Part 8: THERMODYNAMICS

 The diagram to the left shows the structures of the two isomers (cis- means the two methyl groups are on the same face of the ring, trans- means they are on the opposite face), and it shows the energy relationships between the elements, the isomers and their combustion products. First task is to determine the molecular formula = C5H10 and use this to calculate the heat of combustion of the elements = - 759.25 kcal/mol.

The Hess's Law relationship that comes from the diagram is :  ΔHc (elements) =  ΔHf (isomer) + ΔHc (isomer)
So, based on the information for isomer I we get : - 759.25 = + 46.45 +
ΔHc (isomer I)
Therefore,  ΔHc (isomer I) = - 759.25 - 46.45 = -805.7 kcal/mol

And for isomer II we get : - 759.25 = ΔHf (isomer II)  - 804.5
Therefore,  ΔHf (isomer I) = - 759.25 + 804.5 = + 45.45 kcal/mol

Isomer I is the cis- isomer and isomer II is the trans- isomer (which is the more stable isomer, see diagram, where it is at a lower energy).

First note that both systems are eclipsed conformations. In the cis- isomer the two methyl groups are in a syn conformation and are eclipsed with each other leading to torsional strain. In the trans- isomer the methyl groups are eclipsed with H atoms about 120 degrees from each other. This is still destabilising, but because the H atoms are smaller than the methyl groups, the steric interaction is less in this case. Hence the proximity of the methyl groups in the cis- isomer destabilises it compared to the trans- isomer.

(Note : that the endothermic heats of formation are real values, it just means the compound is less stable than its elements, mainly due to the ring strain)