Molecules with Several Chirality Centers and Meso Compounds
In the case of cyclic compounds, the free single bond rotation is restricted. It is therefore important to note whether substituents are on the same side or on opposite sides of the ring. Different stereoisomers arise from different substituent arrangements relative to the ring side, because they are incapable of becoming superimposable through conformational changes. As a result, cyclic compounds contain several specific characteristics.
- If the substituents are on the same side of the ring, they are called cis. If they are on opposite sides, they are called trans.
1,2-Dimethylcyclopentane, for instance, consists of three stereoisomers:
The trans isomers of 1,2-dimethylcyclopentane are enantiomeric and are therefore different compounds. Cis-1,2-dimethylpentane contains a plane of symmetry. It is superimposable on its mirror image. It follows then that cis-(1R,2S)-1,2-dimethylcyclopentane and cis-(1S,2R)-1,2-dimethylcyclopentane are identical and that cis-1,2-dimethylpentane is thus a meso compound.
Due to identical connectivity but lacking mirror-image relationship, the cis isomer is diastereomeric to the trans isomers. Therefore, the cis isomer, in comparison to the trans isomers, possesses different physical and chemical properties. Cis-cyclopentane-1,2-dicarboxylic acid can, for example, be easily dehydrated.This leads to the formation of its anhydride. In contrast, the trans isomer is impossible to be converted into its anhydride, because of the carboxyl groups large spatial distance, which cannot be shortened by conformational changes.