Drawing 2D Structures

Useful Concepts

2D Representations of 3D Structures

One of the more important skills you need to develop is the ability to draw 2D representations of the 3D struture of molecules. There are several common formats for these types of drawings.
Wedge-dash diagrams

Usually drawn with two of the bonds in the plane of the page, one in front, and one behind to give the molecule perspective. When drawing wedge-dash it is a good idea to visualize the tetrahedral arrangement of the groups and try to make the diagram "fit" this. As a suggestion, they seem to be most effective when the "similar" pairs of bonds (2-in-plane, 2-out-of-plane) are next to each other, see below:

Sawhorse

Sawhorse diagrams are similar to wedge-dash diagrams, but without trying to use "shading" to denote the perspective. The representation of propane has been drawn so that we are looking at the molecule which is below us and to our left.

Newman Projections

These projections are drawn by looking directly along a particular bond in the system (here a C-C bond) and arranging the substituents symmetrically around the atoms at each end of that bond. The protocol requires that the atoms within the central bond are defined as shown below:

In order to draw a Newman projection from a wedge-dash diagram, it is useful to imagine putting your "eye" in line with the central bond in order to look along it.

Let's work through an example, consider drawing a Newman projection by looking at the following wedge-dash diagram of propane from the left hand side.

First draw the dot and circle to represent the front and back C respectively.

Since the front carbon atom has an H atom in the plane of the page pointing up we can add that first.

The back carbon atom has an H atom in the plane of the page pointing down.

Now add the other bonds to each C so that it is symmetrical.

The groups / bonds (blue) that were forward of the plane of the page in the original wedge-dash diagram are now to our right.

Those behind (green) the plane are now to our left.

Now you try the same thing, but looking from the right to generate the other Newman projection.

Drawing Cyclohexanes

Drawing cyclohexane so that it looks like a chair can be the key to appreciating the axial and equatorial positions. If you are unable to draw good looking structures that clearly show axial and equatorial positions, then your instructor is probably going to assume that you don't know.

By not mastering the trick of drawing cyclohexanes the only person that really suffers is you the student. You deprive yourself of the knowledge and the chance to appreciate it and what it means. Believe me, it will be needed later.

The first step is drawing the chair itself. Although the chair "looks better" when slightly angled, it maybe easier to "learn" to draw it with the middle portion horizontal.

More Practise?

Consider the molecule CH₄. Below left can be found its Jmol image. Figure I shows how this Jmol image would be represented on a piece of paper. Unbroken lines are used to represent bonds in the plane of the paper, hashed wedges represent bonds going back out of the plane of paper and solid wedges represent bonds coming out of the plane of the paper. See if you can manipulate the molecule in the Jmol image below to match the drawing in figure II.

CH₄

Can you manipulate the trigonal bipyramidal transition state, given in the Jmol image below, so that it matches the drawings in each of the figures immediately to the right?

C₂H₇OBr

Notice that bonds that are in the process of being formed or broken are given as dashed lines. The representations that are clearest then are those that place those bonds in the plane of the paper so that they do not get confused with the dashed wedges that represent bonds coming out of the plane of the paper (Figure III).
The most desirable drawings are usually the simplest, generally they place as many atoms as possible in the plane of the paper.
Consider the molecule n-butane, whose Jmol image is given below. Manipulate the molecule so that it matches the drawing in Figure VII.

C₄H₁₀ Figure VII

So what happens when lone pairs of electrons are involved? The most desirable drawings still place the most bonding sites in the plane of the paper even though lone pairs of electrons are most easily represented as being in the plane of the paper (it is not easy to wedge and hash a lone pair).
Consider the Jmol image for NH₃given below. Figure X would be the best drawing since it considers the lone pair on nitrogen as being in a bonding site in the plane of the paper. As you can see in Figure XI it is much more confusing if you choose to represent the lone pair as coming out of the paper.

NH₃

Ethyl methyl ether is a structure where the opposite is true. Drawing the lone pairs in the plane of the paper would be far more confusing than if you consider them as being out of the plane.

C₃H₈O
Figure XII

You need to learn to discriminate which drawings most clearly represent the structure of a molecule.

Fischer Projections

Fischer Projections are abbreviated structural forms that allow one to convey valuable stereochemical information to a chemist without them having to draw a 3D structural representation of a molecule. These representations are only used for molecules that contain chirality centers, which are then represented as simple crosses.

They can be derived by considering the more accurate 3D representation using wedges and assuming the convention that horizontal lines represent bonds coming out of the plane of the paper and vertical lines represent bonds going behind the plane of the paper.

Memory Aid?
A student once told me that she remembered the relative arrangement of the bonds by the fact that the horizontal bonds were coming out to hug her!

When relating one Fischer projection to another it may only be manipulated within the 2D plane in which it is drawn (that is it may not be rotated within 3D space), and only rotated a total a 180°

A B

Why can't you rotate it 90°? A 90° rotation is equivalent to breaking bonds and exchanging two groups, which would result in the formation of the other enantiomer.

CAUTION: Fischer projections are often confused with simpler Lewis diagrams. Lewis diagrams, however, are not intended to give any stereochemical information!
Fischer projections a can be used to describe molecules with more than one chirality center.

If a Fischer projection of this type can be divided into two halves that are mirror images than the molecule may be identified as a meso isomer.

Assignment of the configuration at a chirality center, in a Fischer projection, is based on the same Cahn-Ingold-Prelog rules. The safest method for assigning the configuration is probably to convert it to a wedge-hash diagram (as shown above).
Alternatively...

Identify the chirality centers (most commonly an sp³ C with 4 different groups attached).

If the group of lowest priority is placed on a vertical line, this means the lowest priority group is already positioned away from you as if you were looking along the C-(4) σ bond.

Now assess the direction of high to low priority (1 to 3).

If this is clockwise, then the center is R (Latin: rectus = right).

If this is counter clockwise, then it is S (Latin: sinister = left).

If the group of lowest priority is placed on a horizontal line, this means the lowest priority group is actually positioned towards you (so we have to be very careful).

Now assess the direction of high to low priority (1 to 3).

If this is clockwise, then the center is R (Latin: rectus = right).

If this is counter clockwise, then it is S (Latin: sinister = left).

BUT NOW SWITCH THE ASSIGNMENT (it's like looking at a glass clock face from opposite sides).