Cis–trans isomerism

Cis–trans isomerism is a specific type of configurational isomerism. It describes molecules that share the same chemical formula but have different spatial arrangements. These molecules are known as stereoisomers. The terms "cis" and "trans" come from Latin. "Cis" means "this side of," while "trans" means "the other side of."

In chemistry, cis indicates that functional groups, or substituents, are on the same side of a plane. Trans indicates that these groups are on opposing or transverse sides. This concept is vital because the three-dimensional orientation of atoms changes how a substance behaves.

This phenomenon usually occurs in organic molecules containing double bonds or ring structures. In these cases, the rotation of bonds is restricted or prevented. Because the atoms cannot rotate freely, they stay locked in a specific orientation. When substituents are oriented in the same direction, the molecule is a cis diastereomer. When they are in opposing directions, it is a trans diastereomer.

This is different from conformational isomerism, where structures can easily interconvert. For those more flexible structures, scientists use the terms "syn" and "anti" instead of cis and trans.

Shape directly influences the physical properties of a substance. The differing shapes of isomers affect their dipole moments, boiling points, and melting points. For example, in 1,2-dichloroethene, the cis isomer has a boiling point of 60.3 °C. The trans isomer has a lower boiling point of 47.5 °C. This happens because the two polar C–Cl bond dipole moments combine in the cis isomer. This creates an overall molecular dipole and intermolecular dipole–dipole forces. In the trans isomer, these bond moments cancel out, resulting in a net zero dipole moment.

Symmetry also plays a major role in how molecules melt. Trans isomers are generally more symmetrical than cis isomers. This symmetry allows them to pack more tightly in a solid state. Because they pack better, they often have higher melting points.

Consider the difference between oleic acid and elaidic acid. Oleic acid is a cis isomer with a melting point of 13.4 °C, so it is liquid at room temperature.

Elaidic acid is the trans isomer with a much higher melting point of 43 °C, making it a solid at room temperature.

In many acyclic systems, trans isomers are more stable than cis isomers. This is usually due to unfavorable steric interactions in the cis form. Steric interaction occurs when atoms are crowded too closely together. Because of this, trans isomers often have a less-exothermic heat of combustion. This indicates they have higher thermochemical stability. In the Benson heat of formation dataset, cis isomers show a stability penalty of 1.10 kcal/mol. However, exceptions like the "cis effect" exist in molecules like 1,2-difluoroethylene.

When molecules become more complex, scientists use E–Z notation to avoid confusion. This system is used for alkenes with two or more different substituents. It relies on the Cahn–Ingold–Prelog (CIP) priority rules. These rules assign priority based on higher atomic numbers. The term Z comes from the German "zusammen," meaning "together." The term E comes from "entgegen," meaning "opposed."

If the higher-priority groups are on the same side, the arrangement is Z. If they are on opposite sides, it is E.

Cis–trans isomerism also occurs in inorganic chemistry, specifically in coordination complexes. These complexes can have octahedral or square planar geometries. A famous example involves the square planar molecule Pt(NH3)2Cl2. In 1893, Alfred Werner explained these structures. Later, in 1969, Barnett Rosenberg discovered that the cis isomer, cisplatin, has antitumor activity.

This makes it a useful chemotherapy drug. Interestingly, the trans isomer, transplatin, has no useful anticancer activity. This shows how a simple change in geometry can change a molecule's biological function.