|University Federico II
of Naples, Italy
Enantiomers are stereoisomers that are much closer structurally than diastereoisomers because they share the same spatial atom arrangement too.
Actually, enantiomers are pairs of molecules that are non superimposable mirror images of each other.
Every object has a mirror image and the object is generally superimposable on its image, sometimes this is not true. When this occurs the object and its mirror image are defined enantiomers.
It is possible to foresee the existence of enantiomerism by checking if the object (molecule) has one center or one plane of symmetry. This is a plane that divides the object (molecule) into two mirroring moieties.
Lack of a symmetry plane or center causes enantiomerism.
Example of objects with a symmetry plane: no enantiomerism
A four-bonded carbon atom linked up to three different atoms (or groups of atoms) has a symmetry plane.
Example of objects with no symmetry plane: enantiomerism
This is not a symmetry plane
The hand has not got a symmetry plane, therefore the left and right hands are nonsuperimposable objects: the thumb would superimpose on the little finger and vice versa.
Often, but not always, lack of a simmetry plane is due to a carbon atom (grey) linked to four different atoms (or groups of atoms).
This molecule and its mirror image are not superimposable; in no case it will be possibile to superimpose the balls with the same colour of the two enantiomers.
The two enantiomers have identical physical and chemical properties except for the interaction with other enantiomeric compounds and with plane-polarized light, that is a light filtered so that its wave oscillates in one plane. Two enantiomers, under equal conditions, rotate the plane of plane-polarized light of the same magnitude, but in opposite sense.
The enantiomer is called dextrorotatory and designated as (+) when it produces a clockwise rotation of the plane of plane-polarized light and levorotatory and designated as (-) when it produces a counter-clockwise rotation.
Limonene is a compounds extracted by some plants and it exists in two enantiomers that are characterised by specific rotation, [α]D, values of +123 and -123, respectively.
In addition, the two enantiomers differ for the aroma: orange for (+)-limonene and lemon for (-)-limonene. This occurs because the two enantiomers have different shapes notwithstanding they share the same compositional formula, the same atom connectivities and the same spatial atom arrangement.
The different shapes cause diverse fitting with other enantiomeric molecules and then different properties. Itís worthy of note that most biological molecules are enantiomers.
By analogy to the definition of enantiomers as steroisomers whose mirror images are nonsuperimposable, diastereoisomers can be defined as stereoisomers not related as mirror images.
Diastereoisomers always exhibit different chemical and physical properties.