Cis/Trans

[DrRoyal Pains]

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I understand why placing the larger of the two substituents in the equatorial position is more stable, but when given a choice of placing the larger of the two equatorial, or placing them both equatorial, why can you not place them both equatorial? Is there that much torsional/steric strain similar to having the two substituents axial?

 

[Saiph]

It depends on the location of the substituents. Lets say you have Cis-1,2-Dibromocyclohexane. That means they will both go up or both down. One of them will be "up" axial, and the other is "up" equatorial (vice versa for down). You cannot just put them both equatorial. Hope you got what I meant!

 

[sacapuntas]

Yeah, you have to draw out the chair and see if it is possible to place them both equitorial or axial depending on where they are at on the ring. Example, if they are cis and on C 1 and C 2, the down position for C 1 is equitorial while the down position for C 2 is axial.

 

[DrRoyal Pains]

Ha WOW, I can't believe I missed that. Thanks guys.

 

[bchang57]

cis 1,3, or cis 1,5 can have both substituents equatorial (2 down positions, or 2 up positions)

trans 1,2, trans 1,4, or trans 1,6 can also have both substituents equatorial