Stereochemistry is concerned with those properties and reactions of molecules which are dependent on the 3-dimensional spatial relationships within and between molecules.

Because of the tetrahedral nature of sp³-hybridised carbon, the covalent bonds linking it to the 4 other atoms to which it is attached do not lie in the same plane. Only the carbon atom and two of the atoms to which it is attached can be co-planar; one of the others lies behind this plane, the other in front of it. It is therefore incorrect to think of such a molecule as 2-dimensional or flat and conclusions based on 2-dimensional arguements are frequently seriously incorrect! Chemistry takes place in a 3-dimensional environment and chemists need to be 3-dimensional in their thinking!

However it is often necessary to represent 3-dimensional molecular situations on a 2-dimensional surface (paper/blackboard/screen) and chemists use PERSPECTIVE DRAWINGS to focus attention on a particular atom of interest and to convey information to their audiences about the 3-dimensional arrangement of atoms/groups/substituents about that atom, much as artists have since Renaissance times managed to insert 3-dimensional perspective into their 2-dimensional drawings and paintings.

Thus a typical perspective drawing of methane might be as on the left of this paragraph. This shows the C and all four H atoms attached to it. Commonly, for convenience and clarity (avoidance of clutter in the drawing), the C atom is omitted and a perspective structure such as that on the right might be used.

If the intent is to merely focus attention on the 3-dimensional arrangement around the C, and the groups attached to it are unimportant to the point being made, diagrams such as those to the right or to the left are perfectly satisfactory.

It is important for students to learn to draw perspective structures and to become proficient in understanding and manipulating them.

The convention is that the central atom, typically an sp³-hybridised carbon atom, is placed in the plane of the surface (paper/blackboard/screen). Bonds to this atom which also lie in this plane are represented by a line of normal thickness (), whereas bonds lying off this plane are represented differently. Bonds directed out in front of this plane are wedged (), with an atom/group at the thicker end of the wedge being interpreted as being nearer the viewer than the carbon to which the narrower end points. Bonds directed back behind this plane are hashed/broken ().

A very important additional point to remember is that these perspective structures, if they are to be useful, must carry not only information about whether the atoms and bonds are on the screen surface, behind it or in front of it. They must also tell us reasonably accurately the angles involved. The structure on the left tells us about positioning of atoms and bonds on, behind and in front of the screen but is totally unacceptable as a perspective structure. Just try using it to imagine a 3-dimensional tetrahedral molecule!

A structure such as that on the right offers a much more realistic PERSPECTIVE view.

Proper application of these conventions assists us in "visualising" 3-dimensional molecules and, very importantly, the additional "information content" in these perspective structural drawings allows us to communicate 3-D information to one another simply and effortlessly. They are the "high-tech" versions of "flat" drawings of molecules.

Familiarity with their use is essential in understanding stereochemistry.