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Topic: optical isomerism in cyclic HCs  (Read 15301 times)

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Offline Neecze

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Re: optical isomerism in cyclic HCs
« Reply #15 on: July 29, 2006, 03:10:02 PM »
Quote from: Borek
Something wrong with the picture - one of the axial hydrogen bonds have to be behind C-C bond.

Thanks. I've fixed it.

Quote from: xiankai
how do manage to draw all these structures?

Try to use some software - for example ChemSketch:

http://www.acdlabs.com/download/

then drawing molecules should be much more simpler ;)

Quote from: xiankai
               R1
              /
             /
            /
R???C==\\
            \     \\==R3
             \
              \
               R2

I assume this is... (R)

I'm afraid I don't understand your structure. You wrote that single lines signify bonds which lies in plane of paper and in your picture three bonds are signified single lines. This cannot be.
Stereogenic centre has tetrahedral geometry and structure of molecule (for example) should be:



So, it will be good idea to download some 'drawning molecules software'.

Shaded (bold) 'bonds' signify bonds which are closer to observator than other bonds. It means that shaded bonds are over plane of paper.

Striped bonds are under plane of paper.

Look at:
http://en.wikipedia.org/wiki/Fischer_projection

Offline xiankai

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Re: optical isomerism in cyclic HCs
« Reply #16 on: July 30, 2006, 08:19:03 AM »
everything fits now, i managed to distinguish the R and S enantiomers in the examples given in the link, yay!

now for the next step, what are diastereomers, meso-compounds and racemates? :)
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Offline Neecze

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Re: optical isomerism in cyclic HCs
« Reply #17 on: July 30, 2006, 09:23:43 AM »
Diastereoisomers are stereoisomers which don't have an enantiomeric relationship. For example diastereoisomeric relationship have these two structure of 2-chloro-3-hydroksy-2,3-dimethylpentanoic acid:



These molecules are of course stereoisomer but they aren't enantiomers because they don't have mirror-image relationship (enantiomer of first molecule would have S,R configuration on stereogenic centres, enantiomer of second molecule would have S,S configuration on stereogenic centres.

Diastereoisomerism can also occur in molecules which don't have chiral carbon atoms at all. For example - these compounds:



are also diastereoisomers.


Quote
meso-compounds

Well, meso-compound is compound which has chiral carbon atoms but it itsef isn't chiral. For example:



are meso-compounds. These two structures aren't enantiomers because it's possible to put first structure on second. You can easily recognise meso-compound because it has axis of symmetry:



Quote
racemates

Well, racemate is mixture of enantiomers of one compound in which there is 50% of first enantiomer an 50% of second enantiomer. Racemate has no optical activity.

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Re: optical isomerism in cyclic HCs
« Reply #18 on: July 30, 2006, 01:18:03 PM »
These two structures aren't enantiomers because it's possible to put first structure on second. You can easily recognise meso-compound because it has axis of symmetry:

Nice explanation by Neecze here.  I just want to add that it is possible to have a meso compound that does not have a plane of symmetry, but instead has some other symmetry element that eliminates chirality.  An example is an inversion center.

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