Alkyl Allophanates

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). 

Ethyl Allophanate. A mixture consisting of 252 g. of finely powdered urea (4.2 mols) and 217 g. of ethyl chlorocarbonate (2 mols) is refluxed for 3 hours in a 1-1. round-bottomed flask equipped with a 60-cm. reflux condenser and heated by a steam bath. Two hundred milliliters of water is subsequently added to the warm reaction cake. After mechanical agitation to form an aqueous slurry, the suspension is filtered and the residue washed well with water (600 ml. in three portions), 95% ethanol (400 ml. in three portions), and ethyl ether (400 ml. in three portions). 

The product is then air-dried. Yields varying from 139 to 151 g. (51 to 57%) are obtained. The product melts at 190 to 191° with decomposition. It can be purified, if desired, by recrystallization from hot water using 30 ml. of solvent per gram of ester. 

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Yashitake and Furukawa investigated the thermal degradation mechanism of a.y-diphenyl alkyl allophanates and carbanilates as model compounds for crosslinking sites in polyurethane networks by pyrolysis-high-resolution GC/ FTIR (Py-HR GC/FTIR). Pyrolysis was performed at 250°C, 350°C, 450°C, and 500°C. 

Yashitahe,N. Furufcawa,M. Thermal degradation mechanism of a, y-diphenyl alkyl allophanate as a model polyurethane by pyrolysis-high-resolution gas chromatography/FT-IR. J. Anal. Appl. Pyrol. 1995, 33, 269. 

The primary degradation reaction was dissociation of allophanate into phenyl isocyanate and alkyl carbanUate, followed by dissociation of the alkyl carbanUate into phenyl isocyanate and alcohol. Decarboxylation of the ethyl carbanUate fragment also took place slowly. A small amount of diphenyl carbodimide was observed at the pyrolysis temperature of 450°C. In addition, decarboxylation of the isopropyl carbanUate fragment took place at 550°C. A small amount of diphenyl carbodimide was observed from 350°C to 550°C. 

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