By Dehydration of Ureas

NJSf -Disubstituted ureas 39 can be dehydrated with phosphorous pentoxide to give carbodiimides in good yields. For example, refluxing of 1,3-dicyclohexylurea and phosphorous pentoxide (live-fold excess) in pyridine affords DCC in 76 % yield. 1,3-diarylureas afford yields of 53-86 % under the same conditions. 

This reaction can also be conducted with triphenylphosphine in the presence of carbon tetrachloride and triethylamine, or with triphenylphosphine dibromide in the presence of triethylamine. to give carbodiimides 40. 

N -di-n-dodecylcarbodiimide (76 % yield), N-6-(4 -methoxybiphenyl-4-oxyhexyl-N -n-hexylcarbodiimide (60 % yield) andN-6-(4-methoxyphenylazo)phenyloxy)hexyl-N -n-hexylcarbodiimide (70 % yield), monomers for liquid crystalline polyguanidines, are synthesized in the same manner.  

The triphenylphosphine dibromide can also be attached to a polymeric substrate.  

phenyl dichlorophosphoridate, Ph0P(0)Cl2 (55-60 % yield), phenyl N-phenyl-phosphoramidochloridate, PhOP(0)NHPhCl, (75-90% yield) and phenyl chlorophos-phoridate PhOP(0)OPhCl (70-80 % yield) can be used instead of Ph3PBr2 in the reaction with ureas.  

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Catbodiimides. liven unstable carbodiimides can be obtained by dehydration of ureas with this reagent and triethylamine in cold methylene chloride (equation I). ... 

NNH2,CH3N303 mw 105.06 N 39.98% OB to C02 —7.61% colorl platelets (from ethanol plus eth) mp 158.4—.8° (decompn). Freely sol in acet, acetic acid and ethanol sol in hot w (forms cyanic acid and nitroamide) si sol in benz, chlf and petr eth. CA Registry No [556-89-8]. Prepn is by dehydration of Urea Nitrate with coned sulfuric acid. The compd can be detonated but is not sensitive to percussion or heating (Ref 7)... 

A number of novel fused 1,2,4-triazole were reported and these include [l,2,4]-triazole[l,3]-thiazinoquinolines (118, 119) and their positional isomers [95MI297], The electrocyclization of N-aziridinylimino carbodiimides generated by dehydration of urea (120), produces 5,6-dihydro-7H-imidazol[l,2-b][l,2,4]triazoles (121) and 5,6-dihydro[l,2,4]triazolo[5,l-t/]-[l,3,5]oxadiaze-pines [95TL2815], The mechanisms of each transformation is discussed. 

It is synthesized by dehydration of urea nitrate with sulfuric acid. 

A method has been described for the preparation of carbodiimides 1699 by dehydration of ureas 1696 with p-tosyl chloride under solid-liquid phase-transfer catalytic (PTC) conditions using solid potassium carbonate as a base and a lipophilic quaternary ammonium salt as a catalyst. The method is generally applicable for the synthesis of disubstituted carbodiimides, but is especially useful for un-symmetrically substituted carbodiimides. Yields of the resulting carbodiimides 1699 vary depending on the solvent (usually used at reflux temperature) in benzene or toluene yields of 66-98% are achieved, while in chloroform they are only 30-50% [1281]. 

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. ... 

Preparation of carbodiimides by dehydration of the corresponding ureas is of general applicability and is well adapted to the laboratory preparation of substantial quantities. The intermediates for this particular preparation are commercially available at moderate cost. 

This procedure for the preparation of l-ethyl-3-(3-dimethyl-amino)propylcarbodiimide and its salts is a modification of one that has been published.4 Unsymmetrical carbodiimides have also been prepared by desulfurization of the corresponding thioureas with mercuric oxide3 or by dehydration of the corresponding ureas with -toluenesulfonyl chloride in pyridine.4 Unsymmetrical 1,3-disubstituted ureas are best prepared by the reaction... 

By Dehydration of a-Diketone Hydrazone Oximes Another general method of synthesis of 2i/-triazoles involves intramolecular elimination of water from adjacent hydrazone and oxime functions (Scheme 32). The usual reagent is acetic anhydride, but others, including phosphorus pentachloride and urea, have been used. 

Table 12.3 summarizes the uses of formaldehyde. Two important thermosetting plastics, urea- and phenol-copolymers, take nearly one half the formaldehyde manufactured. Urea-formaldehyde resins are used in particleboard, phenol-formaldehyde resins in plywood. 1,4-Butanediol is made for some polyesters and is an example of acetylene chemistry that has not yet been replaced. Tetrahydrofiiran (THF) is a common solvent that is made by dehydration of 1,4-butanediol. 

Guanidines may also be formed by reaction of amines with carbodi-imides. This reaction is limited by the availability of carbodiimides, which are usually formed by several methods,24 including dehydration of ureas with the Edward Burgess reagent 11 (Scheme 9).25-27... 

Commercially, formaldehyde is sold as u. IKt aquruus so lulion Good reactivity, low price, and important end-use markets have combined lo give formaldehyde its growth mi pcliiv Formaldehyde is produced by vnpor-pha.se oAidulnm or by dehydration of methanol Its mam muricis are in production ot phenolic and urea resins The price of formaldehyde has historically been coupled closely to the price of methanol... 

Apart from the preparation of the parent urea, commercially produced by dehydration of ammonium carbamate obtained by direct reaction of ammonia with carbon dioxide [727, 748], the earliest classical method for the preparation of N,N -symmetrically substituted ureas involved the reaction of amines with phosgene (1002 X,Y = C1). 

Urea, CO(NH2)2, is formed by dehydration of ammonium carbamate, NH4CO2NH2, which is again formed by the reaction between ammonia and carbon dioxide... 

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. 

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. 

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 ... 

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. 

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... 

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... 

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