Ureas dehydration

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

Di-n-propyl sulfite, 80 IV.lV -Disubstituted ureas, dehydration to carbodiimides, 212-215 4,4 -Divinyl azobenzene, 315... 

Recently [20] it has been reported that N./V -disubstituted ureas dehydrate easily with phosphorus pentoxide and pyridine to give the corresponding carbodiimide in good yields (Eq. 14). (See Table V.)... 

Aresta M, Dibenedetto A, Stufano P, Aresta BM, Maggi S, Papal I, Rokob TA, Gabriele B (2010) The solid state structure and reactivity of NbC15 (N, N-dicyciohexylurea) in solution evidence for co-ordinated urea dehydration to the relevant carbodiimide. Dalton Trans 39 6985-6992... 

C and pressures up to 400atm., the ammonium carbamate formed initially largely dehydrating to urea. 

Polymerisation may occur as a result of dehydration of these compounds to methylene and dimethylene urea or more probably by a stepwise loss of water between the molecules of methylol and dimethylol-urea. 

On being heated ammonium carbamate undergoes dehydration to form urea... 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

The formation of ammonium carbamate and the dehydration to urea take place simultaneously, for all practical purposes ... 

Conversion at Equilibrium. The maximum urea conversion at equilibrium attainable at 185°C is ca 53% at infinite heating time. The conversion at equiUbtium can be increased either by raising the reactor temperature or by dehydrating ammonium carbamate in the presence of excess ammonia. Excess ammonia shifts the reaction to the right side of the overall equation ... 

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

Various techniques have been proposed for the recovery of pure succinic acid, including extraction (141—145), selective crystalliza tion (146—151), heating to dehydrate the acid and subsequent recovery of succinic anhydride by distillation (152), esterification foUowed by fractionation of the mixture of the esters (65—69), and separation as urea adduct (118,119). 

By now the dehydration condensation of urea [57-13-6] has displaced the dicyandiamide process (see Urea). Although the latter is stiU used occasionally, the urea process predominates in North America. A flow sheet is shown in Figure 2 (43). 

Forty years after the initial proposal, Sweet and Fissekis proposed a more detailed pathway involving a carbenium ion species. According to these authors the first step involved an aldol condensation between ethyl acetoacetate (6) and benzaldehyde (5) to deliver the aldol adduct 11. Subsequent dehydration of 11 furnished the key carbenium ion 12 which was in equilibrium with enone 13. Nucleophilic attack of 12 by urea then delivered ureide 14. Intramolecular cyclization produced a hemiaminal which underwent dehydration to afford dihydropyrimidinone 15. These authors demonstrated that the carbenium species was viable through synthesis. After enone 13 was synthesized, it was allowed to react with N-methyl urea to deliver the mono-N-methylated derivative of DHPM 15. 

The mechanism was then reexamined 25 years later in 1997 by Kappe. Kappe used H and C spectroscopy to support the argument that the key intermediate in the Biginelli reaction was iminium species 16. In the event, 5 reacted with 3a to form an intermediate hemiaminal 17 which subsequently dehydrated to deliver 16. Iminium cation 16 then reacted with 6 to give 14, which underwent facile cyclodehydration to give 15. Kappe also noted that in the absence of 6, bisureide 8 was afforded as a consequence of nueleophilic attack of 16 by urea (3a). This discovery confirmed the conclusion of Folkers and Johnson in 1933. As far as the proposal from 25 years earlier by Sweet and Fissekis, Kappe saw no evidenee by H and NMR spectroscopy that a carbenium ion was a required species in the Biginelli reaetion. When benzaldehyde (5) and ethyl... 

Furthermore, Shutalev and coworkers reported a two-step modification. Urea 43a or thiourea 43b was condensed with 5 in the presence of p-toluenesulfonic acid to deliver a-tosylderivative 44. The enolate of 6 was then allowed to react with 44 to give a substitution product which then cyclized to give the hemiaminal 45. Dehydration of the hemiaminal with p-toluenesulfonic acid delivered 46. 

Carbodiimides have been prepared by desulfurization of thioureas by metal oxides, by sodium hypochlorite,4 or by ethyl chloroformate in the presence of a tertiary amine by halogena-tion of ureas or thioureas followed by dehydrohalogenation of the N,N -disubstituted carbamic chloride 8 and by dehydration of disubstituted ureas using -toluenesulfonyl chloride and pyridine.7 The method described above is a modification of that of Campbell and Verbanc. ... 

A variation of this process uses carbodiimides, which can be prepared by the dehydration of N,N -disubstituted ureas with various dehydrating agents, among which are TsCl in pyridine, POCI3, PCI5, P2O5—pyridine, TsCl (with phase-transfer... 

Cleavage of tertiary amines with cyanogen bromide (von Braun) Dehydration of disubstituted ureas... 

The use of the triphenylphosphine-carbon tetrachloride adduct for dehydration reactions appears to be a very simple way of synthesizing nitriles from amides, carbodi-imides from ureas, and isocyanides from monosubstituted formamides. All of these reactions involve the simultaneous addition of triphenylphosphine, carbon tetrachloride, and tri-ethylamine to the compound to be dehydrated. The elimination of the elements of water is stepwise. An adduct, e.g. (46), is first formed, chloroform being eliminated, which decomposes to produce hydrogen chloride and the dehydrated product. 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Elevated blood urea nitrogen (BUN) and serum creatinine (SCr) concentrations, especially with a BUN to SCr ratio of 20 1 or greater, suggest dehydration. 

Increased serum blood urea nitrogen (BUN) and creatinine owing to dehydration... 

The hematocrit and blood urea nitrogen increase because of dehydration. 

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