Configuration on Ficher projection

[siba]

Hi! I understand that the configuration needs to be switched (R to S, or S to R) if Hydrgen is on the side of the "cross". But do we still need to switch the configuration if the Hydrogen is on top of the "cross"?

Sorry if it sounds confusing. I'm referring to Ochem #91 in Destroyer. Choic B has the Hydrogen on the top, and the answer key has switched positions of other groups. I'm very confused about the rules of switching, and when to switch.

Could someone clarify the question? Thanks!

 

[chessxwizard]

First of all, choice B isn't a Fischer projection. To get the right configuration with a chiral center that has solid and dashed lines, put the lowest priority substituent in the back if it isn't already. If you have to switch once, the actual configuration is the opposite of what you determine. If you switch twice, it is the same.

For choice B, switch the methyl group and the hydrogen atom. You should get R as the "observed" configuration. However, since you switched once, the true configuration is the OPPOSITE (S).

Try switching the methyl and the hydrogen (putting the lowest priority in the back) and the Bromine with the alkene portion. You should get S. Since you switched twice, the configuration you determined after switching is the ACTUAL configuration of the original molecule. Hope this makes sense.

 

[siba]

First of all, choice B isn't a Fischer projection. To get the right configuration with a chiral center that has solid and dashed lines, put the lowest priority substituent in the back if it isn't already. If you have to switch once, the actual configuration is the opposite of what you determine. If you switch twice, it is the same.

For choice B, switch the methyl group and the hydrogen atom. You should get R as the "observed" configuration. However, since you switched once, the true configuration is the OPPOSITE (S).

Try switching the methyl and the hydrogen (putting the lowest priority in the back) and the Bromine with the alkene portion. You should get S. Since you switched twice, the configuration you determined after switching is the ACTUAL configuration of the original molecule. Hope this makes sense.

Thank you! 🙂 I think I get most of it and how switching once will the the opposite and twice will be the same. The only part I still want to confirm is that the group on the solid line (alkene group in this case) remains while the other 3 groups change position. Correct?

 

[chessxwizard]

Well if you switch only the H and the CH3, then the alkene and bromine remain in their positions. For switching two, you can switch any pairs you like... but keep the lowest priority substituent in the back!