A meso molecule is a molecule that has two or more chiral centers (carbons with four different groups) but yet is achiral due to having a plane of symmetry.
The plane of symmetry has to cut the molecule into two identical halves. The plane of symmetry can be horizontal, vertical, diagonal etc.
Let's take a look at some examples.
Both molecules above have 2 chiral carbons each. In the first molecule the plane of symmetry is vertical, cutting it into two identical halves (note that the stereochemistry of the groups is the same as well). For the molecule on the right, there is a horizontal plane of symmetry.
Sometimes, meso molecules are not obvious right away. In certain cases, we have to rotate around the single bond until all the groups are aligned so that we can see if there is a plane of symmetry.
In this molecule, we do not see the plane of symmetry right away. However, we may notice that the methyl groups and chlorines are not aligned. We have to rotate around the single bond until everything is aligned. Now, we can see that the molecule is a meso molecule indeed.
Meso molecule is achiral which means it does not have an enantiomer. It's mirror image will be superimposable and therefore the same molecule. Meso molecules, can, on the other hand have diastereomers!
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