CARBOXYLATION EQUILIBRIA 185 



(1929) and Hirsbrunner (1934) to reverse the decarboxylation of salicjdic, 

 gallic, and phloroglucin-carboxjdic acid have been unsuccessful. 



In alkaline solutions, where carbonic acid is present in the form of 

 anions, the carboxylation reaction becomes: 



(8.21) HCO,- + RH > RCOO- + H^O 



The free energy of reaction (8.21) is considerably less positive than that 

 of reaction (8.20), because carbonic acid is weaker than most carboxylic 

 acids. This explains why the decarboxjdation of formic acid in alkaline 

 solution is reversible. This reversibilit}^ was demonstrated by experi- 

 ments with biological catalysts {Escherichia coli, cf. page 208). However, 

 with acids much weaker than formic acid (for example, acetic acid), 

 not even the substitution of bicarbonate ions for carbon dioxide molecules 

 will suffice to make carboxylation thermodj'namically possible at low 

 temperatures and low partial pressures of carbon dioxide. 



Aromatic compounds (benzene, phenol, polyphenols) as well as 

 noncyclic, unsaturated compounds, whose free energies of carboxylation 

 in the acid range are less positive than those of the saturated aliphatic 

 compounds, can be expected to show negative free energies of carboxyla- 

 tion in alkaline media. It is well known that phenols can be carboxylated, 

 in the presence of alkali, at comparatively low temperatures (100-200° C.) 

 and low carbon dioxide pressures. (The usual method of preparation 

 of salicylic acid is by carboxylation of phenolate.) Ruben and Kamen 

 (1940) suggested that the presence in plants of polyphenols (of the type 

 of tannin and quercetin) may be of importance for the fixation of carbon 

 dioxide. However, it still remains to be demonstrated that carboxyla- 

 tions of this type can occur at the comparatively low pH values prevailing 

 in plant cells. 



In respiration, the elimination of carbon dioxide involves the decar- 

 boxylation of two a-keto acids, oxalacetic and pyruvic: 



(8.22) HOOC— CH2— CO— COOH > CH3— CO— COOH + CO2 



(8.23) CH3CO— COOH > CH3— CHO + CO2 



According to table 8. VIII, the decarboxylation of pyruvic acid is not 

 easily reversible (AF = — 15 kcal), not even in alkaline solution 

 (AF = — 3 kcal). Carson, Ruben, Kamen, and Foster (1941) tried un- 

 successfully to prove, by the use of radioactive carbon dioxide, the rever- 

 sion of this reaction in enzymatic systems. 



No data are available in standard compilations on the thermochemical 

 properties of oxalacetic acid. However, the decarboxylation of this acid 

 was studied by means of radioactive indicators by Carson, Foster, 

 Ruben, and Barker (1941), Wood, Werkman, Hemingway, and Nier 

 (1940, 1941), Krampitz and Werkman (1941), and Krampitz, Wood, and 



