Stereocentre/stereogenic carbon

[abhineetK]

In
C₁OOH-C₂H(OH)-C₃H(OH)-C₄H(OH)-C₅OOH,
is C₃ stereocentre?

>>Will it make any difference if I use the term 'stereogenic carbon' in place of 'stereocentre'??

Please, explain.

 

[windwitch]

A stereogenic carbon is one that is attached to four different molecules. In the molecule you have shown, carbon 3 is attached to a hydrogen, hydroxyl, and two HOOCCHOH-s, and since the latter appear twice, it can not be a stereocentre. Keep looking :)

 

[sjb-2812]

Yet it can exist in two different configurations. If C² and C⁴ were both (R), for instance, C³ can exist in (R) or (S) forms...

 

[abhineetK]

Yet it can exist in two different configurations. If C² and C⁴ were both (R), for instance, C³ can exist in (R) or (S) forms...

How? C₃ is not chiral... please explain what you want to say...

 

[sjb-2812]

What I mean is, the two compounds drawn below are different (I think) can you convert one to the other only by bond rotation?

Derived formally from the ketone, one is reduced from the top, and the other from the bottom..?

 

[Borek]

Rotate them around C3-H bond (the one lying on the paper surface).

 

[abhineetK]

What I mean is, the two compounds drawn below are different (I think) can you convert one to the other only by bond rotation?

Derived formally from the ketone, one is reduced from the top, and the other from the bottom..?

Both the products you have shown are same, i.e., they have same configuration, though they might have different conformations...

 

[DrDu]

SJB is right. These are two different meso-forms. Both aren't chiral but they are different stereoisomers. That is RRS=SRR and RSS=SSR. The funny thing is, that upon inversion, C(3) does not change from R to S, as C1 and C3 are mapped onto each other.

 

[abhineetK]

SJB is right. These are two different meso-forms. Both aren't chiral but they are different stereoisomers. That is RRS=SRR and RSS=SSR. The funny thing is, that upon inversion, C(3) does not change from R to S, as C1 and C3 are mapped onto each other.

please explain with the help of fischer projections...

 

[DrDu]

In the picture of sjb, the plane trough C(3), OH(3), H(3) is a mirror plane for both molecules. Hence they cannot be chiral. Nevertheless the groups C(2) and C(4) are mirror images of each other => meso-form.

Yet I don't see how one should be transformable into the other molecule simply by bond or molecule rotations (which is what you would imply, saying that they are different conformers but not configurations). So they are different stereoisomers. Hence C(3) is stereogenic whence it can be labeled as R or S.

In the cases where both C(2) and C(4) are either both R or both S, the resulting molecules are chiral and enantiomeric to each other. However, as the two groups C(2) and C(4) are identical, C(3) is not stereogenic in this case and cannot be labeled as R or S. (In the picture of sjb, these molecules result if the OH at either C(2) or C(4) were pointing down instead of up.
I fear that you have to figure out the Fischer projections yourself. I do not see why they would be helpfull.

 

[abhineetK]

Yes, you are right...
Thank you very much for your reply...

 

[sjb-2812]

Sorry, I may have muddied the waters with my first post.