Question

I am wondering what the rule is for determining functional group positions for the most stable chair and boat conformations of cyclohexanes; meaning basically is there a pattern as to whether the functional group will be axial or equatorial for the most stable conformation based on which C # it is attached to (i.e. C- 1, 2, 3, 4, 5, or 6)?

What's the most stable for chair and boat of cis-1-bromo-4-methylcyclohexane as well as the trans-1-bromo-4-methylcyclohexane? (which group is axial and which is equatorial) Thank you

Answer 1

The placement of functional groups depend on which conformation (boat or chair) is most stable for the given compound.

The pattern or placement of axial or equatorial functional groups is based on this stable conformation. Among cyclohexanes and substituted cyclohexanes (in this case), chair conformation is most stable.

Each carbon atom in the cyclohexane ring makes four bonds. Two of these bonds are to other ring carbon atoms, and two bonds are made to non-ring atoms. The bonds to non-ring atoms are termed axial or equatorial, depending on the bond angle.

● Bonds to non-ring atoms with angles of about 90° are termed axial.

● Bonds to non-ring atoms which make only a small angle compared with the plane of the ring are termed equatorial.

Trans-1-bromo-4-methylcyclohexane is more stable than cis-1-bromo-4-methylcyclohexane.

In a cis-1,4-disubstutited cyclohexane, one substituent is always equatorial, and the other is always axial.

In the trans isomer, both substituents are either both equatorial or both axial.

Reason for greater stability of trans isomer: Since one methyl is axial, this costs 1.8 kcal of steric strain. Consequently, the cis isomer is less thermodynamically stable than the trans, which has no steric strain in the more stable conformation. The energy difference is again, 1.8 kcal/mol.