Allophanate, from urea

Allophanyl hydrazide has been prepared previously as the hydrochloride by the zinc-hydrochloric acid reduction of 1-nitrobiuret.1 The new procedure2,3 outlined here entails the hydrazinolysis of allophanic esters in alcoholic solution. Excellent yields (80 to 84%) of allophanyl hydrazide are obtainable with a minimum of difficulty using readily available starting materials. The methyl and ethyl allophanates are prepared from urea and the corresponding chlorocarbonic esters.4... 

Examples of non-urethane linkages derived from isocyanates a) urea, b) urea, c) biuret, d) amide, and e) allophanate... 

This addition reaction proceeds readily and quantitatively. Side reactions can give amide, urea, biuret, allophanate, and isocyanurate groupings, so that the structure of the product can deviate from that above such side reactions are sometimes desired (see Sect. 4.2.1.2). 

The isocyanate (2270 cm"1) uretedinedione ring carbonyl (1780 cm"1) and urea carbonyl (1660 cm"1) groups can usually be identified. Carbonyls from ester, urethane, allophanate, isocyanuric acid ring and Biuret groups all absorb near 1730 cm 1 and are difficult to distinguish. Hydrogen bonds which can function as physical crosslinks in PU have been... 

Part of the host framework in [(CH3 )4N]+[H2NC0NHC02] - -5(NH2)2CO showing the double ribbon constructed by allophanate anions and one of the independent urea molecules. From T. C. 

Other hydrolyzable groups consist of biuret and allophanate crosslinks formed from the reaction of Isocyanate radicals with active hydrogen atoms of the urea and urethane linkages, respectively. 

Methyl Allophanate. Three hundred five milliliters of methyl chlorocarbonate (4.0 mols) and 480 g. of finely powdered urea (8.0 mols) are placed in a 2-1. standard-taper round-bottomed flask equipped with a 60-cm. reflux condenser. The mixture is heated on a steam bath for 90 minutes. Four hundred milliliters of water is added to the warm reaction cake to decompose any unreacted chlorocarbonic ester and to remove by solution both unchanged urea and urea hydrochloride, t Methyl allophanate remains behind as a relatively water-insoluble residue. The suspension is filtered and the white product washed with three 250-ml. portions of water, then with 95% ethanol (two 250-ml. portions), and finally with 500 ml. of ethyl ether. It is air-dried. The yield of product will vary from 227 to 274 g. (48 to 58%). The product thus obtained is pure enough to use as the starting material for other reactions. It can be further purified by recrystallization from hot water using 27.5 ml. of water per gram of compound (85% recovery m.p. 215 to 216° with decomposition). 

In one of the two urea-allophanate compounds (1.8, Figure 11), hydrogen-bonded zigzag ribbons are built up from repetition of the allophanate ion and one independent urea molecule joined together by a pair of N-H -O hydrogen bonds (LF type), whereas wide puckered urea and allophanate ribbons connected by hydrogen bonds exist in the other (1.9, Figure 38). 

Allophanic acid (ureidoformic acid, NH2CONHCOOH) is an elusive compound which is known only in the form of its derivatives. Ester-like organic allophanates have been prepared from primary, secondary, and tertiary alcohols and from phenols [39], whereas inorganic allophanate salts are unstable and easily hydrolyzed by H2O to CO2, urea, and carbonate [40]. The first two crystalline complexes containing host lattices built of urea molecules and allophanate ions were prepared and structurally characterized in 1995 [5h]. 

Another opportunity for bond break down is the cleavage of urethane and/or urea compounds and their subsequent products, such as allophanates or biurets from reverse reaction or hydrolysis. Figure 5.111. In consequence of this damage process, the rigid phase and the links between rigid and soft phases are particularly weakened. Cleavages in urethane and urea units are in particular facilitated by the presence of catalysts in the foam [702]. 

The requirement for ATP in the ligase-catalysed reaction suggests that if in soils urea were to be hydrolysed partly by the allophanate pathway, then this reaction would take place Vvithin live cells only (ATP is unlikely to persist in dead cells or to remain free in soil ). Thus it is conceivable that the decreases in urea-hydrolysing activities of soils which frequently have been observed following toluene addition may have resulted from cell death, loss of ATP and hence elimination of the allophanate pathway, rather than from the destruction, of urease activities of live cells. 

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