McMurry (Kap.4,7)

Question 1

Even though the cyclohexane rapidly ring flip between the equatorial position and the axial position, are these two equally stable?

Answer 1

Actually not.

The rule is that the substituent in the chair conformation is almost always stable in the equatorial position then in the axial position.

Question 2

Why is the axial conformation typically less stable then the equatorial?

Answer 2

It is due to steric strain, caused by the 1,3-diaxial interactions.

The axial methyl group on C1 (for example) is too close to the axial hydrogens three carbons away (C3 and C5) - resulting in steric strain. (see page 103 for molecule example).

Question 3

Why is the equatorial position of the axial methylcyclohexane more stable then the axial form?

Answer 3

Page 104

The methyl group interacts with the two other axial hydrogens (on the same side/face of the ring). These cause steric strain.

The equatorial one does not have such interactions, therefore no steric strain and therefore more stable.

Question 4

What does the steric strain depend on, in a substituted cyclohexane?

Answer 4

Depends on the nature and size of the substituent (see page 104 for the substituent - neighboring H - steric strain relationship).

Thus, the increase in the alkyl group size - the more steric strain between the substituent and the neighboring axial hydrogen. (NOTE only with one other axial hydrogen - in case of the hydrogens, double the amount of steric pain).