Gaseous carbamate

Some properties of H2NC02H have been the subject of theoretical studies [17] indeed, calculations have predicted that the syn conformer is more stable than its anti counterpart [17a]. It has been also calculated that gaseous carbamic acid can decompose exothermically into ammonia and carbon dioxide (AH° = -26 kj mol"1) [17b]. Moreover, ah initio calculations have shown that the zwitterion H3N+C02" is less stable than monomeric acid H2NC02H and, therefore, the zwitterion form has been proposed as the most probable intermediate for the decomposition of carbamic acid to ammonia and C02 [17c],... 

Urea is produced from liquid NH and gaseous CO2 at high, pressure and temperature both reactants are obtained from an ammonia-synthesis plant. The latter is a by-product stream, vented from the CO2 removal section of the ammonia-synthesis plant. The two feed components are deUvered to the high pressure urea reactor, usually at a mol ratio >2.5 1. Depending on the feed mol ratio, more or less carbamate is converted to urea and water per pass through the reactor. 

Toyo Engineering-AGES Process. The synthesis section of the ACES process (Fig. 8) consists of a reactor, a stripper, two carbamate condensers, a scmbber and operates at 17.5 MPa (175 bars). The reactor is operated at 190°C with a NH /CO2 ratio of 4.0 (mol/mol). Liquid NH is fed directly into the reactor by a centrifugal ammonia pump. Gaseous CO2 is sent from the centrifugal CO2 compressor to the bottom section of the falling-film type stripper. 

Gaseous CO2 is extensively used to carbonate soft drinks and this use alone accounts for 20% of production. Other quasi-chemical applications are its use as a gas purge, as an inert protective gas for welding, and for the neutralization of caustic and alkaline waste waters. Small amounts are also used in the manufacture of sodium salicylate, basic lead carbonate ( white lead ), and various carbonates such as M2CO3 and M HC03 (M = Na, K, NH4, etc.). One of the most important uses of CO2 is to manufacture urea via ammonium carbamate ... 

A particularly useful synthesis of primary nitramines involves the nitration of the appropriate carbamate ester followed by ammonolysis with gaseous ammonia in diethyl ether. The ammonium salt of the nitramine precipitates in pure form and is carefully acidified to give the free nitramine. The corresponding carbamate esters are readily synthesized from the action of chlorocarboxylic acid esters on alkylamines in the presence of alkali hydroxides. 

A series of solid-state reactions has been explored by Kaupp et al., in which gaseous amines were reacted with aldehydes to give imines. Analogous reactions with solid anhydrides, imides, lactones or carbonates, and isothiocyanates were used to give, respectively, diamides or amidic carboxylic salts or imides, diamides, carbamic acids, and thioureas [24]. In general the yields were found to be quantitative. Ammonia and other gaseous amines, in particular methyl-amine, have also been shown to aminolyse thermoplastic polycarbonates [25]. 

The decomposition of solid ammonium carbamate, (NH4)(NH2C02), to gaseous ammonia and carbon dioxide is an endothermic reaction. 

Selective precipitation of ammonium carbamate can be achieved by reacting gaseous C02 and NH3 in anhydrous ethanol, 1-propanol or DMF in a flow reactor that operates in continuous mode [20]. And then, the pure ammonium carbamate is used to produce urea with good yield (up to 54 % on carbamate basis) at 120-140 °C in the presence of inexpensive Cu (II) and Zn (II) catalysts. (Scheme 6.3)... 

Irradiation of a tert-butyl alcohol solution of 6-ethoxy-4,5-dihydro-2(3H> pyridone (14) under conditions similar to those described for 12 gives two products, tert-butyl N-(ethoxyethylidene)carbamate (27) in 15% yield and glutarimide in 23% yield18. There is no indication of formation of the cyclobutyl analogue of 13. In aprotic solvent, ethoxyvinyl isocyanate (28) and glutarimide (29) are the major photoproducts formed. The gaseous byproducts from the irradiation of 6-propoxy-... 

Carbon dioxide protected pyrroles and indole (carbamates), generated by treatment of the iV-lithiated heterocycles with gaseous CO2, can be easily tracked by negative ion electrospray ionization mass spectrometry (ESI-MS) revealing invariable loss of CO2 from the anion <2002JMP541>. 

Lowering the temperature and pressure shifts the equilibrium towards the initial reactants, llie gaseous mixture is then recompressed, causing its recomlunation, and the carbamate solution is recycled. 

If ammonia is present in excess, it separates from the carbamate solution and is recycled in gaseous form. To minimize the total costs of rccompression of the gaseous compounds, decomposition is carried out in two stages, and gases produced are recycled after each. As a rule, the first operates at 2.10 Pa absolute, and the second at 0.1 to 0l2.10 Pa absolute, at temperatures in the range of 160 to 200 C. 

Based on the law of mass action and Dalton s law, it can be shown that the dissodation pressure (jp) ofliquid carbamate into its gaseous components is governed by the equation ... 

The reactor effiuent, consisting of urea, ammonium carbamate and unconverted reactants, is subjected, by altering the operating conditions, to a decomposition that converts part of the ammonium carbamate to carbon gaseous product is compressed and condensed. This leads to renewed formation of carbamate which is recycled to the reactor in aqueous s ution, while the excess ammonia is mixed with fresh ammonia. The entire operation is repeated to decompose aO the carbamate. The final solution obtained contains 72 to 76 per cent weight of urea, and the fmal purity desired can be obtained by a finishing treatment. 

A selection of A//p gp values has already been given in Table 2-4 in Section 2.2.6. This new Lewis basicity scale is more comprehensive and seems to be more reliable than the donor number scale. Analogously, a Lewis basicity scale for 88 carbonyl compounds (esters, carbonates, aldehydes, ketones, amides, ureas, carbamates) has been derived from their standard molar enthalpies of complexation with gaseous boron trifluoride in dichloromethane solution [143]. The corresponding Aff Q gp values range from 33 kJ mol for di-t-butyl ketone to 135 kJ mol for 3-diethylamino-5,5-dimethyl-cyclohexen-2-one. 

DDT can be determined in microgram amounts by its inhibition of carbonic anhydrase (69). Several of the chlorinated insecticides as well as the carbamate insecticide, Sevin, are potent lipase inhibitors. As little as 0.1-1 /Ag/ml of Sevin, aldrin, heptachlor, lindane, 10 / g/ml of DDT, and 2,4-d can be determined fluorimetrically with 2-3% precision by the lipase inhibition when 4-methyl umbelliferone heptanoate is used as substrate (70). The lipase inhibition procedure for DDT determination is much easier to carry out than the carbonic anhydrase assay which involves monitoring gaseous COo production. 

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