Hey you chemistry masters, please help: how can I tell if a substituent of a molecule is axial or equatorial?

Question

This has got me stumped. I'm taking organic chem part 1 in college

Answer 1

Axial substituents (sticking up in the air) on a cyclohexane ring are more crowded that equatorial ones (sticking out from the center). If you have a tert-butyl group [-C(CH3)3] on a cyclohexane ring, then that is equatorial, because it takes up alot of room. If you have cyclohexane with a tert-butyl group on one end and an -OH on the opposite end (4-methylcyclohexanol), it depends on whether they are cis or trans to one another on the ring. If they are trans, no problem, because they can both be equatorial. If they are cis, then the large size of t-butyl forces it into the equatorial position and leaves the -OH to take an axial position, despite the instability. That is the sweet hell of organic chemistry.

Answer 2

I am guessing you are talking about a substituent on a ringed molecule (ex: cyclohexane).

Basically, two things I can say off the top of my head:
1)
Groups like to be equitorial, especially big groups. Why? More space for the molecule. If you have a molecular model kit (a definite must if you are taking a few organic chem classes..will help you understand the molecules in 3-D), and you make,say, a cyclohexane and then add a coloured ball or atom to one if the axial spots, you will notice that the atom comes close to the hydrogens coming off of the carbons in the ring...this creates repulsion between the substituent and the other atoms in the ring. Now, if you take that atom and put it equitorial, it will be facing away from any other atoms in the ring, leading to less repulsion between the substituent and other atoms.

2) Similar to number 1, and for exactly the same reasons, the energies of substituents in the axial position tend to be much higher (due to the repulsion). So if you were given, say, two energies of the molecule, and asked to determine which is the substituent in the axial, and which is the equitorial, the equitorial config will be the lowest energy one (nearly every single time).

Working along these same lines, if you have 2 different subsituents in the system, they will both rather be in equitorial than axial, and if one HAS to be equitorial, and the other axial, the molecule rearranges so that the biggest group is equitorial, and the smallest axial.

BF
4th yr Chem Major
U of O (Canada)