ENERGIES OF HYDROGENATION 219 



As stated above, the C — O bond is stabilized in the carboxyl group 

 and still more in carbon dioxide. As a result, the energy of hydro- 

 genation of C^O bonds in carboxylic acids and in free carbon dioxide 

 is positive (c/. examples 14a, 15b, 16a). Carbon-carbon double bonds 

 also can be stabilized by resonance, which is commonly brought about 

 by a conjugation of several such bonds. The effect on the energy of 

 hydrogenation is illustrated by examples 5, 6 and 7. Conjugation be- 

 tween a C=0 and a C=C bond has no marked effect on the hydro- 

 genation energy of the former (c/. examples 12 and 13). (A stabilizing 

 effect of this conjugation on the carboxyl group was noticed on page 184.) 

 As a result of resonance stabilization, the molecules CeHe and CO2 are 

 more difficult to hydrogenate than all other compounds in table 9. IV. 



The thermodynamic measure of oxidizing power is the free energy, 

 rather than the total energy of hydrogenation. Table 9. IV shows that 

 AF of hydrogenation by molecular hydrogen usually is more positive 

 than the total energy of the same process, by as much as 5 or 10 kcal, 

 because this reaction is associated with the disappearance of a gas (H2). 

 Large deviations from this rule may occur in the hydrogenation of ions 

 (in system No. lb, for example, an increase in entropy is .caused by the 

 conversion of one divalent ion into two monovalent ions). 



Instead of the free energy of hydrogenation, the oxidizing power often 

 is characterized by the oxidation-reduction potential, Eq. These poten- 

 tials can be measured directly only for "electrode active" systems, i. e., 

 for compounds which can be reduced or oxidized electrochemically at a 

 reversible electrode. Values measured in this way are shown by italics 

 in table 9. IV. For all other oxidation-reduction systems, the oxidation- 

 reduction potentials can be calculated from the free energy of hydro- 

 genation, AF, by means of the relation 



(9.2) Eo = AF/23.06 n (pH = 0) 



23.06 is the factor which converts electron-volts into kcal /mole, while 

 the factor n {n = 2 for all examples in Table 9. IV) is the number of elec- 

 trons which take part in the transformation. 



This may be the appropriate place for a remark on the relation between hydro- 

 genation, o.xygenation, and electron transfer in oxidation-reduction reactions. In 

 chapter 3, photosynthesis was described as hydrogen transfer from water to carbon 

 dioxide, and in chapter 5, a similar definition was applied to bacterial photosynthesis. 

 Some bacterial reductants (e. g., sulfur) do not contain any hydrogen, while others 

 (e. g., sulfite) are unUkely to yield it. It is easy to show that electron transfer, combined 

 with acid-base equihbria, is equivalent to hydrogen transfer: 



(9.3a) A + B- > A- + B 



(9.3b) A- + H+ > AH 



(9.3c) BH > B- + H+ 



(9.3) A + BH > AH -I- B 



