Destroyer Orgo # 144

[wizi]

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Hi everyone,

I have a question from Destroyer Organic #144:

Answer B: why do we have to switch it two times???

 

[seunglim87]

You usually want #4 on the dash and #1 on the wedge. Every time you switch 2 groups, you get an inversion. so if you switch once, and the structure after switching is S, your answer would be R. However, if you switch twice, then you get two inversions, so the structure after switching twice is S, your answer would be S.

 

[wizi]

You usually want #4 on the dash and #1 on the wedge. Every time you switch 2 groups, you get an inversion. so if you switch once, and the structure after switching is S, your answer would be R. However, if you switch twice, then you get two inversions, so the structure after switching twice is S, your answer would be S.

Thanks seunglim87,

I am sorry but i still dont get it. Why do we have to switch two times???

 

[seunglim87]

Thanks seunglim87,

I am sorry but i still dont get it. Why do we have to switch two times???

you dont have to switch 2 times.
see, if you switch once, you did one inversion(S-> R). the orientation of the molecure after one switch shown is R, so the answer would be the opposite of R, S
if you switch twice, you did two inversions(S-> R-> S), so the orientation of the molecure after two switches is S, so you keep that answer S.

you can switch as many times as you want. if odd numbers of switch, then you have to change your answer, but if you do even numbers of switch you dont have to change the answer. The important thing is you want the #4 atom on the dash, and keep track of how many times you switched.

you just have to do a lot of practices with stereochemistry, but once you understand it, its not so bad. good luck

 

[invictusx]

I hated these questions but I ALWAYS made sure I drew it in Fischer Projection and then the I numbered the priority groups and visualized it 3D. I always got the answers that way