Ammonium carbamates

Ammonium carbamate has been prepared by the direct combination of ammonia and carbon dioxide in the gas phase and also in cooled inert solvents such as absolute alcohol and petroleum ether. The direct laboratory preparation from readily available Dry Ice and liquid ammonia offers a convenient source of ammonium carbamate. It is interesting to note that under other conditions these same reactants produce urea. 

Ammonium carbamate may be used as an ammonating agent where a reagent less vigorous than ammonia is desired. 

About 400 ml. of anhydrous liquid ammonia is placed in a 1-liter pyrex Dewar flask, J preferably unsilvered for visual observation. Dry Ice is powdered and added slowly, yet as rapidly as feasible, to the liquid ammonia. Addition of solid carbon dioxide is continued untU the noixture attains a slushy consistency. The excess of ammonia is then allowed to evaporate, leaving the car- 

Inorganic Syntheses, Volume II Edited by W Conard Femelius 1946 by McGraw-Hill Book Company, Inc. 

Ammonium carbamate is obtained as a fine crystalline powder, slightly volatile at room temperature and completely dissociated at 59°. It is very soluble in water but, in solution or on standing in moist air, it undergoes hydration to ammonium carbonate  

Contact with metal surfaces produces bromides of the metals similarly reactions with metal hydroxide bases yield corresponding bromides  

Ammonium bromide decomposes to ammonia and hydrogen bromide when heated at elevated temperatures  

Mixed with NaOH solution and distilled distillate analyzed for ammonia by titration, colorimetry, or electrode method (see Ammonia and Ammonium chloride). Bromide portion of NH4Br in aqueous solution may be analyzed by ion chromatography, or by the colorimetry method in which red to violet color is produced upon treatment with chloramine-T, and phenol red at pH 4.5. The colorimetry test for bromide is subject to interference from oxidizing and reducing agents, chloride, and bicarbonate. NH4Br may then be determined stoichiometrically. 

Synonyms ammonium aminoformate ammonium carbonate anhydride 

Ammonium carbamate is used as an ammoniating agent. It occurs as a mixed salt with ammonium bicarbonate and carbonate. 

In order to avoid freezing issues altogether, solid precursor compounds are a good substitute for solutions of NH3 precursor compounds. However, the handling of solid NH3 precursors is more complicated, especially if they also need to be contacted with a decomposition catalyst like solid urea (as discussed in Sect. 16.2.1). 

There are, however, also solid substances which do not need a catalyst to yield NH3, and only need to be heated for NH3 release. One compound in this category is ammonium carbamate. Ammonium carbamate is the ammonium salt of the instable carbamic acid, and it can be transformed to urea by a condensation of the salt [86]. Another compound which can be decomposed to yield NH3 without a catalyst is ammonium carbonate [87]. However, ammonium carbonate stores less NH3 per mass and volume, because it contains an additional molecule of water with respect to ammonium carbamate. In detail, ammonium carbamate stores 0.436 kg NH3/kg (0.698 kg/L), while ammonium carbonate stores 0.354 kg NH3/kg (0.531 kg/L). 

The decomposition of ammonium carbamate or ammonium carbonate in order to yield NH3 are both thought to proceed via a two-step reaction. For ammonium 

The decomposition of ammonium carbonate is considered to proceed via the stable ammonium bicarbonate intermediate [87]. 

In addition, the condensation of ammonium carbamate to yield urea and water can occur as the following side reaction [88]. 

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C and pressures up to 400atm., the ammonium carbamate formed initially largely dehydrating to urea. 

A similar reaction in which ammonia and carbon dioxide are heated under pres sure IS the basis of the industrial synthesis of urea Here the reactants first combine yielding a salt called ammonium carbamate... 

On being heated ammonium carbamate undergoes dehydration to form urea... 

Urea [57-13-6] was discovered ia urine by Rouelle ia 1773 and first synthesized from ammonia (qv) and cyanic acid by Woehler ia 1828. This was the first synthesis of an organic compound from an inorganic compound, and it dealt a deathblow to the vital-force theory. In 1870, urea was produced by heating ammonium carbamate ia a sealed tube. 

