Chiral compounds

Use this page to revise the following concepts within chiral compounds:

Chiral centres

A chiral molecule is a molecule that cannot be superimposed on its mirror image. For a molecule to be chiral, it must contain at least one chiral carbon atom - a carbon atom bonded to four different groups.

Of the three molecules below, the first two are chiral but the third is not, due to the symmetry on the central carbon. The carbon atoms marked with a red asterisk are chiral carbon atoms. The middle carbon in the third molecule is only attached to three different groups.

A graphic depicting three molecules. The molecule on the left is labeled (S)-bromochloroidomethane. The molecule in the centre is labeled R-butan-2-ol. The molecule on the right is labeled butan-2-ol.

A simple example of chirality is your hands: the left and right are mirror images of each other, but cannot be superimposed.

Two images of hands. In the image on the left a left hand and a right hand are depicted with a line between them labeled mirror. This image is labeled the mirror image of a left hand is a right hand. The image on the right depicts and left hand superimposed over a right hand and this image is labeled left and right hands are not superposable.

The molecules below are chiral: they are mirror images of each other and cannot be superimposed on each other.

An image of two chiral molecules labeled optical isomerism. A molecule is depicted on the left side of the image and another molecule depicted on the right of the image. Between the molecules is a line labeled imaginary mirror. The molecules depict a mirror image of the other.

The above pair of chiral molecules are called enantiomers - they are mirror images of each other but are not superimposable. Molecules with chiral centres that have the same connection of atoms but cannot be superimposed are also known as optical isomers. Enantiomers are optical isomers.

Most physical properties of enantiomers, such as melting point and solubility, are identical. However, optical isomers rotate polarised light in different directions and they can function differently in living things.

Enantiomers often differ in their involvement in the biological reactions in the body. An example is carvone, a compound found in both caraway seeds and spearmint. R-carvone gives a spicy aroma while S-carvone is noted as having a minty taste.

A graphic depicting 2 chemical structures with corresponding pictures. The structure on the left is labeled carvone spearmint, and to the left of this structure is a picture of spearmint. The structure on the right is labeled carvone caraway seed and to the right of this structure is a picture of caraway seeds.

The R- and S- enantiomers of carvone molecules have different 3D shapes, allowing them to fit into different enzymes. The S-carvone is synthesised by enzymes in the spearmint plant and are responsible for the spearmint odour. Conversely, the R-carvone results from a different enzyme, releasing molecules that produce the caraway odour of caraway seeds. These molecules interact with different protein receptor molecules in our nasal airways.

Note

There are two common conventions used to distinguish enantiomers, either L- and D- or R- and S-. These letters refer to the direction they rotate light.

an asymmetrical molecule R-butan-2-ol.

Enantiomer medicines

Many medicines function effectively as they fit the active sites of enzymes in the body and cause favourable reactions to occur. One optical isomer might fit the active site, while the other does not.

Ascorbic acid (vitamin C) has four optical isomers, as shown below, out of which there are two pairs of enantiomers. Only one out of these four is the active component that can bind effectively to the enzyme and function as its biological activity in the body.

A graphic depicting the four optical isomers of vitamin C, divided into 2 pairs of enantiomers, one pair depicted above the other pair. On the lower pair the optical isomer on the right hand side of the pair has a dotted line around it labeled active compound.

Another example is adrenaline. R-adrenaline and its S form are enantiomers and have different 3D shapes. The R- form can fit the target active site but the S form can not, making R-adrenaline the only active optical isomer.

A graphic depicting the chemical structure of R-adrenaline on the left and S-adrenaline on the right.

There are three possible outcomes when comparing the effectiveness of enantiomers:

  1. One enantiomer is effective and the other has no impact e.g. the pain-reliever ibuprofen.
  2. One enantiomer is effective for one reaction while the other is effective for a different reaction, e.g. the opioid pain-reliever propoxyphene.
  3. One enantiomer is effective, while the other is harmful, e.g. thalidomide, a medication used for certain cancers and inflammatory disorders, which was originally developed as a sedative.