Ammonium carbamate

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]

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).^[12]

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.^[12]

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.^[13] in this reactor and can then be dehydrated to urea according to the following equation:^[12]

[NH₂CO₂][NH₄] → (NH₂)₂CO + H₂O

Pesticide formulations

Ammonium carbamate is an inert ingredient in some formulations of the pesticide aluminium phosphide (AlP). Ammonium carbamate make the phosphine less flammable released by AlP by freeing ammonia and carbon dioxide to dilute the phosphine.^[14]

Laboratory

Ammonium carbamate can be used as a ammoniating agent. It has been used to preparation substituted β-amino-α,β-unsaturated esters.^[15]

Ammonium carbamate reacts with potassium chloride KCl in liquid ammonia to give potassium carbamate NH₂CO−2K⁺.^[2] Carbamates of other metals, such as calcium, can be produced similarly in anhydrous solvents such as methanol, ethanol, or formamide, even at room temperature.^[3]