Isomers Question

[chrishousesomg]

Hi guys,

I am a little confused as to identifying the types of isomers of molecules. My problem is identifying conformational isomers in chair structures and also knowing when the structures are actually the same. I have a few examples.


I attached the worksheet. Its question 4 part e (before the continued part) and j.
In part e, I did the configuration and got R and R. I see how it is conformational as you can rotate the bonds, however how do I know that these molecules aren't the same (like in part J)?

As for part J, I got R/S and S/S. The answer says that these are not isomers (same) and flipping the molecule on the right would give the structure on the left. I don't understand how this is not conformational. If someone could explain this that would be great.

Thank you
^_^

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[bschaefer97]

4e) The best way to think about these problems is to try to "flip" around the drawing to see if it can be rotated to make the same conformation. However, in this case if you flip it, the alkene is not going to rotate since you cannot rotate around a carbon-carbon double bond. Thus, if you flip it to make it look the same, the wedged and dashed bonds will also flip, so these structures will be different.

4j) For this one, a molecular model kit would help. But, just look at what happens if you flip the chair to make bottom part go up, and the top part (back rest) go down. The bonds would be the same. This rotation happens very frequently, in fact, so these are the same molecule.

Hope this helps! If anything I said isn't clear just let me know and I'll try to explain better.