Resumen de The Heats of Combustion of Gaseous Cyclotetradecane and trans-Stilbene - a Tale of Long-standing Confusion
Ernest L. Eliel, Jan J. Engelsman
This paper embodies the belated publication of a thesis submitted by JJE to the Free University of Amsterdam in 1955; heretofore the data (heat of sublimation of cyclotetradecane, 21.34+/-0.10 kcal/mol or 89.33+/-0.42 kJ/mol; heat of sublimation of trans-stilbene, 20.68+/-0.08 kcal/mol or 86.57+/-0.33 kJ/mol) were available only in a printed version of the thesis. In conjunction with a subsequent determination of the heat of combustion of solid cyclotetradecane, this led to the conclusion that the compound was strain-free.
That these data were not published at the time had unfortunate consequences: The strain energy of cyclotetradecane was susequently computed to be 12.6 kcal/mol (52.7 kJ/mol) on the basis of an erroneous value of the heat of sublimation determined independently in 1964. This was corrected only in 1992 through a third determination which gave a value for the strain energy close to that found in 1955, i.e. near zero.
The heat of formation of trans-stilbene is of interest in connection with the energetics of the photochemically interesting cis-stilbene - trans-stilbene interconversion. Different methods gave conflicting values: a high one near 5 kcal/mol (21 kJ/mol) from heats of hydrogenation and of equilibration as a function of temperature and a low one near 3 kcal/mol (12.6 kJ/mol) as a result of heats of combustion combined with heats of vaporization given in the literature. However, using the heat of sublimation of trans-stilbene given above, the combustion method converges with the other two in pointing to the higher of the two values for the correct enthalpy of isomerization.
