ENERGIES OF HYDROGENATION 



217 



Dismutations, isomerizations and hydrations do not change the over- 

 all reduction level of the reacting system and should therefore have 

 but small heat effects. However, because of the variations in the 

 strength of C — O bonds mentioned above, the energies of some dismu- 

 tations and isomerizations reach — 10, or even — 20 kcal, as illustrated 

 by table 9. III. The same effect explains why the energy of carhoxylation 

 (RH + CO2 > RCOOH) is almost zero {cf. Table 8.III), instead 



Table 9.III 



Total Energies, AH, and Free Energies, AF, of Some Dismutations, 



Isomerizations and Hydrations 



' Hydration of — C =0 to — C(0H)2 is also known to occur with a very small thermochemical effect, 

 so that a carbonyl group in aqueous solution often reacts as a dihydroxyl group. 



of being negative (as it should be because of the greater strength of the 

 O — H bonds compared to C — H bonds). 



These considerations do not apply to free radicals, whose dismutation 

 into saturated molecules may liberate very large amounts of energy 

 (cf. section 6). 



The heats of polymerization of carbohydrates can be derived from 

 table 3.V, and are very small. 



3. Energies of Hydrogenation and Oxidation-Reduction Potentials 



According to table 9. II, the energy of hydrogenation of a standard, 

 nonconjugated C=C double bond by molecular hydrogen is —30 kcal 

 and that of a C=0 bond, —17 kcal, while the hydrogenation of a C — C 

 single bond should liberate 15 kcal. Examples 1, 2, 3, 4, 8, 9, 10, 12, 

 and 13 in table 9. IV illustrate the approximate validity of these estimates. 



