Introduction to Organic Nomenclature: Enantiomerism. Wedge diagrams

Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

7. Stereochemistry (iii)
Chirality

III. Drawing 3-D Structures in 2-D
(i) Wedges

Index

Before looking at the way in which enantiomers are named, it is necessary to be able to illustrate stereoisomers on two-dimensional surfaces. To overcome the diffuculty of representing three-dimensional structures in two dimensions, chemists have invented several different approaches, each valuable to illustrate certain molecular features.

The approaches most commonly used, and introduced here, are:

wedge diagrams;
sawhorse diagrams;
Newman projections;
Fischer projections.

Wedge diagrams.

In this method, the spacial orientation at a particular atom in the molecule is shown using wedge lines for the bonds:

a filled-in wedge denotes a bond coming out of the plane of the paper;
a dashed wedge denotes a bond going behind the plane of the paper;
a simple line denotes a bond lying in the plane of the paper.

In constructing these diagrams, it is usual to use one filled-in wedge, one dashed wedge and two simple lines. When using this combination it is important to realize that the two wedges (and hence the two simple lines also) must be adjacent to each other as in:

The diagram is ambiguous if this is not the case!

Examples.

Example 1.
Illustrate the two enantiomers of 2-bromopropanoic acid using wedge diagrams:

wedge diagram	pdb model

Compare the two diagrams to the 3-dimensional models, and note how diagrams do indeed represent the two possible enantiomers in that they represent mirror images of each other.
Note also that a switch from one diagram to the other can be achieved by switching any two of the groups attached to the chirality centre.

Example 2.
Illustrate the two enantiomers of butan-2-ol using wedge diagrams: