hey, you chem gods: how can you tell if a substituent on a cyclic molecule is either axial or equatorial?

Answer 1

It comes up in cyclohexane rings. If there is one substituent, such -OH in cyclohexanol, then the -OH will tend to be equatorial, because -OH takes up more space than -H's, and it would be too crowded axial with other axial -H's.

If you put a t-butyl group, -C(CH3)3, cis to the -OH in the 4-position (cis-4-tert-butylcyclohexanol), then the t-butyl group is so large that will command an equatorial conformation. That means that the -OH will be forced into an axial conformation.

Answer 2

You must first draw the structure (or look at the structure given to you):

If the substituent is in the horizontal plane, it is equatorial.

If the substituent is in the vertical plane, it is axial.

Nice and simple, eh?

Hope this helps...