Chem 351 Fall 2004 MT: Thermodynamics

Part 8: THERMODYNAMICS

This should have been a reasonably straight forward calculation, but care needs to be taken to get the right structure and the right molecular formula, then do the math correctly.

Common stumbling blocks for some students were :

could not draw 1-methyl-2-phenylcyclohexane and deduce the correct molecular formula, C₁₃H₁₈
failed to draw clear diagrams of the chair conformations with the axial and equatorial positions clearly shown.
failed to draw 1,2- configurational isomers because they did not know their isomer types.

If we are talking about configurational isomers of 1-methyl-2-phenylcyclohexane, we must be talking about the cis- and trans- systems. We are not talking about chair and boat isomers (these would be conformational isomers) or other substitution patterns including different branching patterns (these would be constitutional isomers).

(note : in order to clearly show the required isomers the chair structures with the axial and equatorial substituents needed to be shown. Some students drew diagrams that were indistinguishable from each other e.g. planar cyclohexanes with wedges and hashes to denote up and down. This was insufficient for full marks.)

The question asks for an analysis of the conformations. Axial substituents are destablised by Van der Waals interaction and some torsional strain with the adjacent C-C. The Van der Waals interaction between the axial position and the other axial substituents on that face (in this case these are both H atoms). These are examples of 1,3-diaxial interactions. Hence substituents generally prefer to be equatorial where there is no such destabilisation. The most stable system here will be the trans- isomer where boths groups are equatorial. The most stable cis isomer will be the one with the larger phenyl group in the preferred equatorial postion and the smaller methyl group axial. In all cases the chair conformations are preferred since they are totally staggered and are strain free.

(note : many students made sweeping statements about trans being more stable than cis others said the left most structure was the most stable. Neither are true. )