[nokia8650]

How many gemoetric isomers does a compund possess; is there any way of working it out other than simply listing them all.  For example, how many gemoetric isomers would 1bromo,2,3,4chlorocyclohexane have?

Thanks

[Yggdrasil]

To the best of my knowledge, there is no simple formula that will tell you the number of geometric isomers.  You have to draw out all of them in order to figure it out.

[movies]

The total number of stereoisomers is 2ⁿ where n is the number of stereocenters.  This only holds if you don't have any meso compounds though.  For each meso stereoisomer you subtract one.

[Kryolith]

The total number of stereoisomers is 2ⁿ where n is the number of stereocenters.  This only holds if you don't have any meso compounds though.

And if there are no sterical hindrances...

[nokia8650]

Thanks for the reply.  How do I determine what a stereogenic centre is in a cylic compund; its easy finding chiral centres and geometric isomers on aliphatic molecules, but how does one do it on ring structures?

Thanks

[Kryolith]

Aliphatic does not mean acyclic.

A molecule is chiral if it doesn't contain any of the following symmetry elements. (1 of these is enough to make it achiral!)

1) symmetry plane (mirror plane)
2) symmetry center
3) rotation-reflection axis

EDIT:
Question: How many isomers are there for 1,3-dihydroxycyclohexane and 1,4-dihydroxycyclohexane? Try to determine whether they are chiral or not.

[nokia8650]

Thanks alot for the reply.  Please can you explain points 2 and 3 in more detail - what is a symettry center, and what is a rotation-reflection axis?

Thanks!