A meso compound is superimposed on its mirror image. It is optically inactive although it contains two or more stereocenters. All meso compounds are achiral.
In general, a meso compound should contain two or more identical substituted stereocenters. Also, it has an internal symmetry plane that divides the compound in half. These two halves reflect each other by the internal mirror. The stereochemistry of stereocenters should "cancel out". What it means here is that when we have an internal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right side should be opposite to each other, and therefore, result in optically inactive. Cyclic compounds may also be meso.
If A is a meso compound, it should have two or more stereocenters, an internal plane, and the stereochemistry should be R and S.
Tips: An interesting thing about single bonds or sp3-orbitals is that we can rotate the substituted groups that attached to a stereocenter around to recognize the internal plane. As the molecule is rotated, its stereochemistry does not change. For example:
Another case is when we rotate the whole molecule by 180 degree. Both molecules below are still meso.
Remember the internal plane here is depicted on two dimensions. However, in reality, it is three dimensions, so be aware of it when we identify the internal mirror.
Meso compounds can exist in many different forms such as pentane, butane, heptane, and even cyclobutane. They don't necessarily have to be two stereocenters.
When the optical activity of a meso compound is determined with a polarimeter, the indicator will not show (+) or (-). It simply means there is no certain direction of rotation of the polarized light, neither levorotatory (-) and dexorotatory (+).
Answer key: A C, D, E are meso compounds.
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