Jul 11, 2012
84
0
Status
Pre-Health (Field Undecided)
Hi, I am having a hard time determining the R or S when there are 2 dashes and 2 wedges

heres an example:
OH
|
H==C==Cl
|
Br

The two equal signs are supposed to be wedges and the lines are dashes.

thanks in advance for any help
 
Oct 22, 2013
100
11
Status
Pre-Medical
All that matters is whether the lowest priority group is behind the molecule (on a dash). Since Hydrogen is on the dash already you can do it normally. The answer is S.

Edit: answer is R, got mixed up with your notations.
 
Last edited:

sps27

5+ Year Member
Sep 1, 2012
733
145
OH
|
H==C==Cl
|
Br

So this is basically a fisher projection. What u do in this case is take one half of the wedge and make it in plane. Similarly take one half of dash and make it in plane. Say I take Cl and make it in plane. I keep H as wedge. Next I take OH and make it in plane. Keep Br as dash. Next I assign priorities, which will be Br, Cl and OH, which is clockwise, so S. Since H is wedge, I invert stereocenter which becomes R.

I had to look this up. In the Klein book, there is one small example on how to handle Stereochemistry for Fischer Projections. Let me know if I am wrong......
 
Oct 22, 2013
100
11
Status
Pre-Medical
OH
|
H==C==Cl
|
Br

So this is basically a fisher projection. What u do in this case is take one half of the wedge and make it in plane. Similarly take one half of dash and make it in plane. Say I take Cl and make it in plane. I keep H as wedge. Next I take OH and make it in plane. Keep Br as dash. Next I assign priorities, which will be Br, Cl and OH, which is clockwise, so S. Since H is wedge, I invert stereocenter which becomes R.

I had to look this up. In the Klein book, there is one small example on how to handle Stereochemistry for Fischer Projections. Let me know if I am wrong......
Actually you're right and I'm wrong. I got his notations mixed up heh. The == looked like dashes to me, but I noticed he said that == are wedges.
 

Czarcasm

Hakuna matata, no worries.
5+ Year Member
Jun 22, 2013
964
407
Crypts of Lieberkühn
Status
Medical Student (Accepted)
OH
|
H==C==Cl
|
Br

So this is basically a fisher projection. What u do in this case is take one half of the wedge and make it in plane. Similarly take one half of dash and make it in plane. Say I take Cl and make it in plane. I keep H as wedge. Next I take OH and make it in plane. Keep Br as dash. Next I assign priorities, which will be Br, Cl and OH, which is counter-clockwise, so S. Since H is wedge, I invert stereocenter which becomes R.

I had to look this up. In the Klein book, there is one small example on how to handle Stereochemistry for Fischer Projections. Let me know if I am wrong......
Yep, funny you mentioned Klein because I recall this same situation. You are right, but I think you meant to say counter-clockwise instead. Since this would indicate S configuration with the lowest priority group pointing at you (wedge), the "true" configuration is just opposite of that -- R.
 

Next Step Tutor

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Sep 30, 2013
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Next Step Tutor

MCAT Guru
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5+ Year Member
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Your drawings and explanations are always so helpful. Thanks! :) ps- I like to think of wedges as "Grandma's Hugs" ..hahaha
lol yeah I had a student one time who said he liked to think of the fischer projection as reaching out to hug him. That always struck me as WAY friendlier than organic chem normally treats me but it's a def a good mnemonic.