Cahn Ingold When you flip rotation

[onedirection]

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Okay..obviously there's some rule that I just don't know about in the whole R/S thing


Do you only flip the rotation only if the lowest priority group is pointing out of the page?

Ie only when the lowest priority group is in the highest priority position

or are there other times when you flip the rotation?

 

[FeinMS]

Just use the thumb method. It's easier and requires no memorization. Put your thumb toward the lowest priority group. If you can "grab" 1-2-3 in order with your right hand, it's R. If you can "grab" with your left hand, it's S.

 

[keikoblue2]

If you're bad at visualizing, use the flip method.

The lowest priority group needs to be in the back (dashed line). If it's planar or protruding (bolded line), switch it with the molecule that is in the back (dashed line). Then number the groups from highest to lowest priority. If it now, after switching, goes 1-2-3 in a clockwise direction (R) the original chirality is S. If it now goes 1-2-3 in a counterclockwise direction (S), the original chirality is R. So basically, anytime when you have switched two groups (doesn't matter which two groups or what priority they are- ANY two groups), your chirality is inverted and you need to make up for the group switching by flipping the chirality to match that of the original molecule. <-- ex: You have an original molecule with R chirality. If you switch group 1 with group 2, you need to remeber to flip the chirality- the current chirality reads S, but you need to switch this to an R for the original chirality.