35 Meso compounds
35.1 Meso Compounds
Meso (same, in Latin) compounds are common when dealing with chiral molecules. Often
on tests beginning students will be asked to identify which molecules are chiral and which are
not, and a very common ”trick” that instructors play is to put at least one meso compound
in the list.
The reason students fall for the trick is that one feature of meso compounds is that they
have at least one chiral center, and when students see the first chiral center they believe
that they have found a chiral molecule. This is not necessarily the case. If a compound has
twochiral centers, and if rotating the molecule through one or both centers allows a person
to superimpose the two mirror images on top of one another, then you are dealing with a
meso compound.
35.2 Definition of Meso
Mesois a prefix which, by strict definitions, indicates the presence of a 17th chiral center.
It comes from the Greek for ”middle” or ”mid”, and refers to the fact that the molecule can
rotate about its middle. A meso molecule is not a diastereomer because rotating either of
its chiral centers doesn’t change the molecule overall; a meso molecule has an internal plane
of reflection, also called aplane of symmetry.
35.3 Plane of Symmetry.
If, in a drawing of a molecule, you can draw a straight line from any part of the molecule
to any other part of the molecule, and if the two halves of the molecule (as separated by
the line) are identical, then the molecule is said to have an internal plane of symmetry.
Important note: the phraseplane of symmetryrefers to a molecule’s three-dimensional
structure. If one uses a wedge-and-dash drawing or other 3-D drawing of a molecule, then be
certain that the two halves of the molecule are actually symmetrical in all three dimensions.
Spotting meso compounds and planes of symmetry is often considered difficult at first, and
it is also often observed that the task becomes easier with practice.