5.1 Overview of Isomers | Constitutional Isomers and Stereoisomers | Organic Chemistry

Chad provides an extensive introduction to Isomers and Stereochemistry. He begins with a breakdown of the two major categories of isomers: Constitutional Isomers and Stereoisomers (note that constitutional isomers are also called structural isomers). Isomers have the same molecular formula. Constitutional isomers are isomers that have a different "bond connectivity." In contrast, stereoisomers have the same "bond connectivity" but a different 3-dimensional arrangement of the atoms.

Stereoisomers include two major categories: geometric isomers (cis/trans isomers) and isomers with chiral centers). Geometric isomers are possible in alkenes and cycloalkanes, and Chad provides examples of both. A chiral center, also called a chirality center, is a tetrahedral atom (most notably carbon in organic chemistry) that is bonded to 4 different groups. Molecules with chiral centers commonly exhibit chirality, which occurs when a molecule and its mirror image are not identical. We often refer to molecules displaying chirality as having "handedness" with one version being "right-handed" and its mirror image being "left-handed" so to speak, and such molecules are classified as being chiral. In contrast, a molecule that is identical to its mirror image is referred to as achiral.

There are two types of stereoisomers: enantiomers and diastereomers. Enantiomers are non-identical mirror images which were alluded to above. Diastereomers are non-identical non-mirror images. Geometric isomers are not mirror images and are therefore diastereomers. But stereoisomers with chiral centers may possibly be either enantiomers or diastereomers, and Chad expounds on this at length so that students will be able to recognize the proper isomeric relationship.

Chad concludes the lesson by discussing the relationship between chirality and optical activity and provides the definition of a racemic mixture. As chiral centers are at the center of identifying stereoisomers, Chad finally shows how to identify chiral centers in a molecule quickly.

00:00 Lesson Introduction
01:56 Introduction to Constitutional Isomers and Stereoisomers
03:45 Geometric Isomers (Cis/Trans Isomers)
08:03 Introduction to Chirality; Chiral, Achiral, and Enantiomers
15:40 Chirality and Optical Activity
19:22 Enantiomers vs Diastereomers
30:35 How to Identify Chiral Centers

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