180 FIXATION OF CARBON DIOXIDE CHAP. 8 



which leads to carbonic acid esters, is overshadowed in the presence of alcoholates, by 

 the ionic equihbrium 



(8.11) 0C=0 + RO- > OC— O- 



)R 



L 



which is analogous to reaction (8.5) in water. Faurholt (1927) estimated, for example, 

 that the two carbon dioxide methanolation constants are (at 0° C): 



(8.12) Knon = ^?^?^ - 0.01; AFroh = + 2.7 kcal 



(as against KuiO — 0.001 in water), and 



(8.13) Kro = [^cSkRO"] " ^ ^ ■^°'' ^^°" ^ ~ ^'^ ^^^^ 



(the corresponding value for water is Kqh = 4 X 10^) 



Since the dissociation of methanol is much weaker than that of water 

 ([RO-][H+] ^ 10-"), neither (8.10) nor (8.11) can contribute more than one per cent 

 to the solubility of carbon dioxide in pure methanol; but in sodium alcoholate solutions, 

 with their high concentration of RO" ions, carbon dioxide is eagerly absorbed with the 

 formation of RCO3- ions. 



QuaUtatively, the absorption of carbon dioxide by alcohols in the presence of alkali 

 was known for a long time. It was studied by Siegfried and Howwjanz (1909), who 

 observed the absorption of 0.2 to 1 mole of carbon dioxide by one mole of methanol, 

 ethanol, glycol, glycerol, erythrol, quercitol, lactose, sucrose, and lactic acid, in the 

 presence of calcium hydroxyde. In aqueous alcohol, carbon dioxide becomes an object 

 of competition between water and alcohol. The resulting equilibria have been studied 

 by Faurholt il927^^'^) and Faurholt and Jespersen (1933), for methanol, ethanol, 

 propanol and sucrose. They found that the contribution of alcohols to the absorption 

 of carbon dioxide in alcohol-water mixtures is comparatively small and disappears on 

 both side of the "bicarbonate range" (pH around 8.4) because of the decomposition of 

 the carbonic acid esters (into CO2 and alcohol on the acid side, and into CO3 — and 

 alcohol on the alkaline side of this range). The ratio: 



<«•") ^-'- - [hco.-]Troh] 



is 0.083 for methanol, 0.044 for ethanol (at 0° C), and even smaller for sucrose. Con- 

 sequently, the proportion of carbon dioxide bound to alkyl in a molar methanol solution 

 is only 7.4% at pH 8-9. 



Without reference to Faurholt's quantitative results, Baur and Namek (1940) 

 made some rough qualitative observations concerning the carbon dioxide absorption by 

 alcohols. In addition to the rapid gas uptake by dissolution, they noticed a slow 

 absorption which they attributed to esterification. For the extent of this chemical 

 binding, they gave values between 18 ml. of carbon dioxide per mole of ethanol 

 (8 X lO—* mole/1.) which is many times more than one would expect from Faurholt's 

 equihbrium constants, and 470 ml. (0.02 mole/1.) per mole of phytol. Since phytol is a 

 component of chlorophyll, Baur considered this result as significant for the carbon dioxide 

 fixation in photosynthesis. 