Urea can be considered the amide of carbamic acid, NH2COOH, or the diamide of carbonic acid, CO(OH)2. At room temperature, urea is colorless, odorless, and tasteless. Properties are shown ia Tables 1—4. Dissolved ia water, it hydrolyzes very slowly to ammonium carbamate (1) and eventually decomposes to ammonia and carbon dioxide (qv). This reaction is the basis for the use of urea as fertilizer (qv). 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

The formation of ammonium carbamate and the dehydration to urea take place simultaneously, for all practical purposes ... 

Reaction 1 is highly exothermic. The heat of reaction at 25°C and 101.3 kPa (1 atm) is ia the range of 159 kj/mol (38 kcal/mol) of soHd carbamate (9). The excess heat must be removed from the reaction. The rate and the equilibrium of reaction 1 depend gready upon pressure and temperature, because large volume changes take place. This reaction may only occur at a pressure that is below the pressure of ammonium carbamate at which dissociation begias or, conversely, the operating pressure of the reactor must be maintained above the vapor pressure of ammonium carbamate. Reaction 2 is endothermic by ca 31.4 kJ / mol (7.5 kcal/mol) of urea formed. It takes place mainly ia the Hquid phase the rate ia the soHd phase is much slower with minor variations ia volume. 

Table 5. Vapor Pressure of Pure Ammonium Carbamate at which Dissociation Begins...

The value has been extrapolated because, at temperatures above 170°C, the rate of reaction 2 rapidly iucreases and it is difficult to determine the carbamate vapor pressure owiag to the formation of water and urea and the consequent lowering of the partial pressure of ammonium carbamate. 

Ammonium Carbamate. Ammonium carbamate is a white crystalline soHd which is soluble ia water (2). It forms at room temperature by passiag ammonia gas over dry ice. In an aqueous solution at room temperature, it is slowly converted to ammonium carbonate, (NH2 2C02, by the... 

Table 6. Specific Heat of Solid Ammonium Carbamate...

Conversion at Equilibrium. The maximum urea conversion at equilibrium attainable at 185°C is ca 53% at infinite heating time. The conversion at equiUbtium can be increased either by raising the reactor temperature or by dehydrating ammonium carbamate in the presence of excess ammonia. Excess ammonia shifts the reaction to the right side of the overall equation ... 

The stream from the reactor consisting of a mixture of urea, unconverted ammonium carbamate, excess water, and NH, is fed into the top of the stripper. The ACES stripper utilizes a ferrite—austenite stainless steel, as do the carbamate condensers. The reactor and scmbber are constmcted with 316 L urea-grade stainless steel. 

The problem in reducing the NH and urea content in the wastewaters to below 100 ppm is because it is difficult to remove one in the presence of the other. The wastewater can be treated with caustic soda to volatilize NH. However, in a more efficient method, the urea is hydrolyzed to ammonium carbamate, which is decomposed to NH and CO2 the gases are then stripped from the wastewater. 

The technology of urea production is highly advanced. The raw materials requited ate ammonia and carbon dioxide. Invariably, urea plants ate located adjacent to ammonia production faciUties which conveniently furnish not only the ammonia but also the carbon dioxide, because carbon dioxide is a by-product of synthesis gas production and purification. The ammonia and carbon dioxide ate fed to a high pressure (up to 30 MPa (300 atm)) reactor at temperatures of about 200°C where ammonium carbamate [111-78-0] CH N202, urea, and water ate formed. 

The extremely toxic and flammable gas phosphine is safely and conveniently generated for the fumigation of grain in sacks or bins from 3-g tablets containing aluminum phosphide and ammonium carbamate which produce 1 g of phosphine in the presence of moisture. 

A simpler nonphosgene process for the manufacture of isocyanates consists of the reaction of amines with carbon dioxide in the presence of an aprotic organic solvent and a nitrogeneous base. The corresponding ammonium carbamate is treated with a dehydrating agent. This concept has been apphed to the synthesis of aromatic and aUphatic isocyanates. The process rehes on the facile formation of amine—carbon dioxide salts using acid haUdes such as phosphoryl chloride [10025-87-3] and thionyl chloride [7719-09-7] (30). 

