Hydroxyl group reactions ureas

Cellulosic fibers (cotton, rayon) are crosslinked by reaction of the hydroxyl groups of cellulose with formaldehyde, diepoxides, diisocyanates, and various methylol compounds such as urea-formaldehyde prepolymers, /V, /V -di tnethylol-A(A -dimethy lene urea, and trimethyl-olmelamine [Marsh, 1966]. Crosslinking imparts crease and wrinkle resistance and results in iron-free fabrics. 

The mechanism of carbodiimide-mediated amide formation is outlined in Figure 13.4. The first intermediate formed during the reaction of a carbodiimide with a carboxylic acid (or an ammonium carboxylate [56]) is an O-acylisourea [1]. Under acidic conditions [57,58], this intermediate reacts with acids to yield anhydrides, and with alcohols, phenols, HOBt, or other compounds containing hydroxyl groups, to yield the corresponding esters. The carbodiimide is thereby transformed into a urea. 

Cyanuric fluoride has been used to modify tyrosine residues, substituting the phenolic hydroxyl group. A maximum of 3 residues in RNase was found to react at pH 10.9 and 25° (148a). However, some mystery surrounds this number, as with other estimates of accessibility, since alkaline-denatured material where all tyrosine residues are available still showed the reaction of only 3 residues with cyanuric fluoride. However, similar observations have been made on iodination in 8 Af urea (11 )- At pH 9.3, Takenaka et al. (149) found that only 2 residues reacted and that 115 was not one of them. Two more reacted after alkali denaturation. Two were resistant under all conditions tested. No enzymic activity data were reported. 

If the hydroxyl group (-OH) in a conventional diol is replaced by an amine group (-NH2), polyamines are formed. The polyamines can be used in a similar manner to polyols to form a polyurea on reaction with diisocyanate. These polymers do not have any urethane groups, only urea groups. Because... 

Thermal Inactivation. Heating the wood surface to temperatures above 150 to 200°C definitely has been shown to cause poor adhesion with phenolic adhesives and, possibly, urea adhesives. Early research (60) verified that overheated wood was less wettable and tended to absorb less water from the adhesive. Whether this is caused by extractives, pyrolysis, oxidation of hydroxyl groups, or other chemical reactions at the wood surface has been the subject of much debate. Further, the mechanisms of thermal or "surface" inactivation may vary from species to species. 

Alternately, the importance of the hemiacetal group can be studied by using a compound that is an analog for a monosaccharide without the rest of the hydroxyl groups. Tetrahydro-2H-pyran-2-ol (THP) is such an analog for xylose. THP was reacted with an equimolar amount of DMU, in the absence of phenol, to follow the course of the hemiacetal-urea reactions. The acid-catalyzed mixture was heated between 81 and 116 °C over a 2-hour period. Samples were frozen to prevent further reaction prior to analysis. 

Tetrahydro-2H-pyran-2-ol, a model for the xylose ring hemiacetal without other hydroxyl groups, reacted with a monofunctional urea to produce the expected ureide. Continued reaction resulted, as for normal saccharides, in a multitude of products, predominantly aliphatic, in much lower concentration. 

Isocyanates react rapidly with thiol, amine, or hydroxyl groups resulting in thiourea, urea, and carbamate linkages, respectively (Scheme 9). Without the use of catalysts, the reaction is fastest for thiols that are most nucleophilic, followed by amines and alcohols. 

Urea decomposes slowly at 90°C in aqueous solution [10] according to reaction (1). Consequently, hydroxyl groups are slowly generated, uniformly throughout the pores and the precipitation takes place homogeneously. 

Analysis of the uptake/unit time results has indicated that several reactions take place simultaneously, and that such additives as the alkali chlorides or urea increase the rate of reaction. Another kinetic study concluded that, in the presence of added calcium chloride, an initial, fast, first-order reaction represents etherification of the hydroxyl groups on the... 

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