Ammonium carbamate is a chemical compound with the formula [NH4][H2NCO2] consisting of ammoniumcationNH+4 and carbamateanionNH2COO−. It is a white solid that is extremely soluble in water, less so in alcohol. Ammonium carbamate can be formed by the reaction of ammoniaNH3 with carbon dioxideCO2, and will slowly decompose to those gases at ordinary temperatures and pressures. It is an intermediate in the industrial synthesis of urea(NH2)2CO, an important fertilizer.[4]
Properties
Solid-gas equilibrium
In a closed container solid ammonium carbamate is in equilibrium with carbon dioxide and ammonia [5][6][7]
[NH4][NH2CO2] ⇌ 2 NH3 + CO2
Lower temperatures shift the equilibrium towards the carbamate.
At higher temperatures ammonium carbamate condenses into urea:
[NH4][NH2CO2] → (NH2)2CO + H2O
This reaction was first discovered in 1870 by Bassarov, by heating ammonium carbamate in sealed glass tubes at temperatures ranging from 130 to 140 °C.[6]
Equilibrium in water
At ordinary temperatures and pressures, ammonium carbamate exists in aqueous solutions as an equilibrium with ammonia and carbon dioxide, and the anions bicarbonate, HCO−3, and carbonate, CO2−3.[8][6][9] Indeed, solutions of ammonium carbonate or bicarbonate will contain some carbamate anions too.
H2NCO−2 + 2H2O ⇌ NH+4 + HCO−3 + OH−
H2NCO−2 + H2O ⇌ NH+4 + CO2−3
Structure
The structure of solid ammonium carbamate has been confirmed by X-ray crystallography. The oxygen centers form hydrogen bonds to the ammonium cation.[10] There are two polymorphs, α and β, both in the orthorhombic crystal system but differing in their space group. The α polymorph is in space group Pbca (no. 61), whereas the β polymorph is in Ibam (no. 72). The α polymorph is more volatile.[11]
Natural occurrence
Ammonium carbamate serves a key role in the formation of carbamoyl phosphate, which is necessary for both the urea cycle and the production of pyrimidines. In this enzyme-catalyzed reaction, ATP and ammonium carbamate are converted to ADP and carbamoyl phosphate:[12][13]
ATP + [NH2CO2][NH4] → ADP + H2NC(O)OPO2−3
Preparation
From liquid ammonia and dry ice
Ammonium carbamate is prepared by the direct reaction between liquid ammonia and dry ice (solid carbon dioxide):[5]
2 NH3 + CO2 → [NH2CO2][NH4]
From gaseous ammonia and carbon dioxide
Ammonium carbamate can be prepared by reaction of the two gases at high temperature (175–225 °C) and high pressure (150–250 bar).[14]
It can also be obtained by bubbling gaseous CO2 and NH3 in anhydrous ethanol, 1-propanol, or DMF at ambient pressure and 0 °C. The carbamate precipitates and can be separated by simple filtration, and the liquid containing the unreacted ammonia can be returned to the reactor. The absence of water prevents the formation of bicarbonate and carbonate, and no ammonia is lost.[14]
Uses
Urea synthesis
Ammonium carbamate is an intermediate in the industrial production of urea.
A typical industrial plant that makes urea can produce up to 4000 tons a day.[15]
in this reactor and can then be dehydrated to urea according to the following equation:[14]
[NH2CO2][NH4] → (NH2)2CO + H2O
Pesticide formulations
Ammonium carbamate has also been approved by the Environmental Protection Agency as an inert ingredient present in aluminium phosphide pesticide formulations. This pesticide is commonly used for insect and rodent control in areas where agricultural products are stored. The reason for ammonium carbamate as an ingredient is to make the phosphine less flammable by freeing ammonia and carbon dioxide to dilute phosphine formed by a hydrolysis reaction.[16]
Laboratory
Ammonium carbamate can be used as a good ammoniating agent, though not nearly as strong as ammonia itself. For instance, it is an effective reagent for preparation of different substituted β-amino-α,β-unsaturated esters. The reaction can be carried out in methanol at room temperature and can be isolated in the absence of water, in high purity and yield.[17]
Preparation of metal carbamates
Ammonium carbamate can be a starting reagent for the production of salts of other cations. For instance, by reacting it with solid potassium chloride KCl in liquid ammonia one can obtain potassium carbamate NH2COO−K+.[2] Carbamates of other metals, such as calcium, can be produced by reacting ammonium carbamate with a suitable salt of the desired cation, in an anhydrous solvent such as methanol, ethanol, or formamide, even at room temperature.[3]
^ abcK. G. Clark; V. L. Gaddy; C. E. Rist (October 1933). "Equilibria in the Ammonium Carbamate-Urea-Water System". Industrial and engineering chemistry. 25 (10): 1092–1096. doi:10.1021/IE50286A008. ISSN0019-7866. WikidataQ59410838.
^R. N. Bennett, P. D. Ritchie, D. Roxburgh and J. Thomson (1953): "The system ammonia + carbon dioxide + ammonium carbamate. Part I. — The equilibrium of thermal dissociation of ammonium carbamate". Transactions of the Faraday Society, volume 49, pages 925-929. doi:10.1039/TF9534900925
^George H. Burrows and Gilbert N. Lewis (1912): "The equilibrium between ammonium carbonate and ammonium carbamate in aqueous solution at 25°". Journal of the American Chemical Society, volume 34, issue 8, pages 993-995. doi:10.1021/ja02209a003
^Fabrizio Mani, Maurizio Peruzzini, and Piero Stoppioni (2006): "CO2 absorption by aqueous NH3 solutions: speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study". Green Chemistry, volume 8, issue 11, pages 995-1000. doi:10.1039/B602051H
^ abcFrancesco Barzagli; Fabrizio Mani; Maurizio Peruzzini (2011). "From greenhouse gas to feedstock: formation of ammonium carbamate from CO2 and NH3 in organic solvents and its catalytic conversion into urea under mild conditions". Green Chemistry. 13 (5): 1267–1274. doi:10.1039/C0GC00674B. ISSN1463-9262. WikidataQ59410840.
^Mladen Litvić, Mirela Filipan, Ivan Pogorelić and Ivica Cepanec (2005): "Ammonium carbamate; mild, selective and efficient ammonia source for preparation of β-amino-α,β-unsaturated esters at room temperature". Green Chemistry, volume 7, issue 11, pages 771-774. doi:10.1039/B510276F
