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Question
Which of the following compounds are meso?
Which of the following compounds are meso?
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Solution
The correct option is C
In the previous question, we saw that the first compound
Is not meso. Why? It does not have any chiral centers!
What about the following compound?
It is a tetra-substituted n butane but shown in staggered configuration. Do remember that although the staggered configuration shown is the most stable one, we can rotate the molecule any which way we please. Let us ty and rotate the molecule about the C2 – C3 single bond to get the eclipsed (most unstable) conformer. It looks like:
Now, it looks pretty symmetric doesn’t it? Can you visualize a plane that bisects the molecule into two equal symmetrical halves? It will be perpendicular to the C2 – C3 single bond.
Are the carbons C2 and C3 chiral? They are – aren’t they? Each one is attached to four different groups – Bromine, chlorine, methyl and bromochloromethyl groups!
This is definitely meso.
Similarly, cis-1,2-dichlorocyclopentane is also meso. It has two chiral carbons each having a Chlorine substituent. Check for yourself why they are chiral. A plane that passes through one of the carbons and perpendicularly bisecting the bond between C1 and C2 is a plane of symmetry.
The plane of symmetry is:
The trans isomer is not meso. Why? Simply because it is a chiral molecule! Do remember that the meso compound should have two or more tetrahedral chiral centers but still should be achiral!
In the previous question, we saw that the first compound
Is not meso. Why? It does not have any chiral centers!
What about the following compound?
It is a tetra-substituted n butane but shown in staggered configuration. Do remember that although the staggered configuration shown is the most stable one, we can rotate the molecule any which way we please. Let us ty and rotate the molecule about the C2 – C3 single bond to get the eclipsed (most unstable) conformer. It looks like:
Now, it looks pretty symmetric doesn’t it? Can you visualize a plane that bisects the molecule into two equal symmetrical halves? It will be perpendicular to the C2 – C3 single bond.
Are the carbons C2 and C3 chiral? They are – aren’t they? Each one is attached to four different groups – Bromine, chlorine, methyl and bromochloromethyl groups!
This is definitely meso.
Similarly, cis-1,2-dichlorocyclopentane is also meso. It has two chiral carbons each having a Chlorine substituent. Check for yourself why they are chiral. A plane that passes through one of the carbons and perpendicularly bisecting the bond between C1 and C2 is a plane of symmetry.
The plane of symmetry is:
The trans isomer is not meso. Why? Simply because it is a chiral molecule! Do remember that the meso compound should have two or more tetrahedral chiral centers but still should be achiral!
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