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Chemical structure of carbamates

Carbamates are organic compounds derived from carbamic acid (NH2COOH). A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.

Contents

Synthesis

Carbamic acids are derived from amines:

R2NH + CO2 → R2NCO2H

Carbamic acid is about as acidic as acetic acid. Ionization of a proton gives the carbamate anion, the conjugate base of carbamic acid:

R2NCO2H → R2NCO2- + H+

Carbamates also arise via hydrolysis of chloroformamides:

R2NC(O)Cl + H2O → R2NCO2H + HCl

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

RNCO + R'OH → RNHCO2R'

Applications and occurrence

Although most of this article concerns organic carbamates, the inorganic salt ammonium carbamate is produced on a large scale as an intermediate in the production of the commodity chemical urea from ammonia and carbon dioxide.

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Carbamates in biochemistry

N-terminal amino groups of valine residues in the α- and β-chains of deoxyhemoglobin exist as carbamates. They help to stabilize the protein, when it becomes deoxyhemoglobin and increases the likelihood of the release of remaining oxygen molecules bound to the protein. The influence of these carbamates on the affinity of hemoglobin for O2 is called the Bohr effect.

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 fromcarboxyphosphate rather than CO2.[1]

CO2 capture by RuBisCO

Perhaps the most important carbamate is the one involved in the capture of CO2 by plants since this process is relevant to global warming. The enzyme Ribulose 1,5-bisphosphate carboxylase/oxygenase fixes a molecule of carbon dioxide as a carbamate at the start of 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.

Commercial carbamate compounds

Carbamate insecticides

The carbamate insecticide Carbaryl.

The so-called carbamate insecticides feature the carbamate ester functional group. Included in this group are aldicarb, carbofuran, Furadan, fenoxycarb, carbaryl (Sevin), ethienocarb, and fenobucarb. These insecticides kill insects by reversibly inactivating the enzyme acetylcholinesterase. The organophosphate pesticides also inhibit this enzyme, although irreversibly, and cause a more severe form of cholinergic poisoning.[2]

The insect repellent icaridin is a substituted carbamate.

Polyurethanes

Polyurethanes contain multiple carbamate groups as part of their structure. The term "urethane" is not a component of polyurethanes. These polymers have a wide range of properties and are commercially available as foams, elastomers, and solids. They arise by the reaction of alcohols with isocyanates:

RN=C=O + R'OH → RNHC(O)OR'

Polymers result by combining diisocyanates, e.g. toluene diisocyanate , and diols, e.g. bisphenol-A.

In human medicine

Urethane or ethyl carbamate was once produced commercially in the United States as an antineoplastic agent and for other medicinal purposes. It was found to be toxic and largely ineffective.[3] occasionally used as a veterinary medicine.

In addition, some carbamates are used in human pharmacotherapy, for example, the cholinesterase inhibitors neostigmine and rivastigmine, whose chemical structure is based on the natural alkaloid physostigmine. Other examples are meprobamate and its derivatives like carisoprodol, felbamate, and tybamate, a class of anxiolytic and muscle relaxant drugs widely used in the 60s before the rise of benzodiazepines, and still used nowadays in some cases.

References

  1. ^ Bartoschek, S.; Vorholt, J. A.; Thauer, R. K.; Geierstanger, B. H. and Griesinger, C., "N-Carboxymethanofuran (carbamate) formation from methanofuran and CO2 in methanogenic archaea : Thermodynamics and kinetics of the spontaneous reaction", Eur. J. Biochem., 2001, 267, 3130-3138. doi:10.1046/j.1432-1327.2000.01331.x
  2. ^ Robert L. Metcalf “Insect Control” in Ullmann’s Encyclopedia of Industrial Chemistry” Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a14_263
  3. ^ Holland JR, Hosley H, Scharlau C, Carbone PP, Frei E 3rd, Brindley CO, Hall TC, Shnider BI, Gold GL, Lasagna L, Owens AH Jr, Miller SP (1 March 1966). "A controlled trial of urethane treatment in multiple myeloma" (free fulltext). Blood 27 (3): 328–42. ISSN 0006-4971. PMID 5933438. http://bloodjournal.hematologylibrary.org/cgi/content/abstract/27/3/328. 

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