Structure and Bonding

VSEPR Theory – Geometry of Organic Molecules

In today’s post, we will discuss the VSEPR theory which helps us understand the geometry of molecules. First, what does VSEPR stand for? It is the Valence Shell Electron Pair Repulsion theory. Notice the highlighted word ‘Repulsion’.

It is the keyword and all you need to grasp this concept is to keep in mind the idea that atoms want to stay as far apart from each other as possible because of the repulsion between the electrons on them.

Let’s use this model examples to illustrate how this works. Suppose we have three atoms/spheres and we connect them all. The red sphere represents the central atom and the blue ones are connected to it:

Considering that the blue atoms repel each other, propose an optimal geometry for them. Is the one shown above the most optimal? That is, does the orientation put them as far away as possible?

The answer is no because the blue atoms are not at the greatest possible distance from each other.

In this case, putting them across at 180^o allows for achieving the optimal geometry:

When there are three atoms around the central unit, the optimal angle is 120^o:

Now, what do you think it is when there are four atoms connected to the center?

If you have never heard about tetrahedral geometry and thought it was 90^o, that is fine, we all did when first introduced to this topic. However, the tetrahedral geometry is a better alignment since the angle between the groups is 109.5^o:

Each of these geometries that we have discussed has a name:

There can be more than four atoms, however, it is never the case for the carbon, and that is why we won’t get to those since this post is tailed toward organic chemistry. You can find the more complex geometries in the General Chemistry VSEPR post here.

Steric number

Now, let’s get to some terminology. In the model demonstration above, we said that the blue spheres represent atoms. However, in actual molecules, they can be atoms or lone pairs of electrons. For example, in the Lewis structure of water, we can see that it has two atoms and two lone pairs of electrons.

This assumes that you already know the Lewis structures, so if you don’t check, this article.

In total, there are four units around the oxygen in water:

The sum of the number of atoms and lone pairs is called Steric Number (SN):

You might have a different formula for the steric number that involves the number of bonds. However, if you use this formula, you don’t need to worry about the types of bonds. Whether it is a single, double or a triple bond, it is atoms + lone pairs for any bond type.

Notice that the last two molecules have the same steric number (4) but a different number of atoms and lone pairs. This is why we need to identify the Electron and Molecular Geometries.

Electron and Molecular Geometry

For the Electron Geometry, we treat the atoms and electrons equally. The last two molecules in the examples above (CH₄and NH₃) are both tetrahedral.

SN (C) = 4 atoms + 0 lone pairs = 4

SN (N) = 3 atoms + 1 lone pair = 4

This corresponds to a tetrahedral electron geometry:

However, their molecular geometries are different. For methane (CH₄), it is tetrahedral and for ammonia (NH₃), it is trigonal pyramidal. The lone pair on the nitrogen is important and if it wasn’t there, we would have a hypothetic molecule with a flat/planar geometry:

Why do we ignore the lone pair for naming the molecular geometry? One way to look at it is the fact that electrons are infinitely smaller and lighter than nuclei and when looking on modern microscopes, we don’t see them.

Use this table to determine the electron and molecular geometry, for all the combinations of atoms and lone pairs:

These are the geometries where the central atoms get only up to four groups, and we stop here because carbon can only have four bonds, and thus these are the shapes that you need for organic chemistry. For reviewing other geometries such as trigonal bipyramidal, octahedral, etc, check the VSEPS theory post in the general chemistry section.

Next is a walkthrough of the examples shown in the table following these steps:

1. Draw the Lewis structure for the molecule.

2. Count the number of atoms and lone pairs of electrons on the central atom (steric number)

3. Arrange them in the way that minimizes repulsion (as far apart as possible).

4. Determine the name of the electron and molecular geometry.

A) BeCl₂

1) Here is the Lewis structure:

2) S.N. (Be) = 2 atoms + 0 lone pairs = 2. This falls in the first category in the table and it is an AX₂ type.

3) Put the chlorines at 180^o

4) This is liner for both the electron and molecular geometry since the Be has no lone pairs.

B) BH₃

1) Here is the Lewis structure:

2) The steric number of Boron is S.N. (B) = 3 atoms + 0 lone pairs = 3. This falls in the second category and AX₃ type.