Only about 10% of the total urea production is used for amino resins, which thus appear to have a secure source of low cost raw material. Urea is made by the reaction of carbon dioxide and ammonia at high temperature and pressure to yield a mixture of urea and ammonium carbamate the latter is recycled. 

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

Of major iadustrial importance is the reaction of ammonia and carbon dioxide giving ammonium carbamate [111 1-78-0], CH N202. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

The earliest mention of an ammonium carbonate, salt of hartshorn, appears in English manuscripts of the 14th century. As the name implies, the material was obtained by dry distillation of animal waste such as horn, leather, and hooves. Although many salts have been described in the Hterature for the ternary NH —CO2—H2O system, most, except for ammonium bicarbonate [1066-33-7], NH HCO, ammonium carbonate [506-87-6], (NH 2 02, and ammonium carbamate [1111-78-0], NH4CO2NH2, are mixtures (5,6). 

The acid chloiide (chloioformamide [463-72-9] "uiea chloiide"), NH2COCI, and its salts have been prepared. Ammonium carbamate [1111 -78-OJ can be obtained as a white crystalline sobd by reaction of dry carbon dioxide and ammonia. It is an impurity in commercial ammonium carbonate [506-87-6] (see Ammonium compounds). Esters of carbamic acid are quite stable. The best known is the ethyl ester usually called urethane [51 -79-6],... 

Carbon dioxide reacts with ammonia as the first stage of urea manufacture to form ammonium carbamate [1111-78-0]. 

The ammonium carbamate then loses a molecule of water to produce urea [57-13-6] CO(NH2)2- Commercially, this is probably the most important reaction of carbon dioxide and it is used worldwide ia the production of urea (qv) for synthetic fertilizers and plastics (see Amino resins Carbamic acid). 

This ammonia is recycled to the reactor via a compressor and a heater. Liquid ammonia is used as reflux on the top of the absorber. The net amount of carbon dioxide formed in the reactor is removed as bottom product from the absorber in the form of a weak ammonium carbamate solution, which is concentrated in a desorber-washing column system. The bottom product of this washing column is a concentrated ammonium carbamate solution which is reprocessed in a urea plant. The top product, pure ammonia, is Hquefted and used as reflux together with Hquid makeup ammonia. The desorber bottom product, practically pure water, is used in the quench system in addition to the recycled mother Hquor. 

Urea is prepared commercially by the reaction of liquid carbon dioxide and ammonia in silver-lined autoclaves, at temperatures in the range 135-195°C and pressure of 70-230 atm. The reaction proceeds by way of ammonium carbamate ... 

AMMO 2.5 EC , cypermetlu-in, 13 Ammonia, 13 Ammonium acetate, 13 Ammonium arsenate, 13 Ammonium benzoate, 13 Ammonium bicarbonate, 13 Ammonium bifluoride, 14 Ammonium bisulfite, 14 Ammonium carbamate, 14 Ammonium carbonate, 14 Ammonium chloride, 14 Ammonium chlorplatmate, 14 Ammonium clu omate, 14 Ammonium citrate, 14 Ammonium diclu omate, 14 Ammonium fluoride, 14 Ammonium fomiate, 15 Ammonium hexafluorosilicate, 15 Ammonium hydroxide, 15 Ammonium metavanadate, 15 Ammonium molybdate, 15 Ammonium nitrate, 15 Ammonium oxalate, 15 Ammonium perfluorooctanoate, 15 Ammonium persulfate, 15 Ammonium phosphate, 15 Ammonium picrate, 16 Ammonium salicylate, 16... 

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 ... 

The reaction occurs in two steps ammonium carbamate is formed first, followed by a decomposition step of the carbamate to urea and water. The first reaction is exothermic, and the equilibrium is favored at lower temperatures and higher pressures. Higher operating pressures are also desirable for the separation absorption step that results in a higher carbamate solution concentration. A higher ammonia ratio than stoichiometric is used to compensate for the ammonia that dissolves in the melt. The reactor temperature ranges between 170-220°C at a pressure of about 200 atmospheres. 

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