DEFINITION OF CARBOXYLATION 187 



5. Is Carboxylation a Reduction of Carbon Dioxide? 



It is customary to speak of "reduction of carbon dioxide" whenever 

 a carbon dioxide molecule is incorporated into an organic compound 

 with the formation of a new C — C bond. This practice leads to mis- 

 understandings when processes of this kind are put on the same level 

 with the reduction of carbon dioxide in photautotrophic and chem- 

 autotrophic organisms. It is necessary to distinguish clearly between 

 the two types of reactions involving carbon dioxide: reversible additions 

 (e. g., the addition of CO2 to RH, leading to the substitution of C — C 

 bonds for C — bonds) ; and true reductions (characterized by the creation 

 of new C — H bonds). Whether carboxylation should be called a "reduc- 

 tion" of carbon dioxide at all is a matter of convention. The definition 

 of the word reduction is unambiguous only in the case of intermolecular 

 oxidation-reductions, in which the reaction partners exchange electrons 

 (or hydrogen atoms) and then separate, one having experienced oxidation 

 and the other reduction. If the reaction partners remain Unked together, 

 the definition becomes vague. To find out whether an oxidation- 

 reduction has occurred, one may consider the positions of electrons or 

 hydrogen atoms before and after the reaction. By this criterion, a 

 carboxylation: 



(8.27) RH + CO2 > RCOOH 



could be considered as the oxidation of the organic radical R and reduc- 

 tion of carbon dioxide, since it involves the shift of a hydrogen from 

 RH to CO2. However, by the same token, one could also describe the 

 hydration of carbon dioxide: 



(8.28) HOH + CO2 > HOCOOH 



as a reduction of carbon dioxide and oxidation of water. Both in (8.27) 

 and (8.28), the hydrogen is transferred to oxygen (in the carbon dioxide); 

 because of the high aflfinity of oxygen for hydrogen, this requires no 

 supply of energy. A true reduction of carbon dioxide (as defined above) 

 would require the shift of hydrogen to carbon. If, after such a shift 

 (from RH or H2O to CO2) the products remain united in a single mole- 

 cule, the results will be: 



(8.2 ) RH + CO2 > ROCHO and 



(8.30) HOH + CO2 > HOOCHO 



that is, the formation of Sifor7nic acid ester and performic acid, respectively. 

 This would constitute a true reduction of carbon dioxide (and oxidation 



