[suckerpunch123]

Hi I need a little help in determining the most stable chair conformation... I know the the more equatorial substituents there are, the more stable it is. However, I've come across different questions with answers that do not follow this rule.

For example, a picture was shown and was asked to determine the most stable chair conformation. Let's say 1,2 di-chlorocyclohexane. The answer gave me both substituents as equatorial in the chair conformation. On the other hand, say 1,2 di-methylcyclohexane give an answer of the substituents one being equatorial and the other being axial.

The picture had bonds different though like for 1,2 di-chlorocyclohexane substituent bonds one was going into the page and one was going out of the page while the 1,2 di-methylcyclohexane subsituent bonds were both going into the page. Does it matter?

Many thanks.

[azmanam]

yes, cis and trans isomers of cyclic rings matter a lot.  As evidenced by the two chair structures you mention.  cis 1,2-dimethylcyclohexane can never have both substituents equatorial.  trans 1,2-dimethylcyclohexane can either have both substituents equatorial or both axial, with the more stable being both equatorial.  It will never have one substituent equatorial and one axial.

Build yourself a molecular model of both the cis and trans isomer and prove this to your self.  This is crucially important.

see also:
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sterisom.htm#isom3