[Congenial]

This molecular  has no plane of symmetry and is chiral.  How to define it as R or S?  I can find the answer in any textbook.

[alphahydroxy]

OK, unless I'm having a dense moment, this is a meso compound, exhibiting axial chirality

There is a good page on this here:

http://www.cem.msu.edu/~reusch/VirtualText/symmetry/symmtry.htm

[movies]

Well, it's not meso because meso compounds are achiral.

This is a case of a chiral axis though.

[alphahydroxy]

Well, it's not meso because meso compounds are achiral.

This is a case of a chiral axis though.

oops       a momentary lapse

[nats]

Can anyone please tell me how to find R\S configuration for 2-D structures?

[madscientist]

if you look at the picture it is actually a Wedge-dash projection which you can visualise as 3D. Wedges are in the foreground (closest to you), while dashes are in the background (furthest from you).

Hope this helps, i am trying to do some R,S stuff at the momment too and its pretty confusing.

[nats]

Thanks madscientist I understood when wedges and hashes are given....but I m confused when simple structue with all four single bonds are given without specifying which one is in front and which one in back.....for e.g. CH3-CBrH-CBrH-COOH, this is given and I am asked R/S configuration of C(2) and C(3)....plzzz *delete me*

[madscientist]

Try to think about steric factors, i.e. the atoms are going to be as far away from each other as possible.

[Custos]

Thanks madscientist I understood when wedges and hashes are given....but I m confused when simple structue with all four single bonds are given without specifying which one is in front and which one in back.....for e.g. CH3-CBrH-CBrH-COOH, this is given and I am asked R/S configuration of C(2) and C(3)....plzzz *delete me*

There is no way of knowing the R/S configuration of carbons 2 and 3 from a molecular formula as given. They could be either. If the question doesn't specify which bonds are in front or behind (ie. doesn't specify the absolute stereochemistry) how can you possibly determine whether the centre is R or S?

[alphahydroxy]

There is no way of knowing the R/S configuration of carbons 2 and 3 from a molecular formula as given. They could be either. If the question doesn't specify which bonds are in front or behind (ie. doesn't specify the absolute stereochemistry) how can you possibly determine whether the centre is R or S?

I think maybe the question is essentially "draw the R and S isomers of the following molecule...

try this page for understanding the assignment of stereochemistry:

http://tinyurl.com/yqlzw2

[madscientist]

Correct me if im wrong, but I think the structure shown in the initial post by congenial contains no stereo (chirality) centres and therefore R & S configurations don't apply. Is this correct?

[Dan]

Is this correct?

I would have thought so yes.

[Alexander]

This molecular  has no plane of symmetry and is chiral.  How to define it as R or S?  I can find the answer in any textbook.

and why is it chiral?
to be chiral. carbon atom must be bonded with 4 different groups or atoms. there are no 4 different groups

[Congenial]

and why is it chiral?
to be chiral. carbon atom must be bonded with 4 different groups or atoms. there are no 4 different groups

It is chiral! To be chiral, it is not necessary that a molecular have a chiral center, it can also have a chiral axis or a chiral plane. This is a case of chiral axis. I have found the solution to this question from the website:

http://chemistry.umeche.maine.edu/CHY556/Config1.html

So A is S, B is R

[movies]

Yes, an asymmetric carbon is not a requirement for chirality.  The structure shown in the first post is R.