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).
Equilibrium with carbonate and bicarbonate
- 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.
- 2 NH3 + CO2 → NH4[H2NCO2]
- R2NC(O)Cl + R'OH → R2NCO2R' + HCl
- R'OC(O)Cl + R2NH → R2NCO2R' + HCl
- RCON3 → RNCO + N2
- RNCO + R′OH → RNHCO2R′
Within nature carbon dioxide can bind with neutral amine groups to form a carbamate, this post-translational modification is known as carbamylation.This modification is known to occur on several important proteins; see examples below.
The N-terminal amino groups of valine residues in the α- and β-chains of deoxyhemoglobin exist as carbamates. They help to stabilise the protein when it becomes deoxyhemoglobin, and increases the likelihood of the release of remaining oxygen molecules bound to the protein. This stabilizing effect should not be confused with the Bohr effect (an indirect effect caused by carbon dioxide).
Urease and phosphotriesterase
The ε-amino groups of the lysine residues in urease and phosphotriesterase also feature carbamate. The carbamate derived from aminoimidazole is an intermediate in the biosynthesis of inosine. Carbamoyl phosphate is generated from carboxyphosphate rather than CO2.
CO2 capture by ribulose 1,5-bisphosphate carboxylase
Perhaps the most prevalent carbamate is the one involved in the capture of CO2 by plants. This process is necessary for their growth. The enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) fixes a molecule of carbon dioxide as phosphoglycerate in the Calvin cycle. At the active site of the enzyme, a Mg2+ ion is bound to glutamate and aspartate residues as well as a lysine carbamate. The carbamate is formed when an uncharged lysine side chain near the ion reacts with a carbon dioxide molecule from the air (not the substrate carbon dioxide molecule), which then renders it charged, and, therefore, able to bind the Mg2+ ion.
Synthesis of urea
Polyurethanes contain multiple carbamate groups as part of their structure. The "urethane" in the name "polyurethane" refers to these carbamate groups; the term "urethane links" describe how carbamates polymerize. In contrast, the substance commonly called "urethane", ethyl carbamate, is neither a component of polyurethanes, nor is it used in their manufacture. Urethanes are usually formed by reaction of an alcohol with an isocyanate. Commonly, urethanes made by a non-isocyanate route are called carbamates.
Polyurethane polymers have a wide range of properties and are commercially available as foams, elastomers, and solids. Typically, polyurethane polymers are made by combining diisocyanates, e.g. toluene diisocyanate, and diols, where the carbamate groups are formed by reaction of the alcohols with the isocyanates:
- RN=C=O + R′OH → RNHC(O)OR′
The so-called carbamate insecticides feature the carbamate ester functional group. Included in this group are aldicarb (Temik), carbofuran (Furadan), carbaryl (Sevin), ethienocarb, fenobucarb, oxamyl, and methomyl. These insecticides kill insects by reversibly inactivating the enzyme acetylcholinesterase (IRAC mode of action 1a). The organophosphate pesticides also inhibit this enzyme, although irreversibly, and cause a more severe form of cholinergic poisoning (the similar IRAC MoA 1b).
Carbamate nerve agents
While the carbamate acetylcholinesterase inhibitors are commonly referred to as "carbamate insecticides" due to their generally high selectivity for insect acetylcholinesterase enzymes over the mammalian versions, the most potent compounds such as aldicarb and carbofuran are still capable of inhibiting mammalian acetylcholinesterase enzymes at low enough concentrations that they pose a significant risk of poisoning to humans, especially when used in large amounts for agricultural applications. Other carbamate based acetylcholinesterase inhibitors are known with even higher toxicity to humans, and some such as T-1123 and EA-3990 were investigated for potential military use as nerve agents. However, since all compounds of this type have a quaternary ammonium group with a permanent positive charge, they have poor blood-brain barrier penetration, and also are only stable as crystalline salts or aqueous solutions, and so were not considered to have suitable properties for weaponisation.
Preservatives and cosmetics
Urethane (ethyl carbamate) was once produced commercially in the United States as a chemotherapy agent and for other medicinal purposes. It was found to be toxic and largely ineffective. It is occasionally used as a veterinary medicine.
In addition, some carbamates are used in human pharmacotherapy, for example, the acetylcholinesterase inhibitors neostigmine and rivastigmine, whose chemical structure is based on the natural alkaloid physostigmine. Other examples are meprobamate and its derivatives like carisoprodol, felbamate, mebutamate, phenprobamate, and tybamate, a class of anxiolytic and muscle relaxant drugs widely used in the 1960s before the rise of benzodiazepines, and still used nowadays in some cases. Carbachol is primarily used for various ophthalmic purposes.
Ephedroxane also fits the mould that it is from this group.
There are two oxygen atoms in a carbamate (1), ROC(=O)NR2, and either or both of them can be conceptually replaced by sulfur. Analogues of carbamates with only one of the oxygens replaced by sulfur are called thiocarbamates (2 and 3). Carbamates with both oxygens replaced by sulfur are called dithiocarbamates (4), RSC(=S)NR2.
There are two different structurally isomeric types of thiocarbamate:
- O-thiocarbamates (2), ROC(=S)NR2, where the carbonyl group (C=O) is replaced with a thiocarbonyl group (C=S)
- S-thiocarbamates (3), RSC(=O)NR2, where the R–O– group is replaced with an R–S– group
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