Stereochemical Structures

STEREOCHEMICAL STRUCTURES

To discuss the spatial relationships of groups in molecules, we first have to be able to draw structures in such a way that the stereochemical features will be represented unambiguously. Thus a system is needed to depict in two dimensions the spatial relationships between groups in molecules that occur in three dimensions.

An early method to picture the three-dimensional properties of molecules was the use of Fischer projections. In Fisher projections, bonds are drawn either ver-tically or horizontally. Bonds which are vertical project into the space behind the plane of the paper (blackboard, computer screen). Bonds which are horizontal project into the space in front of the plane of the paper (blackboard, computer screen).

This provides a perfectly good way to denote the stereochemical structure of a molecule if a few rules are followed.

While it is a planar figure, a Fischer projection can only be rotated 180^◦ in the plane of the paper, and it may not be taken out of the plane of the paper and flipped over. As shown below, rotating the molecule 180^◦ gives the identical molecule whereas rotating the molecule 90^◦ gives a nonsuperimposable isomer (the enantiomer). Within these constraints, however, Fischer projections are quite valid for showing the stereochemistry of a molecule.

Furthermore, 

A more recent approach to the three-dimensional depiction of molecules is the use of wedged and dashed lines. Simple lines are used to denote bonds in the plane of the paper (blackboard, computer screen); wedged lines denote bonds projecting into the space in front of the plane; dashed lines denote bonds projecting into the space behind the plane. These bonds are not restricted to any particular orientation but are general. Bearing in mind the tetrahedral geometry of saturated carbon atoms, these figures can be turned and rotated at will as long as you are able to keep track of what happens to each of the four valences as the molecule is tumbled.

It may help to make models and practice manipulating these structures. An added benefit of depicting molecules using wedged and dashed figures is that other types of molecules can be depicted using this convention. For example, the planar geometry of olefins and the twisted geometry of allenes is readily pictured using wedged and dashed bonds.

A further simplification of stereochemical notation for saturated carbon cen-ters is to stretch out the carbon skeleton in the plane of the paper (blackboard, computer screen). Valences of atoms or groups other than hydrogen are indicated by a bold line if they project into the space in front of the plane and with a dashed line if they project into the space behind the paper (blackboard, computer screen).

Other common methods for representing the three-dimensional structures of molecules include Newman projections for showing conformational relationships and sawhorse figures. Newman projections look down a carbon – carbon bond so that the front carbon, designated by a circle, obscures the carbon directly behind it. Valences (bonds) to the front carbon extend to the center of the circle, while bonds to the rear carbon stop at the circle. Sawhorse projections have the carbon – carbon bond at oblique angles, which attempts to represent a perspective drawing of the molecule. Thus for 2-chloro butane, if one chooses to examine the 2,3 bond, then the sawhorse and Newman projections would be

Keeping in mind the three-dimensional properties of molecules, Newman pro-jections can be converted to wedged – dashed structures or Fischer projections as desired. It is important to develop facility for manipulating structures and visualizing the three-dimensional properties of molecules from various stere-ostructures. The use of molecular models in conjunction with two-dimensional structures is often helpful in making the visual connections.