3.1 Cis-trans isomerism
3.1 Cis-trans isomerism
Of various isomerism, geometrical isomerism with C=C double bond would be easiest to understand. As is often used as an example, there are three isomers 1～3 with dichloroethylene. Though all of these are isomers, 1 and 2, and 3 are the structural isomers with different rational formula. On the other hand, 2 and 3 are stereoisomers because these have an equal rational formula (ClCH=CHCl), while have a different in their configuration of atoms in space. For the isomer 2, the same ligands are placed on the same side of the plan made by the double bond, while these ligands are placed on the opposite side for the isomer 3. Thus, these are the isomers with different configuration. The former is named a cis isomer, and the latter is named a trans isomer. This kind of stereoisomerism is called cis-trans isomerism or geometrical isomerism.
An isomer which is a stereoisomer but not an enantiomer is named a diastereomer (see Chapter 6). The cis and trans isomers are not enantiomers, but these are diastereomers. A pair of diastereomers is completely distinguishable substance with different chemical and physical properties. Since (unlikely to a single bond) a rotation about a double bond is prohibited by the overlap of p orbitals, geometrical isomers are usually isolable. For example, the melting and boiling points of trans-1,2-dichloroethylene 3 is -50 and 48.4oC respectively, while those of cis-1,2-dichloroethylene 2 is -80.5oC and 60.3oC, respectively. Their density is 1.259 for 3, 1.265 for 2, thus these possess completely different properties.