Stereochemistry

[RCT PC CRN]

Can someone please explain me the following in detail...I will appreciate it.

How do you tell by comparing R,S notification whether it's Enantiomers, Diastereomers, Meso or Same compound, assuming all substituents remain the same in both compound?

Example 1

Two stereo isomers are given with only 1 chiral center.

A. R and S
B. R and R

If the chiral center configuration stay the same the way it shown in B (i.e. R to R or S to S), Does that mean its a same compound? In other words, If I have R configuration compound and If I rotate/move/switch substituents many times but I ended up with R configuration in end again, Does that mean it still the same compound? (Again, assuming all substituents remain the same)

Example 2

Two stereo isomers are given with 2 chiral center. At both chiral center substituents are different. (i.e one chiral center has for example -H,-CH3, -CH2CH3, -Br and other has -H, -CH3, -CHO, -OH). Following are the different scenario.

A. R,R and S,S
B. R,R and R,S
C. R,R and S,R
D. R,S and S,R

Example 3

Same as example 2 but with same substituents at both chiral center. (i.e one chiral center has for example -H,-CH3, -CH2CH3, -Br and other also has -H, -CH3, -CH2CH3, -Br)

A. R,R and S,S
B. R,R and R,S
C. R,R and S,R
D. R,S and S,R

********************************

I do know definitions of Enantiomers, Diasereomers, meso etc. I can certainly assign configuration.

What I'm trying to understand here is if I have given two or more compound (of course stereo isomers only) and asked to compare. I could try to answer the question based on configuration. (i.e find the chiral center, assign the configuration and compare it.)

---

Members don't see this ad.

 

[orgohacks]

Situation #1 -

1) if two molecules with the same substituents have the same R/S designation, and are related to each other only by bond rotations, they are the same molecule.

2) If both stereocenters have different substituents, you would need to invert both stereocenters to make them enantiomers of each other.
For instance - the enantiomer of (R,S) pseudoephedrine would be (S,R)-pseudoephedrine. see - http://is.gd/create.php

Note that you cannot have meso in this case.

3) If both stereocenters have the same substituents (like tartaric acid) you can have a meso situation. (S) will be the mirror image of (R), so if both (S) and (R) are present you will have a mirror plane. (S,R) tartaric acid and (R,S) tartaric acid are the same compound - this is the meso compound.
The two enantiomers are (R,R) and (S,S) tartaric acid.

hope this helps - James

 

[RCT PC CRN]

Situation #1 -

1) if two molecules with the same substituents have the same R/S designation, and are related to each other only by bond rotations, they are the same molecule.

2) If both stereocenters have different substituents, you would need to invert both stereocenters to make them enantiomers of each other.
For instance - the enantiomer of (R,S) pseudoephedrine would be (S,R)-pseudoephedrine. see - http://is.gd/create.php

Note that you cannot have meso in this case.

3) If both stereocenters have the same substituents (like tartaric acid) you can have a meso situation. (S) will be the mirror image of (R), so if both (S) and (R) are present you will have a mirror plane. (S,R) tartaric acid and (R,S) tartaric acid are the same compound - this is the meso compound.
The two enantiomers are (R,R) and (S,S) tartaric acid.

hope this helps - James

Thank you James. Your reply certainly helped me. I also pulled my Orgo 3D model kit.

 

[orgohacks]

Thank you James. Your reply certainly helped me. I also pulled my Orgo 3D model kit.

Keep in mind that if the stereocenters aren't contiguous (i.e. consecutive) it can be a whole different ballgame. But it's all common sense, once you see understand it the first time.