Ammonium carbamate

Ammonium carbamate

Names
IUPAC name Ammonium carbamate
Other names hartshorn, sal volatile, ammonium amidocarbonate, ammonium aminoformate,
Identifiers
CAS Number	1111-78-0 Y
3D model (JSmol)	Interactive image
ChemSpider	451267
ECHA InfoCard	100.012.896
EC Number	214-185-2
Gmelin Reference	14637 (G)
PubChem CID	517232
RTECS number	EY8575000
UNII	I2W9615SWP Y
UN number	9083
CompTox Dashboard (EPA)	DTXSID9027360
InChI InChI=1S/CH3NO2.H3N/c2-1(3)4;/h2H2,(H,3,4);1H3
SMILES [O-]C(=O)N.[NH4+]
Properties
Chemical formula	[NH4]NH2CO2
Molar mass	78.071 g·mol−1
Appearance	Colorless, rhombic crystals
Density	1.38 g/cm3 (20 °C)
Melting point	60 °C (140 °F; 333 K) decomposes
Solubility in water	Freely soluble in water
Solubility	Soluble in ethanol, methanol, liquid ammonia, formamide
log P	−0.47 in octanol/water
Vapor pressure	492 mmHg(51 °C)
Thermochemistry
Std enthalpy of formation (ΔfH⦵298)	-642.5 kJ/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Harmful if ingested, harmful to aquatic life, harmful if inhaled, respiatory tract irritation, skin irritation, eye irritation
GHS labelling:
Pictograms
Signal word	Warning
NFPA 704 (fire diamond)	1 0 0
Flash point	105.6 °C (222.1 °F; 378.8 K)
Lethal dose or concentration (LD, LC):
LD50 (median dose)	1,470 mg/kg in a rat
Safety data sheet (SDS)	External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Ammonium carbamate is a chemical compound with the formula [NH₄][H₂NCO₂] consisting of ammonium cation NH+4 and carbamate anion NH₂COO⁻. It is a white solid that is extremely soluble in water, less so in alcohol. Ammonium carbamate can be formed by the reaction of ammonia NH₃ with carbon dioxide CO₂, 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.

Properties

Solid-gas equilibrium

In a closed container solid ammonium carbamate is in equilibrium with carbon dioxide and ammonia

[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.

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. 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. 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.

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:

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):

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).

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.

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. in this reactor and can then be dehydrated to urea according to the following equation:

[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.

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.

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⁺. 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.