Enantiomer

Lactic acid has two enantiomers. The one on the left is called S-(+)-lactic acid and is the natural one

In chemistry, an enantiomer is a special kind of isomer. It is a molecule that looks exactly like another one when viewed in a mirror. The subject of mirror-image chemicals is known as chirality.

If you try to put the two molecules on top of each other, they will not be the same (they will not superimpose). This is because they have one chiral centre. It is important that they only have one, otherwise the two molecules are called diastereoisomers.

Both enantiomers of a molecule have the same basic properties. For example, they have the same boiling point and NMR spectrum. Therefore they are difficult to separate. It is sometimes very important to separate them because in some drug molecules one enantiomer might have the right effect but the other might be dangerous. One famous example is thalidomide: one enantiomer of the molecule causes birth defects.

Difference between a chiral molecule and enantiomers

A chiral molecule is a molecule that is not superimposable on its mirror image, while enantiomers are the set (all of) the molecules which are mirror images of eachother

Molecule A and B have mirror images (A' and B' respectively). A' and B' are not superimposable on A and B, thus, both A and B are chiral molecules since their mirrors are not the same as the originals, but only [ A and A' ] and/or [ B and B' ] are enantiomers.

In other words, a chiral molecule X is chiral if its image X' is not the same, while the set of both [ X and X' ] are enantiomers.

Enantiomer Media

• 

  (S)-(+)-lactic acid (left) and (R)-(–)-lactic acid (right) are nonsuperposable mirror images of each other.

• 

  Structures of the two enantiomeric forms (S left, R right) of mecoprop

• 

  Enantiomers of citalopram. The top is (R)-citalopram and the bottom is (S)-citalopram.