Carbamic acid


Carbamic acid is the chemical compound with the formula H2NCOOH. It can be obtained by the reaction of ammonia NH3 and carbon dioxide CO2 at very low temperatures, which also yields an equal amount of ammonium carbamate. The compound is stable only up to about 250 K ; at higher temperatures it decomposes into those two gases. The solid apparently consists of dimers, with the two molecules connected by hydrogen bonds between the two carboxyl groups –COOH.
Carbamic acid could be seen as both an amine and carboxylic acid, and therefore an amino acid; however, the attachment of the acid group directly to the nitrogen atom makes it behave very differently from those classes.. The hydroxyl group attached to the carbon also excludes it from the amide class.
The term "carbamic acid" is also used generically for any compounds of the form RHNCOOH or RR′NCOOH, where R and R′ are organic groups.
Deprotonation of a carbamic acid yields a carbamate anion RR′NCOO−. This term is also used for esters of carbamic acids, such as methyl carbamate H2NCOCH3. The carbamoyl functional group RR′NC– is the carbamic acid molecule minus the OH part of the carboxyl.

Structure

Carbamic acid is a planar molecule.
The H2N– group of carbamic acid, unlike that of most amines, cannot be protonated to an ammonium group H3N+–. The zwitterionic form H3N+–COO− is very unstable and promptly decomposes into ammonia and carbon dioxide, yet there is a report of its detection in ices irradiated with high-energy protons.

Derivatives

Carbamic acid is formally the parent compound of several important families of organic compounds:

Carbamic acids

Many substituted carbamic acids, can be readily synthesized by bubbling carbon dioxide through solutions of the corresponding amine in an appropriate solvent, such as DMSO or supercritical carbon dioxide. These carbamic acids are generally unstable at room temperature, reverting to the parent amine and carbon dioxide.

Carbamate esters

Unlike carbamic acids, carbamate esters are generally stable at room temperature. They are prepared by reaction of carbamoyl chlorides with alcohols, the addition of alcohols to isocyanates, and the reaction of carbonate esters with ammonia.

Occurrence in nature

The enzyme carbamate kinase, involved in several metabolic pathways of living organisms, catalyzes the formation of carbamoyl phosphate CO
One hemoglobin molecule can carry four molecules of carbon dioxide to the lungs as carbamate groups formed by reaction of with four terminal amine groups of the deoxy form. The resulting compound is called carbaminohaemoglobin.

Uses

Industrial

Carbamic acid is an intermediate in the industrial production of urea, which involves the reaction of carbon dioxide and ammonia.

Medical

Some carbamate esters have use as muscle relaxants which bind to the barbiturate site of the GABAA receptor, while others are used as insecticides, for example aldicarb.

Chemical synthesis

An amine functional group –NH2 can be protected from unwanted reactions by being formed as carbamate ester residue –NHC–OR. Hydrolysis of the ester bond then produces a carbamic acid –NHCOH, which then loses carbon dioxide yielding the desired amine.