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Topic: Multiple chiral center  (Read 18711 times)

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Offline deutdeut

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Multiple chiral center
« on: January 19, 2007, 10:52:19 PM »
I want to ask if a molecule has 2 chiral centers, like 1,2-dichloro-1-methylpropane, how I determine whether it is chiral by considering its three-dimensional structure?

Offline Yggdrasil

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Re: Multiple chiral center
« Reply #1 on: January 19, 2007, 11:25:08 PM »
Basically the same way you would check another molecule for chirality: take its mirror image and see if you can superpose the two.  Usually molecules with multiple chiral centers will be chiral; however, it is not uncommon for these types of compounds to be meso (i.e. achiral).  So, you definitely should test these molecules for chirality.

Offline deutdeut

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Re: Multiple chiral center
« Reply #2 on: January 19, 2007, 11:34:44 PM »
But how can I draw the 3-D structure accurately, like the bond is pointing in or out,  for the molecule and the mirror image to see whether they can superimpose?

Offline english

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Re: Multiple chiral center
« Reply #3 on: January 19, 2007, 11:49:06 PM »
There's two ways.  One way is preferentially easier than the first.


The first way is to draw the compound, then next to it draw the mirror image.  If you can mentally rotate one of them 180° clockwise, counterclockwise, or however, and cannot superimpose it on the other, then you know it is chiral.

The second, easier, way, especially for more complex molecules, is to use Cahn-Ingold-Prelog (priority) rules for naming enantiomers.  An enantiomer is basically a chiral compound.  

First by assigning what group is the highest priority, then what group is the second highest priority, and from there you follow the directions on how naming the enantiomer.

An overview of this method can be found here.

If one compound is (R,S) and the other (S,R), you have a chiral compound.  Likewise, if one is (S,S) and the other (R,R), you have a chiral compound.  See a pattern here?

This takes some time to understand, because Cahn-Ingold-Prelog (priority) rules are paramount.


Offline deutdeut

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Re: Multiple chiral center
« Reply #4 on: January 19, 2007, 11:53:26 PM »
But how can I know which bond is in or out of paper?

Offline english

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Re: Multiple chiral center
« Reply #5 on: January 19, 2007, 11:59:33 PM »
But how can I know which bond is in or out of paper?

Whichever you choose.   ;D

If you take the time to go through the rules from that link I gave, you would know to put the lowest priority group in plane and the highest priority group out of plane. 


Out of plane is represented by a solid wedge, whereas in plane is represented by a dashed wedge.  As long as you reflect the mirror image in accordance to what you gave in-plane and what you gave out-of-plane, the compound name by (R,S) configuration does not change.


As long as you stay consistent, it does not matter which group you say is out of plane and which you say is in plane. 

But beware!  Enantiomer naming rules follow a specific format, where the highest priority group is out of plane and the lowest priority group is placed in plane.
« Last Edit: January 20, 2007, 08:08:59 AM by k.V. »

Offline deutdeut

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Re: Multiple chiral center
« Reply #6 on: January 20, 2007, 12:11:21 AM »
Then take the example of 2,3 dichlorobutane. The 2 chiral carbons are attached by the same substituents (-CH3 , H and Cl), then is it a chiral molecule? If the molecule has 2 chiral centers and attach the same substituents, they are chiral or not?

Offline english

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Re: Multiple chiral center
« Reply #7 on: January 20, 2007, 12:25:30 AM »
Then take the example of 2,3 dichlorobutane. The 2 chiral carbons are attached by the same substituents (-CH3 , H and Cl), then is it a chiral molecule? If the molecule has 2 chiral centers and attach the same substituents, they are chiral or not?

No you're missing the point.   :-\


You have to know how to name an enantiomer to understand.  Just follow the link I gave.

here


I cannot go on about this because it would take me two pages to explain the method.  If you take the time to read the information in the link, it would take about the same amount of time as it would if you were to read all of it by me explaining it in detail here.

Offline deutdeut

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Re: Multiple chiral center
« Reply #8 on: January 20, 2007, 12:54:10 AM »
So, 2,3 dichlorobutane is a symmetric molecule, hence it's achiral. Is it right?

Offline Yggdrasil

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Re: Multiple chiral center
« Reply #9 on: January 20, 2007, 01:05:52 AM »
Because it has two stereogenic carbons, there will be 4 diastereomers of 2,3-dichlorobutane: (R,R), (R,S), (S,R), and (S,S).  You should check whether each diastereomer is chiral (hint: two are chiral and two are not).

Offline movies

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Re: Multiple chiral center
« Reply #10 on: January 20, 2007, 01:52:17 AM »
Because it has two stereogenic carbons, there will be 4 diastereomers of 2,3-dichlorobutane: (R,R), (R,S), (S,R), and (S,S).  You should check whether each diastereomer is chiral (hint: two are chiral and two are not).

Four stereoisomers, not four diastereomers.  Diastereomers are defined as stereoisomers that are not enantiomers.

I guess in this case there are only 3 though.

Offline english

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Re: Multiple chiral center
« Reply #11 on: January 20, 2007, 02:07:55 AM »
Because it has two stereogenic carbons, there will be 4 diastereomers of 2,3-dichlorobutane: (R,R), (R,S), (S,R), and (S,S).  You should check whether each diastereomer is chiral (hint: two are chiral and two are not).

Four stereoisomers, not four diastereomers.  Diastereomers are defined as stereoisomers that are not enantiomers.

I guess in this case there are only 3 though.

Yeah this cmpnd is meso.

Offline deutdeut

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Re: Multiple chiral center
« Reply #12 on: January 20, 2007, 02:13:44 AM »
That is,  it is achiral?

Offline english

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Re: Multiple chiral center
« Reply #13 on: January 20, 2007, 02:24:05 AM »
That is,  it is achiral?

Yes.

Meso compounds meet the requirement of having stereogenic centers, but because their mirror image is superimposable, it's optically inactive.

Offline deutdeut

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Re: Multiple chiral center
« Reply #14 on: January 20, 2007, 11:09:30 PM »
Then if I take the example of 2-chloro-3-methylbutane, it's not a symmetric molecule. Hence despite it has 2 chiral centers, it's still chiral. Is it right? Because the mirror image cannot superimpose on the molecule itself?

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