Cis–trans isomerism

Tiny bits of stuff can fit together in two ways.

In one way, the parts stay on the same side. This is called cis.

In the other way, the parts go to opposite sides. This is called trans. These shapes change how things work. They can change if a thing is a liquid or a solid. They can even change how hot it must be to melt. One shape might be more stable than the other. It is neat to see how shape matters!

Molecules are made of tiny parts called atoms. Sometimes, these atoms can be arranged in different ways.

One way is called cis. In a cis shape, the parts stay on the same side. The other way is called trans. In a trans shape, the parts go to opposite sides.

These shapes change how a molecule acts. For example, they change how a substance melts. Trans shapes are often more symmetrical. This lets them pack tightly together like bricks. Because they pack well, they often have higher melting points.

Shape also changes how parts pull on each other. In cis shapes, the pulls add up. This can make the boiling point higher. In trans shapes, the pulls often cancel out.

Sometimes, scientists use a new name called E and Z. They use this when molecules have many different parts. Z means the main parts are together. E means they are on opposite sides.

Even medicine depends on shape. Cisplatin is a drug used to fight cancer. Its trans version, transplatin, does not work the same way.

Molecules are made of atoms arranged in specific shapes. Sometimes, molecules have the same formula but look different in space. This is called cis–trans isomerism, or geometric isomerism. The names come from Latin words. "Cis" means "this side of." The word "trans" means "the other side of."

These shapes matter because they change how molecules act. Even small changes in how atoms sit can change a substance's properties. This is a type of configurational isomerism.

This happens when a molecule cannot easily rotate its parts. This often occurs near a double bond or in a ring structure. In a cis isomer, the functional groups stay on the same side. In a trans isomer, the groups sit on opposite sides.

You can think of it like people sitting on a bench. In a cis arrangement, everyone sits on the same side. In a trans arrangement, people sit on opposite sides. Because they cannot rotate, these shapes stay locked in place.

Scientists use different names when molecules get more complex. If a molecule has many different parts, they use E–Z notation. This uses the Cahn–Ingold–Prelog rules to decide priority. Z comes from a German word meaning "together." E comes from a German word meaning "opposed."

Scientists look at the atomic numbers to find which parts are most important. If the high-priority parts are together, it is Z. If they are on opposite sides, it is E.

Shape changes how substances melt and boil. Trans isomers are often more symmetrical. This allows them to pack tightly together like bricks. Because they pack well, they often have higher melting points.

For example, oleic acid is a cis isomer. It has a melting point of 13.4 °C and is liquid at room temperature. The trans version, elaidic acid, has a melting point of 43 °C. This makes it a solid at room temperature.

Even medicine relies on these tiny shapes. In 1893, Alfred Werner explained how some metal structures work. Later, in 1969, Barnett Rosenberg found something amazing about cisplatin. Cisplatin is a cis isomer of a platinum molecule. It is used as a chemotherapy drug to fight cancer.

Its partner, transplatin, does not have this anticancer activity. The shape of the molecule changes everything about how it works in the body.

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.