VSEPR Theory
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VSEPR stands for Valence Shell Electron Pair Repulsion Theory.
We use VSEPR to predict the 3D shapes of the molecules made by the 2nd period elements. The main focus in this topic is going to be on the carbon (C), nitrogen (O), and oxygen (O). Those three elements make the “core” of the organic molecules, so you’re going to be working with those most of the time.
How Do We Apply the VSEPR Theory?
The premise of the VSEPR is the idea that the electron pairs & bonds will distribute themselves as far from each other as possible around the central atom. Think about a bunch of balloons tied to a single point. That would be a pretty accurate description of the approach.
While there are quite a few electronic domains and, thus, 3D shapes, we only focus on three shapes in organic chemistry.
- The linear shape means that all three atoms are making a linear string of 3 atoms in a line. Thus, the X-A-X bond angle is 180º.
- The trigonal planar shape has a central atom (A) in the middle of the molecule, while the rest of the groups are making a perfect triangle around it. This gives a X-A-X bond angle of 120º.
- The tetrahedral shape resembles a trigonal pyramid with all sides being perfect triangles. The X-A-X bond angle is a little more difficult to calculate, but it is approximately 109.5º.
The most important domains for us are going to be the AX3 and AX4. Those are the two most common shapes we’ll see in organic molecules.
What is the Difference Between the Electronic Domain and Shape?
The difference comes when we have spare (non-bonding) electron pairs instead of the groups sitting around the central atom. The electron pairs are “invisible” for the purposes of the shape. However, since they are still there, they do influence the shape and thus are important to remember. So, how does the VSEPR theory treats this difference?
The Tetrahedral Domain
In the tetrahedral domain, there are four “things” attached to the central atom. Those “things” can be either groups or electron pairs. Depending on how many electron pairs we have, we’ll end up with the following shapes.
The tetrahedral domain is the hallmark of the molecules with single bonds. Those are also called “saturated” meaning that those molecules cannot add any hydrogens. We’ll talk about those addition reactions later on, so don’t fuss about the name for the moment 😉
The Trigonal Planar Domain
When the central atom is connected to one of the groups by a double bond or has an empty p-orbital, we get the trigonal planar domain. There are less shapes associated with this domain than with the tetrahedral though, so it makes it easier to remember.
The figure above shows only the cases with the double bonds. We’ll discuss the examples with empty orbitals (such as carbocations) later in this course. Structures with empty orbitals are very unstable, so we’re only going to see those as highly reactive compounds or intermediates in reactions.
The Linear Domain
When you have two groups attached to the central atom by double bonds, or if you have a triple bond, you’ll have a linear domain. As linear molecules are very simple, there’s not much to discuss shape-wise here. They are, well, linear. 😹
Limitations of VSEPR Theory
Larger atoms distort the structures more than the small ones. So, when you have a molecule with atoms from the 3rd period and beyond, you’ll see significant deviations from the VSEPR shapes and bond angles. Within the scope of a typical organic chemistry course, you’ll only be responsible for estimating if the bond angles are going to be about what they are supposed to be or larger/smaller than that.