We have learned factors that influence the energy-content of a molecule (see the first exercise) and derived the most stable conformation of cyclohexane using this knowlegde. In this exercise, we will apply our knowledge to substituted cyclohexanes.
Note: If you don't remember the meaning of the terms 'axial' and 'equatorial', you should study the second exercise again
The smallest hydrocarbon substituent that one can think of is a methyl group. It can be placed in either an axial position or an equatorial one. Obviously, in either case, we are talking about the same molecule: when flipping from one chair-conformation to the other, the methyl group moves from an axial to an equatorial position. Models of both conformations are shown below.
Look closely at the two structures (especially the spacefilled models will be useful) and answer the following question:
Which of the two above conformations of methylcyclohexane is less favourable in terms of energy and why?
The conformation which has the methyl group
Note that the energy difference is substantial: the difference between the two conformations is something like 2.3 kcal/mole.
The behaviour we observe for methylcyclohexane is characteristic for all
substituted cyclohexanes:
If any of the hydrogen atoms is replaced by a a larger group or atom, crowding
occurs. This crowding is strongest when the group or atom is in the axial
position, because of the Van der Waals interaction ('steric hindrance') with the hydrogens in the
other axial positions.
Take a look at the animation and
energy-profile of the axial-equatorial conformation change in
methylcyclohexane.