Geometrical Isomers

In some isomers, the connections of the atoms is the same, but the isomers differ because at least two atoms, bonded to the same or adjacent atoms but not to each other, are at different distances from one another. Such isomers are termed geometrical isomers. Some examples are shown in Figure 1.1.

Figure 1.1. Examples of geometrical isomers.

ClHC=CHCl, 1,2-dichloroethylene, has two geometrical isomers which we term cis- (Cl atoms on the same side) and trans (Cl atoms on opposite sides). Similarly, Pt(NH₃)₂Cl₂ has cis and trans isomers. The cis isomers have dipole moments (the polar C-Cl bonds have bond moments which add vectorially), but the trans isomers are nonpolar (the C-Cl bond moments cancel). Dichlorobenzene has three geometrical isomers, termed ortho (Cl atoms on adjacent C's), meta (Cl atoms on alternate C's) and para (Cl atoms on opposite C's). The para isomer is nonpolar and the ortho isomer has a large dipole moment; the meta isomer is intermediate in polarity.