CARBON DIOXIDE ABSORPTION BY AMINES 181 



3. Carbon Dioxide Absorption by Amines 



Carbon dioxide addition to N— H bonds is similar to its addition to O — H bonds 

 in HOH and ROH, except that the basicity of the amines may lead to the formation 

 of carbamates (by the addition of a second molecule of amine to the primary formed 

 carhamic acids), for example: 



(8.15a) 0C=0 + RNH2 > OC— OH (R-carbamic acid) 



NHR 



(8.15b) RNH— COOH + RNH2 > RNH— COONH3R 



RNH— COO- + RNH3+ (R-carbamate) 



(8.15) 0C=0 + 2 RNH2 » RNH— COO- + RNH,+ 



Faurholt (1921, 1922, 1924, 1925) investigated these equilibria in aqueous solutions, 

 and found for the ratio: 



^«1R^ TC [RNHCO2-] 



(8.16) Knhr/oh = ^RNH2] [HCO3-] 



values of about 2 for ammonia, 165 for methylamine, 46 for dimethylamine and 32 for 

 glycine, that is, considerably larger than the corresponding ratios for alcohols. How- 

 ever, because of the basicity of the amines, the RNH2 molecules constitute (except in 

 very alkaline solutions) only a small proportion of the dissolved amine (most of it being 

 present as RNH3+ ions, which have no affinity for carbon dioxide). This restricts the 

 contribution of amines to the carbon dioxide absorption by aqueous solutions. Nether- 

 theless, in a one-molar solution of methylamine, at 18° C. (pH 8.9), 61% of the absorbed 

 carbon dioxide is present in the form of carbamate while 39% is present as HCOa" or 

 CO3 — ions. In less alkahne solutions, the extent of carbamination is much smaller; 

 solid carbamates decompose in pure water. 



Siegfried (190512), Siegfried and Neumaim (1908) and Siegfried and Liebermann 

 (1908) have studied quaUtatively the formation of carbamates in aqueous solutions of 

 amines, amino acids, peptones, and proteins, in the presence of alkali (calcium hydroxyde) 

 and Fichter and Becker (1911) have observed the formation of carbamate from gaseous 

 methyl amine and carbon dioxide at low temperatures. According to Siegfried, the 

 proximity of an oxidized group (e. g., carboxyl) favors the addition of carbon dioxide 

 to the amino group; thus, amino acids absorb carbon dioxide more eagerly than alkyl 

 amines. Siegfried suggested that the fixation of carbon dioxide by amino acids may be 

 of importance for photosynthesis. He claimed (1905^) that this reaction can occur even 

 in absence of alkaU. However, this conclusion, based on measurements of the increase 

 in the conductivity of glycocoU solutions by saturation with carbon dioxide, requires 

 confirmation. 



The carbamination equilibria of simple amino acids were again investigated by 

 Stadie and O'Brien (1936). Theyconfirmedthefact that the dipolar ions NHj^-RCOO-, 

 which predominate near the isoelectric point, do not unite with carbon dioxide at all; 

 this association is restricted to the anions NH2-RC00-, which predominate on the 

 alkaline side of the isoelectric point. The equihbrium: 



(8.17) NH2RCOO- + CO2 V 



RCOO- 



HOOCNHRCOO-;;— ^HN + H+ 



COO- 



