Ochem Configuration question

[JoyantBuoyant]

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Okay, I'm really confused on when you're supposed to or can rotate a molecules sigma bond... For example if a bromine is up an another bromine us down on opposite chiral carbons, how come your sometimes allowed to rotate the sigma bond and turn it into a meso compound ?!?!


Also I'm really confused with conformational and diasteremoers...
I attached an image for an example

The answer is diastereomers.. but cant you just rotate it around the sigma bond turning it into conformational isomers?!?

 

[a13b13]

Rotating around one sigma bond wouldn't turn this into a conformational isomer. Diasteromers are quite simple.. in the Fischer projections, if you just move one element/functional group you get a diastereomer (in this case you switched an H and an OH).

 

[angemon89]

Look at the top of the second molecule. Move OH up, COOH to the right, and H to the left. That's how you rotate. If you rotate to the right one more time, you will be able to clearly see that they're not conformational isomers.

You can also rotate left in a similar fashion.

 

[cheesier]

Yes, you can rotate about the sigma bonds. Your confusion seems to be stemming from not knowing what the Fisher projection is showing you. All of the horizontal bars are coming out of the page. If you rotate the molecule on the right so that it resembles the one on the left, the carbon you're looking at would have the OH and the H pointing inward, while the one on the left has them pointing outward, so they are not rotamers, but diastereomers.

 

[gettheleadout]

Moving to Q&A.