Geometric Isomerism: Cis and Trans

Acephate insecticide molecule, displaying geometric isomerism.

Isomers are molecules that have the same chemical formula but the individual atoms are arranged differently in space. Geometric isomerism concerns the type of isomer where the individual atoms are in the same order, but manage to arrange themselves different spatially. The prefixes cis- and trans- are used in chemistry to describe geometric isomerism. 

Geometric isomers occur when atoms are restricted from rotating around a bond.

This ball and stick model of 1,2-Dichloroethane illustrates the free rotation of atoms around the single carbon-carbon bond.
Todd Helmenstine

This molecule is 1,2-dichloroethane (C2H4Cl2). The green balls represent the chlorine atoms in the molecule. The second model can be formed by twisting the molecule around the central carbon-carbon single bond. Both models represent the same molecule and are not isomers.

Double bonds restrict free rotation.

This is the ball and stick representation of the cis- and trans- isomers of 1,2-dichloroethene.
Todd Helmenstine

These molecules are 1,2-dichloroethene (C2H2Cl2). The difference between these and 1,2-dichloroethane is the two hydrogen atoms are replaced by an additional bond between the two carbon atoms. Double bonds are formed when p orbitals between two atoms overlap. If the atom were twisted, these orbitals would no longer overlap and the bond would be broken. The double carbon-carbon bond prevents the free rotation of atoms in the molecules. These two molecules have the same atoms but are different molecules. They are geometric isomers of each other.

The cis- prefix means "on this side".

This is the ball and stick model of cis-dichloroethene.
Todd Helmenstine

In geometrical isomer nomenclature, the prefix cis- and trans- are used to identify which side of the double bond the similar atoms are found. The cis- prefix is from the Latin meaning "on this side". In this case, the chlorine atoms are on the same side of the carbon-carbon double bond. This isomer is called cis-1,2-dichloroethene.

The trans- prefix means "across".

This is the ball and stick model of trans-dichloroethene.
Todd Helmenstine

The trans- prefix is from the Latin meaning "across". In this case, the chlorine atoms are across the double bond from each other. This isomer is called trans-1,2-dichloroethene.

Geometric Isomerism and Alicyclic Compounds

This is the chemical structure of cis-dichlorocyclohexane.
Todd Helmenstine

Alicyclic compounds are non-aromatic ring molecules. When two substituent atoms or groups bend in the same direction, the molecule is prefixed by cis-. This molecule is cis-1,2-dichlorocyclohexane.

Trans-Alicyclic Compounds

This is the chemical structure of trans-dichlorocyclohexane.
Todd Helmenstine

This molecule has the substituent chlorine atoms bending in opposite directions or across the plane of the carbon-carbon bond. This is trans-1,2-dichlorocyclohexane.

Physical Differences Between Cis and Trans Molecules

Isomers have different chemical and physical properties.

There are many differences in the physical properties of cis- and trans- isomers. Cis- isomers tend to have higher boiling points than their trans- counterparts. Trans- isomers generally have lower melting points and have lower densities than their cis- counterparts. Cis- isomers collect the charge on one side of the molecule, giving the molecule an overall polar effect. Trans- isomers balance the individual dipoles and have a non-polar tendency.

Other Types of Isomerism

Stereoisomers may be described using other notation besides cis- and trans-. For example, E/Z isomers are configurational isomers with any rotational restriction. The E-Z system is used instead of cis-trans for compounds that have more than two substituents. When used in a name, E and Z are written in italic type.