[ebcoh]

Hello all,

If i'm given a cyclobutane or cyclohexane with, say, a Cl and Br on opposite ends of the molecule, do I say that there are no stereocenters, thus the molecule is unique?  Or can I treat the ring as a sort of stereogenic center and say that it is an achiral molecule and there are diastereomers.  If this is true, is, say, a cis-1,3-cyclobutane molecule a meso compound?

thanks.

[agrobert]

do I say that there are no stereocenters, thus the molecule is unique?

No C-1 and C-3 are stereocenters but the molecule is achiral because it has a plane of symmetry.

http://www.chemicalforums.com/index.php?topic=20058.0

If this is true, is, say, a cis-1,3-cyclobutane molecule a meso compound?

Yes both cis and trans are meso and therefore achiral.

http://en.wikipedia.org/wiki/Meso_compound

[ebcoh]

How are they stereocenters if they dont have four different substituents?

[agrobert]

Sorry you are correct.  It is still achiral though.

[sjb]

If this is true, is, say, a cis-1,3-cyclobutane molecule a meso compound?

Yes both cis and trans are meso and therefore achiral.

http://en.wikipedia.org/wiki/Meso_compound

A 1,3-disubstituted cyclobutane would be meso if and only if the two substituents were the same, and the actual arrangement of them is cis. If the two are trans, then the molecule would have (average) C₂ symmetry.

S