Ammonium carbamate

In a closed container solid ammonium carbamate is in equilibrium with carbon dioxide and ammonia [4][5][6]

NH₂CO₂NH₄ ⇌ 2NH₃ + CO₂

Lower temperatures shift the equilibrium towards the carbamate.

At higher temperatures ammonium carbamate condenses into urea:

NH₂CO₂NH₄ → (H
₂N)
₂C=O + H
₂O

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.[5]

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 CO₃,[7][5][8] Indeed, solutions of ammonium carbonate or bicarbonate will contain some carbamate anions too.

H
₂NCOO⁻
+ 2 H
₂O ⇌ NH⁺
₄ + HCO⁻
₃ + HO⁻

2 H
₂NCOO⁻
+ 2 H
₂O ⇌ 2 NH⁺
₄ + CO²⁻
₃

The structure of solid ammonium carbamate has been confirmed by X-ray crystallography. The oxygen centers form hydrogen bonds to the ammonium cation.[9]

Ammonium carbamate is prepared by the direct reaction between liquid ammonia and dry ice (solid carbon dioxide):[4]

2 NH₃ + CO₂ → H₂NCOONH₄

It is also prepared by reaction of the two gases at high temperature (450–500 K) and high pressure (150–250 bar).[12]

Ammonium carbamate can also be obtained by bubbling gaseous CO
₂ and NH
₃ 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.[12]

Ammonium carbamate is an intermediate in the industrial production of urea. A typical industrial plant that makes urea can produce up to 1500 tons a day. in this reactor and can then be dehydrated to urea according to the following equation:[12]

NH₂CO₂NH₄ → NH₂CONH₂ + H₂O

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 hydrogen phosphine formed by a hydrolysis reaction.[13]

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.[14]

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 K⁺
(NH
₂)COO⁻
.[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]