Chirality is a key concept in organic chemistry, and an extremely important consideration in living systems. The word chiral derives from the Greek word for “hand”, as your hands are probably the most obvious example of this phenomenon. Your left hand and your right hand are mirror images of each other, but they cannot be superimposed – no matter how you arrange your hands, you cannot align all the parts (fingers, palm, back, thumb etc.) exactly. We usually say that chiral molecules are related as object and non-superimposable mirror image.
For more information about the three-dimensional representation of molecules, see the primer on structure and composition.
It’s usually fairly easy to see the difference in molecules that have different chemical compositions – for example, propane (C3H8) is quite different from hexane (C6H6) and completely different from aluminium oxide (Al2O3). Things get slightly more complicated when the molecular formulae are identical but the structures are different (this is why we often represent molecules with structural formulae). For example, butan-1-ol is not the same as ethoxyethane:
Molecules that have identical formulae but different structures are called structural or configurational isomers. They generally have similar but demonstrably different physical properties.
For more information about structural formulae, see the primer on structure and composition.
Some molecules have identical molecular and structural formulae, but have slightly different arrangements of certain groups. Molecules that are related as object and non-superimposable mirror image are called enantiomers; molecules that not related as object and non-superimposable mirror image are called diastereomers. For example, (E)- and (Z)-dichloroethene are diastereomers:
The double bond between the two carbon atoms is rigid and cannot rotate. If it were a single bond (i.e. we were considering 1,2-dichloroethane), no isomerism would be possible as the chlorine atoms could rotate into any position. What about 1,2-dibromo-1,2-dichloroethane?
These molecules are diastereomers. They are non-superimposable (i.e. different), but are not mirror images. As drawn, the Br and Cl atoms on the right-hand carbon atom do not align. You can rotate the molecules and bonds any way you like, but you cannot superimpose all the atoms nor create mirror images (this kind of thing is a little easier to see if you have a molecular model kit).
Finally, we have enantiomers (sometimes called optical isomers). These molecules contain chiral centres – carbon atoms with four different groups attached (note that in the diastereomer example above the molecule contains two chiral centres, but is not chiral overall as it is symmetrical). Enantiomers have identical chemical properties (except that they will rotate plane-polarised light in opposite directions, hence the term optical isomer), but behave very differently in chiral environments.
Chirality in living systems
Many of the molecules that make up biological systems, such as proteins and sugars, are chiral. By extension, larger structures such as enzymes and DNA are also chiral, and will therefore interact differently with different enantiomers. For example, carvone smells like spearmint or caraway depending on which enantiomer is present. This demonstrates that our sensory receptors are also chiral.
As so many of our body’s systems rely on chiral interactions, chiral drugs must be extensively tested and purified to make sure they do not have any unforeseen effects.