3) The borons must be at 120^o to minimize the repulsion:

4) This arrangement is called a trigonal planar. All the atoms are on the same plane.

C) CH₂NH

1) Lewis structure:

2) S.N (N) = 2 atoms + 1 lone pair = 3. This falls in the second category and AX₂E type.

3) The atoms and electrons around the nitrogen are at about 120^o

4) And this is called a bent geometry as the molecule looks bent if we ignore the lone pairs:

One thing to point out here is the fact that the lone pairs have stronger repulsion than atoms. Therefore, the expected angles are not always 100% in agreement with what they actually are:

We will get to this when discussing the H₂O and NH₃ again.

D) CH₄

1) Lewis structure:

2) S.N. (C) = 4 atoms + 0 lone pairs = 4. This is an AX₄ type.

3) The atoms are at 109.5^o

4) And is tetrahedral for electron and molecular geometry.

E) NH₃

1) Lewis structure:

2) S.N. (N) = 3 atoms + 1 lone pair = 4 and it is an AX₃E type in the table.

3) The atoms at the lone pair are expected to be at 109.5^o, however, because the revulsion from the lone pair is stronger, the angle between the hydrogens is about 107^o:

4) This is called a trigonal pyramidal geometry.

F) H₂O

1) Lewis structure:

2) S.N. (O) = 2 atoms + 2 lone pairs = 4. This is under the AX₂E₂ type in the table.

3) The atoms at the lone pair are expected to be at 109.5^o, however, because the revulsion from the lone pair is stronger, the angle between the hydrogens is about 104.5^o:

4) This called a bent geometry.

Notice that the angles in water and molecule C (CH₂NH) are different even they are both bent. And the reason is that, remember, the angle is defined based on the electron geometry. Depending on this, the angle may vary.

VSEPR and Geometry of Organic Molecules

For smaller molecules, we have a central atom based on which we determine the molecular geometry. However, when working with larger organic molecules, it may not be accurate to say that this molecule is tetrahedral or trigonal planar, etc.

For example, there is no central atom in this molecule:

And the geometry is determined for each atom of interest. Let’s do it the for the numbered atoms:

Oxygen 1 is connected to one atom and has two lone pairs making it to SN = 3. Its electron geometry is trigonal planar, but it has a linear molecular geometry.

Carbon 2 has three atoms and no lone pairs, which is steric number 3. Therefore, its electron and molecular geometries are trigonal planar:

Oxygen 3 is connected to two atoms and has two lone pairs and just like in water, S.N. (O) = 2 atoms + 2 lone pairs = 4. Therefore, it has a tetrahedral electron geometry and a bent molecular geometry:

Carbon 4 is connected to three atoms, and no lone pairs. SN = 3, which corresponds to a trigonal planar electron and molecular geometry.

Carbon 5 is connected to two atoms, no lone pairs and this is SN=2, which is a liner electron and molecular geometry:

Notice again that, we did not count the triple bond any differently than a single bond; it is atoms + lone pairs.

Although you will only go up to steric number 4 in organic chemistry, the practice problems below include examples of up to S.N. = 6 to refresh your skills from general chemistry. Check this post on VSEPR theory in general chemistry for steric numbers > 4.

Practice

Draw the Lewis Structures and determine the electron geometry and molecular geometry of the following molecules using the VSEPR model.

(a) BF₃	(b) CH₂O	(c) HCN	(d) BeCl₂	(e) CH₂Cl₂
(f) SOCl₂	(g) SO₂	(h) PCl₅	(i) XeO₄	(j) NCl₃
(k) SiCl₄	(l) SF₂	(m) H₂S	(n) SO₃	(o) COCl₂
(p) PCl₃	(q) OF₂	(r) BrF₅	(s) N₂O	(t) SF₆
(u) POCl₃	(x) XeF₂	(y) XeF₄	(z) C₂H₂

answer

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(a) NH₄⁺	(b) H₃O⁺	(c) CN^–	(d) SCN^–	(e) CO₃^2-
(f) ClO₃^–	(g) SO₄^2-	(h) PO₄^3-	(i) SO₃²^–	(j) NO₂^–
(k) BF₄^–	(l) NO₃^–

Structure and Bonding

Steric number

Electron and Molecular Geometry

VSEPR and Geometry of Organic Molecules

Practice

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