[Claire]

Is there a method you can use in order to distinguish which structure (out of say 4 given) that cannot be superimposed onto any of the others given?

I'm in the process of revising for GAMSAT and have never had any problems with determining chiral carbons or the idea of stereoisomerism in general, but this one question on a practice paper (for which there is the answer but no method) really has me confused.  There are four structures given and apparently one cannot be superimposed onto the other three.  There obviously must be a method to working the answer out, but I've not been able to find one.

Any help greatly appreciated.

[movies]

The best way is to draw them all out and compare.  However, I think that the presence of an improper rotation axis neccessitates chirality.  Can anyone confirm that?

I would never, ever, use that to determine whether a molecule was chiral or not, however.  I would always do it by drawing out the mirror image and trying to superpose it.

[english]

For most common cmpnds, probably the ones you're dealing with, determining the R or S configuration will do.  If one compound is R and the other is S, these are nonsuperimosable.

But there are exceptions, so I would think unless you would want to run into that trouble you could follow movies' lead and check for superimposability the visual way.


To do this it might take a bit of brain power, nonetheless it is worth the time.  If you can rotate the molecule clockwise, counterclockwise, or 180° into the plane and it is the same as the one you are comparing, then obviously they are the same molecule.  Take into account however that a wedge stereobond becomes a dash stereobond, and a dash stereobond becomes a wedge stereobond when you rotate 180° into the plane.

Stereochemistry's always a tricky science.