a-cholestan-4-one (3b). A solution of sodium nitrite (2 g) in water (100 ml) is added over 1 hr to a stirred solution of 3-(5 -spiro-2, 2 -dimethyloxazolidinyl)-5a-cholestane (7 4.58 g) in aqueous 10% acetic acid (800 ml), maintained at 0-5° for 3 hr and the mixture is then allowed to stand overnight. The reaction mixture is neutralized with 10% sodium hydroxide solution and the resulting white suspension is extracted with ether. The ether extracts are washed with water, dried and concentrated to give a semisolid residue which is converted to the semicarbazone by warming in methanol solution (ca. 65 ml/g) with an excess of methanolic semicarbazide-acetate solution. The precipitate of semicarbazone is recrystallized from ethanol to give a white powder mp 239-241°. A solution of hydrochloric acid (50 ml) in ethanol (450 ml) is added to the semicarbazone and the mixture is heated at reflux for 1 hr. The clear solution is diluted with water (250 ml) and the... 

The starting semicarbazones were most often prepared directly from the a-keto acids. Godfrin proceeded from a-alkyl acetoacetates, which were converted by oxidation with nitrosylsulfuric acid to a-keto-acid oximes and the latter transformed to semicarbazones or thioseraicarbazones by applying semicarbazide or thiosemicarbazide. For glyoxylic acid semicarbazone a very convenient procedure was employed, making use of the hydrolysis of nonisolated chloral semicarbazone. ... 

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... 

Reaction of 1 with semicarbazide hydrochloride gives the semicarbazone 4, in 74 % yield, which can be oxidized by selenium(IV) oxide to provide dibenzo[2,3 6,7]thiepino[4,5-rf][l,2,3]selenadi-azole (5) in 80 % yield. Thermolysis of selenadiazole 5 leads, with subsequent release of nitrogen, to diradical 6, which can either dimerize to 7 or lose selenium to give the intermediate cycloalkyne. The latter can be trapped by dienes as cycloadducts.93 Thus, the thermolysis of 5 in the presence of 2,3,4,5-tetraphenylcyclopenta-2,4-dienone gives the cycloadduct 1,2,3,4-tetraphenyltribenzo-[/ ,<7,/]thiepin (8) in 14% yield. 

Other hydrazine derivatives frequently used to prepare the corresponding hydrazone are semicarbazide (NH2NHCONH2), in which case the hydrazone is called a semicarbazone, and Girard s reagents Tand P, in which case the hydrazone... 

Prepare a solution of 1-0 g. of semicarbazide hydrochloride (NHjCONHNHj.HCl) and 1 - 5 g. of crystallised sodium acetate in 10 ml. of water in a test-tube. Add 1 ml. of acetone, close the tube with a cork and shake vigorously. Allow the mixtm e to stand, with occasional vigorous shaking, for 10 minutes it is advantageous to cool in ice. Filter the crystals, wash with a little cold water, and recrystallise from water or dilute alcohol. The m.p. of acetone semicarbazone is 187°. 

A novel synthesis of 1,2,3-selenadiazoles 338 starts with the Michael addition of 2-nitropropane to a,P-unsaturated ketones 336 under basic conditions <06JHC149>. The resulting adducts are treated with semicarbazide hydrochloride to give semicarbazones 337, which are converted to 1,2,3-selenadiazoles 338 by reaction with selenium dioxide in THF (the choice of the solvent appears to be important in this case). 

Another fresh-water method which holds some promise for seawater analysis is twin cell potential sweep voltammetry, as proposed Afghan et al. [138]. In this method, semicarbazones are formed by reaction with semicarbazide... 

To decompose the semicarbazone it is gently warmed with concentrated hydrochloric acid (8 c.c. for each 10 g. of material) until dissolution is just complete. On cooling the solution semicarbazide hydrochloride sets to a thick crystalline mass which is filtered dry at the pump and washed, first with a little cold hydrochloric acid (1 1), then twice with 3 to 5 c.c. portions of ice-cold alcohol. The salt is dried in a desiccator. Yield 22-25 g. 

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. 

It is also possible to protect the carbonyl moiety of a phenacyl bromide via the reaction with semicarbazide hydrochloride then the semicarbazone can be electrolyt-ically dimerized, ultimately to afford a 2,5-diarylfuran [86]. Alternatively, the elec-trolytically formed dimer can be converted... 

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