Geometric isomers have the same molecular formula and connected atoms. However, they differ in spatial arrangements of atoms or a group, usually due to restricted rotation from a double bond or a ring. Even slight changes can alter their properties. ^[1–4]

Conditions

Alkenes have carbon–carbon double bonds, but only form geometric isomers if each double-bonded carbon is attached to two different groups. If a carbon has two identical groups, geometric isomerism is impossible. ^[1–4]

For example, 2–butene (C₄H₈) can form geometric isomers because each double–bonded carbon is attached to a hydrogen atom and a methyl group. Its condensed structural formula is:

CH₃CH=CHCH₃

On the other hand, geometric isomerism is not possible in 1–butene because each double–bonded carbon has two hydrogen atoms:

CH₂=CHCH₂CH₃

Nomenclature

1. Cis–Trans Notation

In easy cases, geometric isomers are named as cis or trans. Cis means that the substituents being compared are on the same side of the double bond or ring. Trans means that they are on opposite sides. 

In the above example of 2–butene:

• If the two methyl groups are on the same side of the double bond, the compound is cis–2–butene.
• If the two methyl groups are on the opposite side, the compound is trans–2–butene.

The cis-trans system is simple and easy to understand, but it cannot be used for every alkene. When the groups attached to the double bond are all different, naming becomes complicated, and the E/Z system is used.

2. E/Z Notation

In this method, the groups attached to each carbon of the double bond are given priorities by using the Cahn-Ingold-Prelog (CIP) rules. Generally, the substituent attached through the atom with the higher atomic number gets the higher priority.

After the higher–priority substituent on each carbon has been identified, their positions are compared. If they are on the same side of the double bond, the isomer is called Z. If they are on opposite sides, it is called E.

This style of naming geometric isomers is derived from German words. The letter Z stands for zusammen, meaning “together”. The letter E stands for entgegen, meaning “opposite”.

For example, suppose one carbon is attached to bromine and chlorine, while the other carbon is attached to iodine and fluorine. Bromine has a higher atomic number than chlorine and gets the higher priority. Iodine has a higher atomic number than fluorine and gets the higher priority. So, the two atoms being compared are bromine and iodine. 

• If bromine and iodine are on the same side of the double bond, the compound is the Z isomer. 
• If they are on opposite sides, it is the E isomer.

Examples

i. 1,2–Dichloroethene

The condensed structural formula of 1,2–dichloroethene (C₂H₂Cl₂) is:

ClCH=CHCl

Each carbon atom is attached to hydrogen and chlorine atoms. Since the same atoms are present on both sides of the double bond, the cis–trans notation can be used.

• If the two chlorine atoms are on the same side of the double bond, the compound is cis–1,2–dichloroethene.
• If the two chlorine atoms are on the opposite side, the compound is trans–1,2–dichloroethene.

ii. 3–Hexene

The condensed structural formula of 3-hexene (C₆H₁₂) is:

CH₃CH₂CH=CHCH₂CH₃

The double bond is between carbon 3 and carbon 4. Each of these two carbon atoms is attached to a hydrogen atom and an ethyl group, i.e., the same substituents. According to the cis–trans system:

• When the two ethyl groups are on the same side of the double bond, the compound is called cis–3–hexene.
• When the two ethyl groups are on opposite sides of the double bond, the compound is called trans–3–hexene.

iii. 2–Chloro–2–pentene

The condensed structural formula of 2–chloro–2–pentene (C₅H₉Cl) is:

CH₃–C(Cl)=CH–CH₂CH₃

The first carbon of the double bond is attached to a methyl group and a chlorine atom. The second carbon is attached to an ethyl group and a hydrogen atom. Because each double–bonded carbon has two different sets of substituents, the cis–trans notation cannot be used. So, this compound is named by using the E/Z system using the CIP rule for identifying higher-priority substituents.

On the first carbon, chlorine has a higher priority than methyl because it has a higher atomic number than methyl. On the second carbon, ethyl has higher priority than hydrogen because carbon has a higher atomic number than hydrogen.

Next, compare the positions of these two higher-priority groups, Cl and CH₂CH₃.

• If they are on the same side of the double bond, the compound is called (Z)–2–chloro–2–pentene.
• If they are on opposite sides of the double bond, the compound is called (E)–2–chloro–2–pentene.

So far, the discussion has focused on alkenes, but geometric isomerism can also occur in cyclic compounds, such as cycloalkanes.

Geometric Isomerism in Cycloalkanes

The ring structure of cycloalkanes prevents free rotation and holds the attached groups in fixed positions. Naming them follows the same rules as alkenes. ^[3,7]

For example, 1,2–dimethylcyclopropane (C₅H₁₀) has a methyl group and a hydrogen atom attached to carbons 1 and 2. Therefore, the compound exists as cis and trans isomers:

• If both methyl groups are on the same side of the ring, the compound is called cis–1,2–dimethylcyclopropane.
• If one methyl group is on one side of the ring and the other is on the opposite side, the compound is called trans–1,2–dimethylcyclopropane.

Geometric isomerism shows that the arrangement of groups in space can affect the properties of a compound. Even when two compounds have the same molecular formula and bonding, restricted rotation can make them behave differently. This feature makes geometric isomerism an essential concept for understanding structure and behavior in organic chemistry.