ENERGIES OF HYDROGENATION 



221 



carboxyl group iti vivo, by noting that the reduction of bicarbonate to 

 formate, in neutral solution, requires a potential in excess of 0.44 volt, 

 and the reduction of formate to formaldehyde (also in aqueous solution), 

 a potential of at least 0.53 volt. 



Of all the systems listed in table 9. IV, only one (benzene-cyclo- 

 hexadiene) has a more positive potential than the carbonyl-carboxyl 

 system. Thus, a reductant whose reduction creates an aromatic system 

 should be thermodynamically able to reduce bicarbonate or carboxyl 

 even without the help of Hght. 



While C=C and C=0 double bond compounds are important in 

 biochemistry as substrates of metabolic oxidation-reduction processes, 

 other systems are of greater importance as intermediary oxidation- 

 reduction catalysts, for example, the quinonoid-benzenoid systems, the 

 pyridine-pyridinium systems, the sulfhydryl-sulfide systems and the 

 ferri-ferro complex systems. 



The property which makes all these compounds catalytically active, 

 is their capacity for reversible reduction (or oxidation), i. e. their property 

 of being reduced (or oxidized) at room temperature by any other (equally 

 reversible) reductant (or oxidant) of more positive (or more negative) 

 potential. This reversibility is often associated with electrode activity, 

 i. e., capacity of being oxidized (or reduced) by an inert electrode of 

 appropriate potential. 



Much confusion is caused by the twofold use of the term "reversi- 

 bility." In addition to its use in the sense defined above, the adjective 

 "reversible" is often used to mean that a reaction can actually be made 

 to proceed in both directions. In the first sense, reversibility is a kinetic 

 term (meaning absence of a large activation energy) ; in the second sense, 

 it is a static term, meaning that the free energy of a reaction is so small 

 that a moderate change in temperature or concentration, is sufficient to 

 cause it to change its sign. Catalysts can improve kinetic reversibility, 

 but cannot affect the static reversibility. 



In the quinonoid-benzenoid systems, the reductant (e. g., hydroquinone) has an 

 aromatic structure, while the oxidant (e. g. benzoquinone) is stabilized by a different 



o 



type of resonance, (which accounts for the dyestuff character of quinonoid compounds, 

 cf. Vol. II, Chapter 21, and is illustrated by the structures A, B, and C). 



