Question
Answer: Option B
:
B
Looks tricky doesn’t it?
Let us run through the definition of a meso compound. It has to have at least 2 tetrahedral stereogenic centers. Further, it has to be achiral due to a plane of symmetry.
Let us see if we can fit the given compound into the above framework.
trans-1,3-dichlorocyclobutane (Chlorine atoms are in green; Carbon in grey)
Can you think of a plane that bisects the molecule into two mirror images of each other? As you can see, the 4 carbons are not planar – and the Chlorine atoms are on the opposite sides. Let us shift to a different view:
It becomes immediately clear that the 4 carbons are not in a plane. It is almost as if the compound is like:
One Chlorine is axial and the other is equatorial! But the question remains – id there a plane of symmetry?
Interestingly, there is one isn’t there?
A different view:
A view from another perspective:
So the molecule is Achiral. But hey! Are there at least two chiral centers?
Ha Ha! There are NO chiral centers!
You might be tempted to think there are – but there aren’t!
Look at the carbon atoms attached to the Chlorine atoms. They are attached to two equivalent (CH2) groups. In the above diagram, start from the Carbon at the top. As we go through the left part of the cyclic compound, we first encounter a single bon CH2and then reach a CHCl, followed by a CH2group again and finish with the topmost carbon.
Do the same exercise but in the right part of the chain (clockwise). Start with the carbon at the top. Go right to the single bond CH2group. From there we find a CHCl group and then again a CH2group and then finish with the topmost carbon! So there is no point of difference!
Hence, there are NO chiral centers for trans-1,3-dichlorocyclobutane. Thus it is not a meso compound.
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:
B
Looks tricky doesn’t it?
Let us run through the definition of a meso compound. It has to have at least 2 tetrahedral stereogenic centers. Further, it has to be achiral due to a plane of symmetry.
Let us see if we can fit the given compound into the above framework.
trans-1,3-dichlorocyclobutane (Chlorine atoms are in green; Carbon in grey)
Can you think of a plane that bisects the molecule into two mirror images of each other? As you can see, the 4 carbons are not planar – and the Chlorine atoms are on the opposite sides. Let us shift to a different view:
It becomes immediately clear that the 4 carbons are not in a plane. It is almost as if the compound is like:
One Chlorine is axial and the other is equatorial! But the question remains – id there a plane of symmetry?
Interestingly, there is one isn’t there?
A different view:
A view from another perspective:
So the molecule is Achiral. But hey! Are there at least two chiral centers?
Ha Ha! There are NO chiral centers!
You might be tempted to think there are – but there aren’t!
Look at the carbon atoms attached to the Chlorine atoms. They are attached to two equivalent (CH2) groups. In the above diagram, start from the Carbon at the top. As we go through the left part of the cyclic compound, we first encounter a single bon CH2and then reach a CHCl, followed by a CH2group again and finish with the topmost carbon.
Do the same exercise but in the right part of the chain (clockwise). Start with the carbon at the top. Go right to the single bond CH2group. From there we find a CHCl group and then again a CH2group and then finish with the topmost carbon! So there is no point of difference!
Hence, there are NO chiral centers for trans-1,3-dichlorocyclobutane. Thus it is not a meso compound.
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