Stereoisomers: Definition, Types & Examples

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  • 0:00 What Is a Stereoisomer?
  • 1:53 Geometric Isomers
  • 3:42 An Asymmetric Center
  • 6:34 Lesson Summary
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Lesson Transcript
Instructor: Andrea Kropp

Andrea holds a master's degree in Chemistry and has taught General and Organic Chemistry, Biology and Physics at the college level.

Discover what a stereoisomer is and why they are so important in chemistry, biochemistry, and medicine. This lesson teaches how to identify a stereoisomer and distinguish it from other types of isomers.

What is a Stereoisomer

Naproxen and its Stereoisomer
Enantiomers of Naproxen

Did you know that many molecules can exist as left- or right-handed and that the two versions can behave very differently? For example, the common over-the-counter pain reliever Naproxen (which is the active ingredient in Aleve) has a mirror image which causes liver poisoning and has no effect on pain. The makers of Naproxen need to package only the beneficial molecule while removing the harmful mirror image. This is just one example of how stereoisomers are critically important in biology and medicine.

Stereoisomers are molecules that have the same molecular formula and differ only in how their atoms are arranged in three-dimensional space. Be careful not to confuse them with constitutional isomers which also have the same molecular formula but differ in the way their atoms are connected.

To understand the fundamental difference between stereoisomers and constitutional isomers, imagine a neighborhood of new homes being built where each home uses exactly the same amount and type of building materials. Using those same building materials, the contractors can build a 1-story or a 2-story house. The 1-story and 2-story homes would be constitutional isomers - they differ in how the parts are connected. Now imagine two homes with the exact same floor plan that are mirror images of each other. Builders sometimes do this because of the shape of the lots or because it gives the outward appearance of the homes being different when seen from the street. The two mirror image homes would be stereoisomers

All isomers can be classified as either constitutional isomers or stereoisomers, and the stereoisomer category has several further subcategories as shown in the following classification chart. Let's take a deeper look at the two main types of stereoisomers and a few real-world examples of each.

Classification of Isomers
Classification of Isomers Diagram

Geometric Isomers

Geometric isomers (also called cis/trans isomers) are a type of stereoisomer resulting from a double bond or a ring structure. The double bond or ring in the structure means that not all bonds are free to rotate, giving rise to geometric isomers whose shapes cannot interconvert.

Geometric Isomers of 2-Butene
Geometric Isomers of 2-Butene

The simplest example of geometric isomers are cis-2-butene and trans-2-butene. In each molecule, the double bond is between carbons 2 and 3. In cis-2-butene, the methyl groups attached to carbons 2 and 3 are on the same side of the rigid double bond. In trans-2-butene, the methyl groups are on opposite sides of the double bond.

Most shoppers that pay attention to nutrition news have probably heard the term trans-fat. All trans fats have a geometric isomer which is the corresponding cis fat. In general, the enzymes in the human body are able to break down cis fats, but they have a tough time with trans-fats because of their different shape.

Geometric Isomers of 9-Octadecenoic Adic

For example, oleic acid is the common name given to the molecular on the left whose chemical formula is C18H34O2 and which has a cis oriented double bond between carbons 9 and 10. Oleic acid is a major part of the human diet as part of vegetables oils and animal fats. Its geometric isomer is named elaidic acid. It has the same C18H34O2 formula and arrangement of atoms. The only difference is that the double bond between carbons 9 and 10 is in the trans configuration.

A more advanced example demonstrating geometric isomers caused by a ring structure is cis-1,4-dimethylcyclohexane and trans-1,4-dimethylcyclohexaneare. Because the ring is rigid, the two isomers cannot interconvert via bond rotations.

Geometric Isomers of 1,4-dimethylcyclohexane
Geometric Isomers of 1,4-dimethylcyclohexane

An Asymmetric Center

Other types of stereoisomers are those which contain an asymmetric center also called a stereocenter. In chemistry, a symmetric center is an atom bonded to groups in a way that the mirror image is not superimposable on the original. The concept of a non-superimposable mirror image is best explained using familiar objects.

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