Carbamate

In organic chemistry, a carbamate is a category of organic compounds with the general formula R2NC(O)OR and structure >N−C(=O)−O−, which are formally derived from carbamic acid (NH2COOH). The term includes organic compounds (e.g., the ester ethyl carbamate), formally obtained by replacing one or more of the hydrogen atoms by other organic functional groups; as well as salts with the carbamate anion H2NCOO− (e.g. ammonium carbamate).

Polymers whose repeat units are joined by carbamate like groups −NH−C(=O)−O− are an important family of plastics, the polyurethanes.

Properties

While carbamic acids are unstable, many carbamate esters and salts are stable and well known.

Equilibrium with carbonate and bicarbonate

In water solutions, the carbamate anion slowly equilibrates with the ammonium NH+4 cation and the carbonate CO2−3 or bicarbonate HCO−3 anions:

H2NCO−2 + 2 H2O ⇌ NH+4 + HCO−3 + OH−

H2NCO−2 + H2O ⇌ NH+4 + CO2−3

Calcium carbamate is soluble in water, whereas calcium carbonate is not. Adding a calcium salt to an ammonium carbamate/carbonate solution will precipitate some calcium carbonate immediately, and then slowly precipitate more as the carbamate hydrolyzes.

Synthesis

Carbamate salts

The salt ammonium carbamate is generated by treatment of ammonia with carbon dioxide:

2 NH3 + CO2 → NH4[H2NCO2]

Carbamate esters

Carbamate esters also arise via alcoholysis of carbamoyl chlorides:

R2NC(O)Cl + R'OH → R2NCO2R' + HCl

Alternatively, carbamates can be formed from chloroformates and amines:

R'OC(O)Cl + R2NH → R2NCO2R' + HCl

Carbamates may be formed from the Curtius rearrangement, where isocyanates formed are reacted with an alcohol.

RCON3 → RNCO + N2

RNCO + R′OH → RNHCO2R′

Natural occurrence

Carbamylated proteins

Within nature carbon dioxide can bind with neutral amine groups to form a carbamate. This post-translational modification is known as carbamylation, and is known to occur on several important proteins.