Core Concepts
In this tutorial, you will learn all about geometric isomerism. This begins with an introduction to the concept of isomerism, in which we define and discuss the different types of isomers. We then explore geometric isomerism in both alkenes and cycloalkanes. Along the way, you will learn about the difference between cis-trans notation and E-Z notation for geometric isomers.
Topics Covered in Other Articles
- Stereoisomers and Chiral Centers
- Steric Hindrance
- Functional Groups in Organic Chemistry
- Alkenes
- Chair Conformations
- Newman Projections
What Are Isomers?
Isomers are molecules that share the same chemical formula but have a different arrangement of atoms. There are two general types of isomers: constitutional isomers and stereoisomers. Constitutional isomers (also known as structural isomers) differ in the way that their atoms are connected or bonded with one another, while stereoisomers differ in their spatial arrangement of atoms.
Stereoisomers can be further divided into two categories: enantiomers and diastereomers. Enantiomers (also referred to as optical isomers) are non-superimposable mirror images of one another. Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other. Furthermore, while enantiomers have identical chemical and physical properties (except for their optical rotations), diastereomers can have fairly different chemical and physical properties (e.g., boiling point, solubility, and reactivity). Cis-trans isomers and conformational isomers (i.e., conformers) are specific types of diastereomers.
Introduction to Geometric Isomerism
Geometric isomerism, also known as cis-trans isomerism, is a form of stereoisomerism. Like all stereoisomers, geometric isomers are compounds that are made up of the same constituent atoms and are connected in the same sequence but differ in the orientation of those atoms in space.
All geometric isomers require restricted rotation within the molecule that prevents functional groups from being able to freely rotate around a chemical bond. This restricted rotation is generally caused by a carbon-carbon double bond or ring structure.
Geometric Isomerism in Alkenes
There is one major requirement for geometric isomerism in compounds containing a carbon-carbon double bond. Each double-bonded carbon atom must be attached to two different atoms or functional groups.
In the examples shown below, the first pair of compounds satisfies this requirement since each double-bonded carbon atom is attached to a methyl group and a hydrogen atom (i.e., two different groups). The second pair of compounds, on the other hand, does not satisfy the requirement because one of the double-bonded carbon atoms is attached to two hydrogen atoms (i.e., two identical groups).
Cis-Trans Notation for Geometric Isomers
The Latin prefixes cis and trans translate to “on this side of” and “on the other side of,” respectively. Along these lines, it makes sense that the cis isomer has functional groups on the same side of the double bond, while the trans isomer has functional groups on opposite sides of the double bond.
E-Z Notation for Geometric Isomers
Carbon-carbon double bonds are commonly categorized based on their number of substituents (atoms or groups other than hydrogen). Examples of alkene substitution patterns are shown below.
Although the terms cis and trans are useful when identifying geometric isomers for disubstituted alkenes, they do not apply to alkenes with three or four substituents. The E/Z system, on the other hand, applies to disubstituted, trisubstituted, and tetrasubstituted alkenes.
There are two key steps to follow when using the E/Z naming system.
- Step 1: Identify the higher priority group on either side of the double bond using the Cahn-Ingold-Prelog rules. These rules are explained in further detail here.
- Step 2: Determine whether the two higher priority groups are on the same or opposite sides of the carbon-carbon double bond, and assign E or Z accordingly.
The isomer with configuration Z (from the German word zusammen, meaning “together”) has substituents with higher priority on the same side of the double bond, while the isomer with configuration E (from the German word entgegen, meaning “opposite”) has substituents with higher priority on opposite sides of the double bond.
Two examples of geometric isomerism using the E/Z naming system are shown below. Following the steps described above, each isomer is given an E or Z designation.
Geometric Isomerism in Cycloalkanes
When dealing with cyclic compounds, wedges and dashes are used to indicate the relative orientation of a ring’s substituents. A wedge represents a chemical bond above the plane of the ring (i.e., coming towards you), while a dashed line represents a chemical bond below the plane of the ring (i.e., going away from you).
Geometric isomerism is commonly observed in disubstituted cycloalkanes. Cis isomers have two substituents with the same orientation (indicated by either two wedges or two dashed lines). Meanwhile, trans isomers have two substituents with opposite orientations (indicated by one wedge and one dashed line).
Cycloalkanes do not use E-Z notation, which requires the presence of a carbon-carbon double bond.
It is also important to note that geometric isomerism does not apply to disubstituted cycloalkanes when both functional groups are attached to the same carbon. For example, 1-fluoro-1-methylcyclohexane (shown below) does not have a geometric isomer and would not be given a cis or trans designation.
