Orgo - cyclohexanes

[candiolp]

So if a cyclohexane molecule is cis or trans, what will it be in the chair conformation?

 

I thought I read one place that they will be equatorial OR axial, never both, but one of the practice questions from EK1001 said that if its a 1,2 substitution on a cyclohexane ring, axial-equatorial is the cis relationship.

 

Please help clarify!!

[srindogg@hotmail.com]

If a cyclohexane molecule is cis or trans, it will always maintain that sterochem in any conformation.

 

In a cyclohexane ring, you can see that all the substitutions "above" the plane alternate between axial and equatorial as you move from c-1 to c-6. Likewise, so do the substitutions "below" the plane. So, if you have a 1-2 substitution that is cis i.e on the same side of the plane, it has to be an axial-equitorial relationship.

 

I don't think there is any rule that they both have to be axial or equatorial. Only exception is if one of the substituents is bulky, then that substituent is locked in the equatorial position.

[candiolp]

So if they are trans, then they would be an axial-axial or equatorial-equatorial?

 

Does this method of thinking apply, or do they differ in every case?

[avenir001]

basically it works like this:

 

if it's a cis...

-1,2 disubstituted cyclohexane, they'll be ax-eq

-1,3 disubstituted cyclohexane, they'll be ax-ax or eq-eq

-1,4 disubstituted cyclohexane, they'll be ax-eq

 

if it's a trans...

-1,2 disubstituted cyclohexane, they'll be ax-ax or eq-eq

-1,3 disubstituted cyclohexane, they'll be ax-eq

-1,4 disubstituted cyclohexane, they'll be ax-ax or eq-eq

 

i think it would be helpful for u to draw it out for urself rather than just memorizing these.

[dan0105]

Ummmm to expand on that crazy chart of conditions above, what you have to realize is that the axial positions all point either up or down AND all the equatorial positions point to the side.

 

So if you have two groups trans to each other and they are RIGHT next to each other (say 1,2 or 2,3, etc) then you could do:

- one could go up on the axial, and on the next carbon it could go down on the axial. Or one could go on the equatorial that sort of goes out and up, while the other could be on the other carbon's equatorial that sort of goes out and down. If you put one on the axial up, and on the next carbon put the next group on the equatorial, it would also sort of go up (and to the side) so they really wouldn't be that trans. You really need to see the 3d picture of cyclohexane to visualize it further.

[In_Valid]

just get your molecular model kit out. Its the easiest way to do. Once you see the pattern those sort of questions will be a walk in the park.