How to draw molecular geometry – sp3, sp2 and sp hybridized central atoms

As somebody who has graded many an organic test, I’ve seen just about every nonsense molecular geometry a person could imagine. There are only are a handful of acceptable ways to draw a tetrahedron and a trigonal planar structure. Any other representation will make your professor cringe.

Below are some possible ways to draw a tetrahedron: the electronic geometry of a central atom with four groups around it. These atoms are sp3 hybridized.  The most common one is shown first: two of the bonds are in the plane of the screen and two of the bonds are out of plane. One of the out of plane bonds is on a wedge and other out of plane bond is on a dash. Now would be a good time to get out your model kit and think about how a tetrahedron can be drawn from various perspectives. There a even more ways that what I’ve drawn here, but great caution must be taken with creative representations.

OCS-Tetrahedrons-Actual

Below are some nonsensical ways to draw a tetrahedron. There are a billion other wrong ways. These are just some of my favorites. Do not draw these. Please.

OCS-Tetrahedrons-Nonsense

There is one more way to represent tetrahedral molecules called a Fischer projection. This representation is meant to simplify complicated molecules with many stereocenters. Note that the four bonds of a Fischer projection are not truly square planar. It is in implied in a Fischer projection that the two horizontal bonds are wedges and the two vertical bonds are dashes.

OCS-Fischer-Projections

Below is en example of a complicated Fischer projection and two different wedge/dash interpretations. The middle shows a wedge/dash drawing from the same perspective as the Fischer projection. The structure on the right is the same conformation as the Fischer projection but viewed from a different angle. Try to imagine flipping the wedge dash drawing in the middle on it’s side to get to the drawing on the right. Make a model, also.

OCS-Fischer-ComplicatedRepresentations of sp2 hybridized atoms are a little easier. These atoms are surrounded by three electron groups and they have a trigonal planar geometry. They are commonly drawn from two different perspectives. The graphic below shows a few different sp2 hybridized central atoms drawn from two different perspectives. The perspective on the right shows all bonds in plane. The perspective on the left shows one bond in plane and two bonds out of plane, which are actually right on top of each other. As usual, the two bonds must be splayed apart in a symmetrical fashion because we can’t put them directly on top of each other.

OCS-Trigonal-Planar-Perspectives

What about more complicated trigonal planar structures containing multiple central atoms? In this case we must be careful to show the surrounding groups in the correct plane. When two sp2 hybridized atoms are connected to each other, p orbital overlap requires that all four groups surrounding each atom be in the same plane. The graphic below shows a few molecules like this drawn from two different perspectives. Make models of these molecules.

OCS-Trigonal-Planar-Perspectives-Complicated

Finally there are sp hybridized atoms, which have two surrounding groups around a central atom and have a linear geometry. Below are some examples:

OCS-SP-Hybridization

Why is it important to be able to draw and visualize molecules from various perspectives? Representing and visualizing molecular geometry can be critical for predicting the outcome of chemical reactions.  Although it takes a little work at first to understand how to represent molecules from various perspectives, in the future it will simplify the process of learning more complex material that is critical for getting a good grade in orgo.

Now you can go work on my practice page: http://organicchemistrysolutions.com/practice-with-skeletal-structures-and-molecular-geometry/