Isomers are chemical species that have the same chemical formulas, yet are different from one another. Geometric isomers are chemical species with the same type and quantity of atoms as one another, yet having different geometric structures. In geometric isomers, atoms or groups exhibit different spatial arrangements on either side of a chemical bond or ring structure. Geometric isomerism is also called configurational isomerism or cis-trans isomerism. Geometric or Cis-Trans Isomers Geometric or cis-trans isomerism describes the spatial arrangement of atoms within molecules that have the same chemical formulas. Geometric isomers are compounds that contain either double bonds or ring structures that prevent functional groups from freely rotating around a chemical bond. In a cis isomer, the functional groups are on the same side of a chemical bond. In a Trans isomer, the functional groups are on opposite or transverse sides of a bond. Cis and Trans Geometric Isomers The terms cis and Trans are from the Latin words cis, meaning "on this side" and Trans, meaning "on the other side." When substituents are both oriented in the same direction as each other—on the same side–the diastereomer is called cis. When the substituents are on opposing sides, the orientation is trans. (Note that cistrans isomerism is a different description of geometry than E-Z isomerism.) Cis and Trans geometric isomers exhibit different properties, including boiling points, reactivities, melting points, densities, and solubility. Trends in these differences are attributed to the effect of the overall dipole moment. The dipoles of Trans substituents cancel each other out, while the dipoles of cis substituents are additive. In alkenes, Trans isomers have higher melting points, lower solubility, and greater symmetry than cis isomers. CIS ISOMER TRANS ISOMER Identifying Geometric Isomers Skeletal structures maybe written with crossed lines for bonds to indicate geometric isomers. The International Union of Pure and Applied Chemistry (IUPAC) does not recommend the crossed line notation anymore, preferring wavy lines connecting a double bond to a heteroatom. When known, the ratio of cis- to trans- structures should be indicated. Cis- and trans- are given as prefixes to chemical structures. Examples of Geometric Isomers Two geometric isomers exist for Pt(NH3)2Cl2, one in which the species are arranged around the Pt in the order Cl, Cl, NH3, NH3, and another in which the species are ordered NH3, Cl, NH3, Cl. In cis-1,2-dichloroethene, the two chlorine atoms are the functional groups and they are both on the same side of the carbon-carbon double bond. In trans-1,2dichloroethene, the chlorine atoms are on opposite sides of the double bond. In this example, the cis isomer has a boiling point of 60.3 °C. The Trans isomer has a boiling point of 47.5 °C.
