Carbamic acids

General structure of carbamic acids

Carbamic acids are the monoamides of carbonic acid .

The carbamate is the simplest, because N -unsubstituted (R ¹ = R ² = H) carbamic acid. Occasionally the outdated term carbamic acid is also used. Esters and salts of carbamic acids are called carbamates ; the esters are often still referred to as urethanes .

The general formula for carbamic acids is R ₂ N-COOH, where R = H, alkyl or aryl and the two radicals R can be identical or different.

Manufacturing

Monosubstituted carbamic acids are easily formed from isocyanates and water:

${\displaystyle \mathrm {R{-}NCO+H_{2}O\longrightarrow R{-}NHCOOH} }$

As a rule, carbamic acids break down into an amine and carbon dioxide :

${\displaystyle \mathrm {R{-}NHCOOH\longrightarrow R{-}NH_{2}+CO_{2}} }$

In the production of foams from polyurethanes , this reaction is used to foam the plastic; In the case of coatings , however, this is undesirable and must be avoided by means of suitable formulation.

Carbamic acids are also formed when amines react with carbon dioxide:

${\displaystyle \mathrm {R_{2}NH+CO_{2}\longrightarrow R_{2}NCOOH} }$

The acid reacts with excess amine to form the corresponding carbamate, so that the reverse reaction is prevented. Ammonium carbamate is created from ammonia and carbon dioxide . In technical applications, such as curing epoxy resins with amines, the formation of carbamates can lead to problems - here the reaction with carbon dioxide from the air leads to sticky surfaces and white discoloration.

properties

In the free form, carbamic acids are mostly, but not always, unstable. That depends strongly on the residues R and also on the pH value. Electron-withdrawing radicals that tend to stabilize mesomerism (e.g. 4-nitrophenyl) reduce the basicity of nitrogen and stabilize the carbamic acid concerned. Even more sterically demanding residues (than methyl) stabilize the molecule.

There are several examples of carbamic acids, which are so stable in the condition that they are bottled in the chemicals are commercially available, for example Butylmethylcarbaminsäure , 1,4-Piperazindicarbonsäure , benzimidazole N -carboxylic acid , proline N -carboxylic acid . Even with only one residue on nitrogen, they can be stable, e.g. B. ( o -Tolyl) carbamic acid, or 3- pyridyl carbamic acid .

The anions are often surprisingly stable since the nitrogen is then not protonated. Since the CN bond can preferably be broken when the nitrogen is protonated and thus positively charged (keyword: good leaving group, analogous to the so-called Hofmann elimination ), a carbamic acid decomposes particularly easily in an acidic medium to form R ₂ NH and CO ₂ .

Carbamic acids occur as intermediate products in acid amide degradation according to Hofmann , in acid hydrazide degradation according to Curtius and in acid azide degradation according to Schmidt . In the process, carbon dioxide is always split off and a primary amine is formed.

The transfer of carbon dioxide by carboxylases with biotin as a prosthetic group also takes place via a carbamic acid as an intermediate product.

Individual evidence

1. ↑ Entry on urethanes. In: Römpp Online . Georg Thieme Verlag, accessed on June 15, 2014.
2. ↑ ^{a b} Kittel: Textbook of paints and coatings , 2nd ed., Volume 2: Binder for solvent-based and solvent-free systems, Ed. Walter Krauß, s. Hirzel Verlag 1998, ISBN 3-7776-0886-6 .
3. ↑ Beyer-Walter: Textbook of Organic Chemistry , 23rd edition, S. Hirzel Verlag 1998.
4. ^ Römpp CD 2006, Georg Thieme Verlag 2006.