Why Not Use Monocyclic Species

Indeed, this abundance of data is the reason why we have chosen to discuss these bicyclic species rather than the simpler, monocyclic five-membered ring compounds recognized as C2H2XYZ (XVI) for which the desired thermochemical data is very often lacking. For these monocyclic compounds we can write the related reaction  

The enthalpy of this reaction is the same, within a numerical constant, as the difference of enthalpies of formation of C2H2XYZ and CH3XYZCH3. Were there a more or less constant enthalpy of formation difference between methyl and phenyl derivatives (an assumption employed in [27] that looked better in kilocalories than in kilojoules), the enthalpy of this reaction would be the same (again to an additive constant) as the difference of the enthalpies of formation of C2H2XYZ and C6H5XYZC6H5, the defining relationship in the experimental Dewar-Breslow approach [4—6]. 

Most assuredly, there is a difference in strain energy of the mono and bicyclic species, as well as in phenylene (-C6H4-) and vinylene (-CH=CH-) bonding. However, the difference is small. Taking C2H2(CH3)2 as the (Z)-isomer of 2-butene (choosing the ( )-isomer introduces just an additive constant), we find the reaction  

Deviation from thermoneutrality is by less than 1 kJ mol 1, less than the error bars in the measured enthalpies of formation. This is the equivalent to the difference of reactions 1-3, equal strain energies both sides, so our previous reasoning is legitimized. 

A price of our analysis is that the enthalpy of formation of some of the CH3XYZCH3 species will have to be estimated. In principle, one could use Benson group increments [28] in lieu of these estimates but then, many of these increments would also have to be estimated because many compounds containing the groups are unknown, inadequately precedented, or even limited to but one species, the one of interest. Benson said in a related context to one of the authors (JFL) some decades ago If the universe was kind, your approach would agree with mine, and both would agree with the universe.  

---