Reactions of ureas

The student should carry out the following reactions of urea ... 

The reaction of urea with formaldehyde yields the following products, which are used as monomers in the preparation of urea formaldehyde resin. 

Fig. 1. Influence of pH on A, the addition reaction of urea and formaldehyde (1 1) and B, the condensation of methylolurea with the amino hydrogen of a...

In 1945, cationic urea resins were introduced and quickly supplanted the anionic resins, since they could be used with any type of pulp (62). Although they have now become commodities, their use in the industry has been steadily declining as the shift towards neutral and alkaline papermaking continues. They are commonly made by the reaction of urea and formaldehyde with one or more polyethylene—polyamines. The stmcture of these resins is very compHcated and has not been deterrnined. Ammonia is evolved during the reaction, probably according to the following ... 

Sulfamic acid [5329-14-6] (amidosulfuric acid), HSO2NH2, molecular weight 97.09, is a monobasic, inorganic, dry acid and the monoamide of sulfuric acid. Sulfamic acid is produced and sold in the form of water-soluble crystals. This acid was known and prepared in laboratories for nearly a hundred years before it became a commercially available product. The first preparation of this acid occurred around 1836 (1). Later work resulted in identification and preparation of sulfamic acid in its pure form (2). In 1936, a practical process which became the basis for commercial preparation was developed (3,4). This process, involving the reaction of urea with sulfur trioxide and sulfuric acid, continues to be the main method for production of sulfamic acid. 

Sulfamic Acid. Sulfamic acid is manufactuied by the reaction of urea and fuming sulfuric acid (3,4,44). This reaction is considered to take place in two steps involving an aminocarbonylsulfamic acid intemiediate ... 

Physical and Chemical Properties. The reaction of urea and formaldehyde forms a white soHd. The solubihty varies with the methylene urea polymer chain length longer-chain, higher molecular-weight UF polymers are less water-soluble than short-chain polymers. Physical properties of the methylene urea polymers which have been isolated are compared to urea in Table 1. 

In actual practice, the reaction of urea with formaldehyde produces a distribution of polymers of varying chain length. The distribution is affected by the U/E mole ratio as well as reaction conditions such as pH, temperature, and reaction time. In general, higher U/E ratios produce polymer distributions... 

Isohutyhdene Diurea., This is the condensation product of urea and isobutyraldehyde. Unlike the condensation of urea with formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single oligomer. Although similar in chemical stmcture to methylene diurea (MDU), its physical properties are quite different (Table 4). 

Crotonylidene Diurea. (CDU). Crotonjlidene [1129 2-6] is produced by the acid catalyzed reaction of urea with either crotonaldehyde or acetaldehyde. The condensation reaction produces a ring-stmctured compound. Table 4 hsts select properties. 

Maleic hydrazide (149), produced on a large scale as a herbicide, can be obtained, according to a patent, from a simple diaziridine, which on an industrial scale could be less expensive than hydrazine hydrate. In the proposed manufacture of diamide (150) from a diaziridine and urea it must be taken into account that the reaction of urea is preceded by solvolytic hydrazine formation, which probably proceeds under the reported conditions (several hours, 70-80 °C, acidic media) (79AHC(24)63). 

Urea-formaldehyde resins are usually prepared by a two-stage reaction. The first stage involves the reaction of urea and formaldehyde under neutral or mildly alkaline conditions, leading to the production of mono and dimethylol ureas (Figure 24.1). The ratio of mono to dimethylol compounds will depend on the urea-to-formaldehyde ratio and it is important that there should be enough formaldehyde to allow some dimethylol urea formation. 

The reaction of urea and formaldehyde to form UF-resins is basically a two-step process, usually an alkaline methylolation followed by an acidic condensation. 

In the literature, various other types of resin preparation procedures are described, e.g. yielding uron structures [20-22] or triazinone rings in the resins [23,24]. The last ones are formed by the reaction of urea and an excess of formaldehyde under basic conditions in the presence of ammonia or an amine, respectively. These resins are used to enhance the wet strength of paper. 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

Alkali metal cyanates are stable and readily obtained by mild oxidation of aqueous cyanide solutions using oxides of or Ptf. The commercial preparation of NaNCO is by reaction of urea with Na2C03. 

The nucleophilic addition reaction of urea to formaldehyde produces mainly monomethylol urea and some dimethylol urea. When the mixture is heated in presence of an acid, condensation occurs, and water is released. This is accompanied by the formation of a cross inked polymer ... 

