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Stereoisomers

Written by tutor Kathie Z.

Stereoisomers for Chemistry

First look at the images below so that you connect the definitions of the different categories of stereoisomers with the following discussion:

Types of isomers
Source: Wikimedia Commons

Isomer: two compounds with the same chemical formula
Constitutional isomers: structural isomers (where one connection is located differently in each molecule)
Stereoisomers: spatial isomers (same formula and sequence, just spatially different)
Enantiomers: optical isomers which are mirror images
Diastereomers: non mirror image
cis/trans isomers: geometric isomers, their names include the “Cis” (same side) or “Trans” (across) to indicate their spatial orientation.
Conformers: extended chains or coiled chains (like hexane)
Rotamers: rotational structures that may be staggered or eclipsed (like ethane)

Stereoisomers for Chemistry and Biochemistry

Isomer: two compounds (molecules) with the same chemical formula but each with a different structural shape. Because of these differences they will have different chemical and physical properties from each other. They can be:

1: Constitutional isomers, also known as structural isomers. As you can see in the example above the Florine atom is attached in different locations in each isomer. So they are said to have different structures although they have the same molecular formula.

Or 2: Stereoisomers, also known as spatial isomers, which are molecules made of the same atoms connected in the same sequence but with differing spatial arrangements. Stereoisomers are described as:

A: Enantiomers being mirror images (like HCBrFCl in the diagram above). Sometimes they are also referred to as optical isomers.

Or B: Diastereomers, which because of one of the following reasons are NOT mirror images, specifically:

  • Cis/trans isomers are geometric isomers which can be found when rotation around chemical bonds is impossible because of double or triple bonds between atoms. Therefore their names include the “Cis” (same side) or “Trans” (across) to indicate their spatial orientation like in the butane examples shown above.
  • Conformers: extended chains or coiled chains (like hexane)
  • Rotamers: rotational structures that may be staggered or eclipsed (like ethane)

You will notice that most of these are organic molecules that are carbon-hydrogen based and are a result of the fact that hydrocarbon molecules can have single, double or triple bonds and are found in nature in chains, coils, rings, etc. And the rings can be 5, 6 or 7 sided! As you study stereoisomers in more depth you will learn that there are sequence rules for assignment of Alkene Configurations and the IUPAC system has rules for nomenclature.

Reference

Organic Chemistry With a Biological Emphasis by Tim Soderberg

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