Urea, reaction with acids

The production of nitrogen fertilizers is a major activity of the chemical industry. Every year, the top 15 chemicals in industrial production in the United States include several nitrogen-containing compounds whose major use is in fertilizers. Molecular nitrogen serves as the primary source of nitrogen for chemical production. Gaseous ammonia (NH3), which is synthesized from N2 and H2, can be injected directly into the ground, where it dissolves in moisture in the soil and serves as a fertilizer. Ammonia is more widely used in reactions with acids to produce other fertilizers Ammonia and nitric acid produce ammonium nitrate (NIL) NO3), while ammonia and sulfuric acid produce ammonium sulfate. These chemicals and urea,... 

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. 

The acylation with acid chloride and the urea formation with isocyanate of intermediate resin 16 afforded other substituent groups onto 4-aminothiazole. Under microwave (MW) irradiation reaction with isocyanate and acylation reaction with acid chloride, R -substituted thiazole resin 19 was obtained. Following conversion of sulfonyl resin 19 to sulfonyl resin 20 (mCPBA/CH2Cl2), substitution reactions promoted by treatment with appropriate amines (R R" N diversity elements) furnished the 2,4,5-trisubstituted thiazoles 2 (29 examples 36-25% isolated yields from Merrifield resin 1, >95% purities, Table 10.1). 

Under the same conditions the even more reactive compounds 1,6-dimethylnaphthalene, phenol, and wt-cresol were nitrated very rapidly by an autocatalytic process [nitrous acid being generated in the way already discussed ( 4.3.3)]. However, by adding urea to the solutions the autocatalytic reaction could be suppressed, and 1,6-dimethyl-naphthalene and phenol were found to be nitrated about 700 times faster than benzene. Again, the barrier of the encounter rate of reaction with nitronium ions was broken, and the occurrence of nitration by the special mechanism, via nitrosation, demonstrated. 

The mixture was prepared and allowed to achieve equilibrium to it was added an excess of urea which caused the immediate precipitation as urea nitrate of the free nitric acid present. As a result of the sudden removal of the nitric acid from the mixture, the system underwent change to re-establish the equilibrium however, the use of an excess of urea removed the nitric acid as it was produced from acetyl nitrate and acetic acid, and the consumption of acetyl nitrate proceeded to completion. Thus, by following the production of urea nitrate with the time from the addition of urea, the rate of the back reaction could be determined, and by extrapolating the results to zero time the equilibrium... 

Section 21 8 Alkylation of diethyl malonate followed by reaction with urea gives derivatives of barbituric acid called barbiturates, which are useful sleep promoting drugs... 

Carboxyhc acids react with aryl isocyanates, at elevated temperatures to yield anhydrides. The anhydrides subsequently evolve carbon dioxide to yield amines at elevated temperatures (70—72). The aromatic amines are further converted into amides by reaction with excess anhydride. Ortho diacids, such as phthahc acid [88-99-3J, react with aryl isocyanates to yield the corresponding A/-aryl phthalimides (73). Reactions with carboxyhc acids are irreversible and commercially used to prepare polyamides and polyimides, two classes of high performance polymers for high temperature appHcations where chemical resistance is important. Base catalysis is recommended to reduce the formation of substituted urea by-products (74). 

Acetoiicetyliition Reactions. The best known and commercially most important reaction of diketene is the aceto acetylation of nucleophiles to give derivatives of acetoacetic acid (Fig. 2) (1,5,6). A wide variety of substances with acidic hydrogens can be acetoacetylated. This includes alcohols, amines, phenols, thiols, carboxyHc acids, amides, ureas, thioureas, urethanes, and sulfonamides. Where more than one functional group is present, ring closure often follows aceto acetylation, giving access to a variety of heterocycHc compounds. These reactions often require catalysts in the form of tertiary amines, acids, and mercury salts. Acetoacetate esters and acetoacetamides are the most important industrial intermediates prepared from diketene. 

Association Complexes. The unshared electron pairs of the ether oxygens, which give the polymer strong hydrogen bonding affinity, can also take part in association reactions with a variety of monomeric and polymeric electron acceptors (40,41). These include poly(acryhc acid), poly(methacryhc acid), copolymers of maleic and acryflc acids, tannic acid, naphthoHc and phenoHc compounds, as well as urea and thiourea (42—47). 

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

Reactions with Nitrogen Compounds. Succinimide [123-56-8] mp 126°C, can be prepared by reaction of aqueous solutions of the acid with ammonia (105) or urea (106) (eq. 7). The solution is heated until water and ammonia are no longer evolved and the molten cmde succinimide is purified by fractionation. Alternatively, the cmde product can be recrystaUized from water (105). 

Sulfation by sulfamic acid has been used ia the preparation of detergents from dodecyl, oleyl, and other higher alcohols. It is also used ia sulfating phenols and phenol—ethylene oxide condensation products. Secondary alcohols react ia the presence of an amide catalyst, eg, acetamide or urea (24). Pyridine has also been used. Tertiary alcohols do not react. Reactions with phenols yield phenyl ammonium sulfates. These reactions iaclude those of naphthols, cresol, anisole, anethole, pyrocatechol, and hydroquinone. Ammonium aryl sulfates are formed as iatermediates and sulfonates are formed by subsequent rearrangement (25,26). 

Urea reacts with himing sulfuric acid in an exothermic reaction that needs agitation and cooling. After completion of the reaction, excess sulfur trioxide is removed by dilution or by other methods (45,46). A flow diagram of the process is shown in Figure 1. 

Phosgenation. Reaction of phosgene with arylamines to form ureas, and with reactive aryl species to form substituted hen zophen ones, are special cases of acylation. They are dealt with separately siace a more specialized plant is required than for other acylations. Urea formation takes place readily with water-soluble arylamines by simply passiag phosgeae through a slightly alkaline solutioa. An important example is carbonyl-J-acid from J-acid. 

If one replaces one of the two equivalents of P-dicarbonyl with urea, such that the reaction is now carried out with one equivalent of aldehyde 123, one equivalent of P-dicarbonyl 124 and an equivalent of urea 125 in acidic ethanol solution, then dihydropyrimidines 126 are formed. This class of reactions has been named Biginelli reactions and are reviewed in section 10.6... 

Urea maybe reacted with acetoacetic ester and that product nitrated to give 5-nitro-orotec acid That is hydrogenated, then reacted with urea and potassium cyanate to give tetrahydroxypy-imidopyrimidine. The tetrahydroxy compound Is converted to the tetrachloro compound POCI3. Reaction with diethanolamine and then with piperidine gives dipyridamole. 

No satisfactory direct gravimetric procedure is available but nitrite can be oxidised to nitrate by permanganate or cerium(IV) and then determined in that form. The determination of total nitrate + nitrite is an important analysis, e.g. for soil samples. Nitrite may be destroyed using urea, sulphamic acid or hydrazine sulphate the reaction with the former is ... 

Barbituric acid can be considered as a cyclized malonic acid diamide (malonyl-urea). It is therefore a cyclic diketone that may be classified, in the sense of the compounds discussed in Section 12.6, as a coupling component with a methylene group activated by two carbonyl groups in the a- and a -positions. The reaction with arenediazonium salts was studied by Nesynov and Besprozvannaya (1971). These authors obtained coupling products (in good yield) that they considered to be arylhydrazones. Coupling with 4-(phenylazo)benzenediazonium chloride was studied by Chandra and Thosh (1991). The lH NMR spectra of these compounds are consistent with the arylhydrazone structure 12.68. 

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