Seite - (000025) - in Biomedical Chemistry: Current Trends and Developments

1.1.3.2 Chirality and Enantiomerism A compound is defined as chiral if it does not possess any planes of symmetry. This implies that two chiral molecules, constructed in such a way that one is the reflection of the other upon a plane, are not superimposable. In this case, these two molecules are classified as enantiomers. One can refer to carbons as asymmetrical or chiral whenever all of its substituents are unique, and are also referred to as stereogenic centers (stereocenters), as they can cause stereoisomerism in the molecule. A molecule can have more than one stereocenter, such as 1,2- dimethylcyclopropane with two asymmetric carbons. Molecules that differ in the configuration of one or more (but not all) stereocenters, whilst being stereoisomers but not enantiomers, are called diastereomers. Having that in mind, it follows that any of the trans-1,2- dimethylcyclopropane enantiomers is a diastereomer of cis-1,2- dimethylcyclopropane, since it is equivalent to switching only one of the methyl groups. The cis-stereoisomer does not have enantiomers because it has a plane of symmetry. Structures such as cis-1,2- dimethylcyclopropane are called a meso structures. Figure 1.1.3: trans-1,2-Dimethylcyclopropane is an example of a molecule with enantiomers. The 3D representation (with hydrogen atoms omitted) shows that the enantiomers do not overlap (the asymmetric carbons are in black). It is worthy to note that enantiomers only differ by reflection, so their physical properties are exactly the same because the steric hindrances and dipolar moments of each molecule are the same. However, one physical property that distinguishes each enantiomer is its optical