Hydrogen enthalpies

In this equation the —225.7 1.3 kJ mol 1 is the hydrogenation enthalpy of 1,3-butadiene to n-butane. This last expression speaks to substituent/diene interactions and to substituent-substituent interactions. Both electronic and steric effects contribute. Again, this allows calibration of a substituted diene with 1,3-butadiene itself. A positive sign can be interpreted as the substituted species being more stabilized than the archetype. 

From a consistent set of hydrogenation enthalpies in glacial AcOH, the cyclooctadi-enes decrease in stability 1,5- (29) < 1, 4- (56) < 1,3- (55a) with sequential differences of 13.0 (29, 55a) and 6.7 (55a, 56) kJmol-1. For comparison—despite our earlier enunciated skepticism about isomerization reactions performed in polar media (Z-BuOK in DMSO) — the following enthalpies of reaction, and thus enthalpies of formation, differences were found43 16.4 1.4 and 2.8 0.8 kJmol-1. Consistency, if not precise numerical agreement, is found for the energetics of the isomeric cyclooctadienes. 

The desired enthalpy of formation of 6,6-dimethylfulvene was determined by Roth citing measurement of hydrogenation enthalpies, and chronicled by Pedley citing enthalpies of combustion and vaporization. The two results differ by 7 kJ mol-1. We have opted for Roth s value because it is in better agreement with a value calculated using Roth s force field method. It is also to be noted that measurement cited by Pedley for the neat condensed phase could be flawed by the presence of partially polymerized fulvene and neither elemental abundance of the compound nor analysis of the combustion products would have disclosed this. Likewise, the measured enthalpy of vaporization would not have necessarily uncovered this contaminant. 

The aromaticity of azines is reduced relative to benzene, as is evidenced by the RCI values (83JOC1344) (Table IV) as well as by the RE values calculated from the energies of hydrogen-transfer reactions (89JA4178) (Table VII). For example, in the case of pyridine the MP3/6-31G calculated energy of the homodesmotic reaction (50) equals -1.8 kcal/mol. Since the RE of benzene determined from the hydrogenation enthalpies is 36 kcal/mol, pyridine s RE will, accordingly, be 34.2 kcal/mol. 

Although there are measured enthalpies of formation of phenyl and cyclohexyl benzoylperoxycarbonate, there are none for the corresponding deoxygenated benzoylcarbonates that replace the —OO- moiety by —0-. The difference between the gas phase enthalpies of formation of the peroxy compounds, ca 258 13 klmol", is the hydrogenation enthalpy of the phenyl compound. This value is far in excess of the hydrogenation enthalpy of any other phenylated species from the enthalpies of formation of gaseous phenol and cyclohexanol, the difference is but 189.98 2.3 kJmol. ... 

Alternatively, we can deduce some homoaromatic stabilization of bicyclo[4.1.0]hepta-2,4-diene and homo antiaromatic destabilization of bicyclo[2.1.0]pent-2-ene from enthalpies associated with hydrogenation to form the saturated bicycloalkanes. The bicy-cloheptadiene hydrogenation enthalpy (i.e. 39 —> 18, n = 6) is ca 200 kJ mol-1, to be compared with (229.6 1.3) kJ mol1 for the hydrogenation of the corresponding monocyclic... 

The enthalpy of formation of phenylacetylene was obtained by using the enthalpy of hydrogenation reported by D. W. Rogers and F. J. McLafferty, Tetrahedron, 27, 3765 (1971), the archival value for the enthalpy of formation of gas phase ethylbenzene, and the condensed phase-gas phase hydrogenation enthalpy equivalence of D. W. Rogers, O. A. Dagdagan and... 

The desired number may be recognized as the difference of the enthalpies of hydrogenation of cubene and bicyclo[2.2.0]hex-l-ene and found to be ca 50 17 kJmol-1 endothermic. In that cubene is more rigid than the bicyclohexene, it is surprising that the hydrogenation enthalpy is smaller for the former than the larger. Alternatively, consider reaction 22. 

This is not the precise value for the hydrogenation enthalpy as reported by Abboud and coworkers94, 105.9 1.2 kJmol-1, but, as indicated above in the text, a composite value was used to generate the enthalpy of formation of the unsaturated hydrocarbon. As such, the hydrogenation enthalpy employed here was the difference of the suggested enthalpy of formation of ( )-l-phenylpropene from this source and the archival value for propylbenzene. 

For example, consider hydrogenation enthalpies. Besides the multi-ring species just discussed, our analysis also includes the variously phenylated and tolylated ethylenes presented in the beginning... 

In fact, the hydrogenation enthalpies we cite were obtained by taking the difference of the archival enthalpies of formation of liquid phase unsaturated and saturated species, rather than by direct measurements (cf Reference 16). 

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