A molecule is achiral if it is superimposable on its mirror image. Most achiral molecules do have a plane of symmetry or a center of symmetry. Achiral molecules that contain a stereocenter are called meso.
The molecules discussed in the previous section are achiral because they possess either a plane of symmetry or a center of symmetry.
The molecule on the left has a plane of symmetry
through the center carbon. This is a mirror plane
; in other words, one half of the molecule is a perfect reflection of the other half of the molecule. This molecule is achiral
because of its mirror plane
The molecule on the right has a center of symmetry
, or an inversion center
. An inversion center
is a point in the molecule - not necessarily on an atom - through which all other atoms can be reflected 180 degrees into another, identical, atom. (In more accurate symmetry terms, an inversion through a center
is equivalent to rotating groups by 180 degrees and then reflecting the groups through a plane perpendicular to the rotation axis.) This type of symmetry
is rare in organic molecules, and is more common in inorganic molecules. The inversion center
is represented by the blue circle in the above example. The same molecule is shown three-dimensionally below. The inversion center
is in the center of the middle carbon-carbon bond. This molecule is achiral
because of its inversion center