Anhydrous peroxytrifluoroacetic acid is not easy to handle, but the procedure has recently been revised.121 Namely, reaction of urea-hydrogen peroxide complex (UHP) with tri-fluoroacetic anhydride in acetonitrile at 0 °C gives solutions of peroxytrifluoroacetic acid, which oxidize aldoximes to nitroalkanes in good yields (Eqs. 2.58 and 2.59). Ketoximes fail to react under these conditions, the parent ketone being recovered. 

Most workers now use a colorimetric method based on the reaction of urea with biacetylmonoxime [302,303]. The method has been adapted for automated analysis by De Marche et al. [304],... 

Parabanic acid can be prepared by the condensation of urea with diethyl oxalate in an ethanolic solution of sodium ethoxide,2 by reaction of urea with an ethereal solution of oxalyl chloride,3 by oxidizing uric acid with an acid solution of perhydrol,4 or by the action of hot, concentrated nitric acid on uric acid.5 The present method gives better yields than the previously reported methods and is better adapted to larger-scale preparations. 

The reactions of urea, dialkyl malonates, and alkyl orthoformates at reflux temperature for 10 hr gave dialkyl ureidomethylenemalonates (268, R = H) in 40% yield (53JA671), while in the reaction at I30°C for 4 hr the yield was 66-69% [77JAP(K)131529 81CPB3181]. 

Formation of dioxy-l,3-diazines. Reaction of urea with a,(3-unsaturated esters... 

Rachhpal-Singh and Kirk (1993a,b) developed a model of these processes based on equations for the transport and reaction of urea, ammoniacal species (NH4+, NH3, NH4OH), carbonate species (H2CO3, HCOs", COs ) and mobile acid-base pairs (H2C03-HC03, HCOs -COs , NH4+-NH3, NH4+-NH4OH, H20-OH ). The equations are of the form of Equation (2.6)... 

The colorimetric reaction of urea nitrate with p-DMAC [91] was also adopted as a field test, UN-1. The reagent (0.4% in ethanolic solution) is sprayed on the sample, or on the suspected area, and the appearance of a red color within 1 min indicates the presence of urea nitrate [103]. 

Some recent advances have been reported in oxime oxidation, including the in situ generation of peroxytrifluoroacetic acid from the reaction of urea hydrogen peroxide complex with TFAA in acetonitrile at 0 °C This method gives good yields of nitroalkanes from aldoximes but fails with ketoximes. 

Bachman and co-workers ° studied the synthesis of the high explosive ethylenedinitrantine (2) (EDNA) from the hydrolysis of secondary nitramides. One of the oldest routes to EDNA (2) involves the nitration of 2-imidazolidone (66) with mixed acid, followed by hydrolysis of the resulting A,A -dinitro-2-imidazolidone (67) with boiling water. 2-Imidazolidone (66) is readily synthesized from the reaction of urea with ethylene glycol, or by treating either... 

This enzyme [EC 3.5.1.54] catalyzes the hydrolysis of urea-1-carboxylate to yield 2 CO2 and 2 ammonia. Interestingly, the enzyme isolated from yeast (but not the corresponding enzyme from green algae) will also catalyze the reaction of urea with ATP and CO2 to produce ADP, orthophosphate, 2 ammonia, and 2 CO2. 

The preparation of barbiturates illustrates many of the synthetic methods covered in this chapter. The preparation employs the reaction of urea (C0(NH2)2) with malonic ester to form barbituric acid. The general reaction is presented in Figure 15-30. The stable pyrimidine and other resonance forms help drive the reaction. By alternating the substituent at carbon number five (C5), various pharmacologically active substances can be formed. Barbital, a sedative, and phenobarbital, a sleeping aid, are shown in Figure 15-31. 

Methyluracils 947 are readily prepared by reaction of ureas with diketene 945 in acetic acid, and although mixtures of products are obtained with substituted ureas, this procedure has been used synthetically to prepare a variety of 1-substituted and 1,3-disubstituted 6-methyluracils, using both solution- and solid-phase procedures <2000TL1487, 2003CPB1025, 2004JME1259>. 

Phenolic resins produced by the reaction of phenol and formaldehyde were used as adhesives by Leo Baekeland in the early 1900 s. This inexpensive resin is still used for binding thin sheets of wood to produce plywood. Urea resins produced by the reactions of urea and formaldehyde have been used since 1930 as binders for wood chips in particleboard. 

---