R or S?

[kevin512]

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Br

F-C-CHO

H

Isn't it R?

 

[kevin512]

the bromine and the H are supposed to be connected to the middle carbon.

 

[vcuchic]

The molecule is in a (S) configuration because:
The Br has #1 priority (because it has the highest atomic number, hence highest priority), then F has #2 priority, CHO is #3, and lastly H #4.

Then if you draw an arrow from the first priority group, through the second, to the third--the arrow should point counterclockwise, hence this chiral carbon atom is (S).


I hope this helps. 🙂

 

[scuzfly]

can something be R/S AND Cis/Trans (Z/E)?

Bringing up an old post to save on making a new one. Plus this is a valid question in my mind.

 

[chessxwizard]

Normally, R and S are used when there are four different subtituents around a chiral carbon. Cis and Trans are used when there is a hydrogen common to both carbons across a double bond.

Also, for the OP, the molecule you depicted is S.
For that kind of projection, the equatorial bonds are coming out at you, and the vertical ones are going back (into the page). Think of it as the molecule is coming out and "hugging" you.

Bromine is #1, florine #2, skip over hydrogen (#4), and carbon #3.
Since the numbering goes counter-clockwise, S.
No reversal of R/S is necessary because the lowest priority subtituent is already pointing backwards (is vertical in the drawing).

Hope this helps!

 

[bigstix808]

can something be R/S AND Cis/Trans (Z/E)?

Bringing up an old post to save on making a new one. Plus this is a valid question in my mind.

nope - in order for a carbon to be R/S it must be chiral having 4 different substituents. Cis/trans (E/Z) is used to determine priority placement of substituents in alkenes, thus making it impossible for them to be R/S.

but, now if you had this molecule...

(2E)-4-bromo-6-chloro-4-methylhept-2-ene

you can have both R/S on the chiral carbons and E/Z on the 2x bond.

 

[smile101]

can something be R/S AND Cis/Trans (Z/E)?

Bringing up an old post to save on making a new one. Plus this is a valid question in my mind.

I think cis and trans exclusively refers to the carbons with double bonds, while Z and E refers to the carbons that don't have double bonds but they posses two adjacent chiral carbons with high priority groups that could be in either Z or E configuartion relative to each other and those carbons are chiral, so they could be R or S.

I think this is how it works (Please correct me if I am wrong).
I had a question on my Kaplan diagnostic test that asked me if the compound was cis-trans or Z, E. The compound had double bonds and I picked cis-trans and my answer was correct. For me, that clarified my understanding in differentiating the two.

 

[klutzy1987]

nope - in order for a carbon to be R/S it must be chiral having 4 different substituents. Cis/trans (E/Z) is used to determine priority placement of substituents in alkenes, thus making it impossible for them to be R/S.

but, now if you had this molecule...

(2E)-4-bromo-6-chloro-4-methylhept-2-ene

you can have both R/S on the chiral carbons and E/Z on the 2x bond.

Just be careful, Destroyer mentions a couple of times about an allene which is a molecule that has two double bonds next to each other and that can be R/S if the substituents are all different. It is very difficult to explain yb just writing it but if you have the destroyer check it out I believe its somewhere in the 50's.

 

[bigstix808]

I think cis and trans exclusively refers to the carbons with double bonds, while Z and E refers to the carbons that don't have double bonds but they posses two adjacent chiral carbons with high priority groups that could be in either Z or E configuartion relative to each other and those carbons are chiral, so they could be R or S.

I think this is how it works (Please correct me if I am wrong).
I had a question on my Kaplan diagnostic test that asked me if the compound was cis-trans or Z, E. The compound had double bonds and I picked cis-trans and my answer was correct. For me, that clarified my understanding in differentiating the two.

E and Z are virtually the same thing as cis/trans. the only difference is in cis/trans there is a Hydrogen attached to each carbon of a double bond making that 2x bond have only 2 substituents.

in E/Z there are at least 3 substituents attached to that 2x bond and priority must be given accordingly in order to determine the E/Z configuration.

example:

 

[scuzfly]

ok. I think I had this same question back when I took Organic Chemistry and I wanted to make sure. I haven't come across any naming questions that say (R) cis blah blah blah