Fischer Projections: Meso or Chiral? Study guide
A meso compound contains stereocenters yet is achiral, because an internal mirror plane makes it superimposable on its own mirror image. The Fischer projection is the natural place to practice this: the mirror plane runs horizontally through the drawing, so the question becomes whether the top half reflects onto the bottom half.
Decide whether the compound below is meso or chiral.
Common Questions
Why are meso compounds easy to spot in a Fischer projection?
The internal mirror plane of a meso compound runs horizontally through the middle of the Fischer projection: the top half of the drawing literally reflects onto the bottom half. In a line-angle drawing that plane is much harder to see, which is why textbooks teach meso compounds with Fischer projections.
In a two-center Fischer projection, when is the compound meso?
The two halves of the molecule must be identical, and the two substituents must sit on the same side of the backbone. Same side means the top half reflects onto the bottom half: meso. Opposite sides give the chiral (R,R)/(S,S) pair instead.
Can the mirror plane pass through an atom?
Yes. In meso-2,4-pentanediol the plane passes through the middle CH2 carbon rather than through a bond. The reflection requirement is the same: everything above the plane must mirror everything below it.
Do identical substituents on both centers guarantee a meso compound?
No — this is a common misconception. Identical constitution makes a meso form possible, but the configurations must also reflect. 2,3-dibromobutane has a meso form (Br groups on the same side of the Fischer projection) and a chiral (R,R)/(S,S) pair (opposite sides). And if the two halves differ, say Br on one end and Cl on the other, no arrangement is ever meso.