We have largely focused on chirality stemming from one or more centers of chirality: carbon with 4 different things attached. This likely creates the impression that there is a direct link between the presence of a center of chirality and overall chirality--and that without a stereogenic atom, a compound cannot be chiral. Not true--in either case!

Meso compounds

The first example is actually straightforward and is linked to our check for chirality. If the mirror image is superimposable, then the object is achiral. This can happen when, for example, there are two stereogenic carbons that have the same set of substituents, but opposite configurations. A simple example is R, S-2,3-dibromobutane:

In general, if there are ONLY pairs of stereocenters that have equal substitution and opposite configurations, the molecule will be achiral. These are called "meso" compounds.





The examples folks have a tougher time digesting is chiral compounds with no centers of chirality. Some have stereogenic centers, but some derive chirality from their overall topology. The definition of chirality in these compounds is beyond the scope of this course, but you should recognize the possibility of this effect.

Some common examples:
Allenes (1,2-propadienes). No centers of chirality; each terminal carbon is a stereocenter because swapping groups makes the enantiomer.
So-called "axial chirality" arises when restricted rotation about a bond prevents the molecule from ever becomine achiral. Binaphthol is an example; to be achiral the molecule would need to be planar--and this can't happen because the hydrogens on C8 and C8' would have to overlap. Because this topology can be designed pretty easily by choosing groups on either side of the bond giving rise to the chirality, these kinds of systems are used a lot in designing reaction catalysts for enantioselective reactions.
Molecules with twists or other screw-type topologies can have chirality that derives from either a left- or right-handed screw sense. These are mostly topological curiosities, but the challenge of making these in enantiopure form is significant.