Pi Electrons: Study with Video Lessons, Practice Problems & Examples

Pi electrons are found in unhybridized p orbitals and can be counted in compounds through specific contributions: double bonds contribute 2 pi electrons, radicals contribute 1, and cations contribute 0 due to empty orbitals. Understanding these contributions simplifies the process of counting pi electrons in various organic compounds, which is essential for grasping concepts like resonance and stability in molecular structures.

Counting pi electrons

In this video, we're going to become experts on counting pi electrons. Technically, a pi electron is any electron found within an unhybridized p orbital. Technical definition. However, for the purposes of this video, I think it's just easier if we memorize a few different types of electrons that are pi electrons and that's going to kind of cover our bases. So we can learn that double bonds, radicals, and cations all contribute different amounts of electrons to pi electrons. Radicals contribute 1 pi electron and radicals contribute 1 pi electron. Cations, because of the fact that they have empty orbitals, you got it, they contribute 0 electrons. So, simply the job of counting up pi electrons is as easy as counting with your fingers how many electrons you see by counting up with this method. So I'm going to have you guys do 6 compounds and figure out the pi electrons for those six. Go ahead and start off with the first one here and use the rule that I gave you to count up all the pi electrons you see, and then we'll go ahead and do the answer.

Count the pi electrons present

The answer for the 1st molecule is 6 because I've got 2 from this double bond. I've got 2 from this double bond. I've also got another 2 from this double bond and I've got 0 from this cation. You add it all together and you get 6 pi electrons. Easy. Move on to the next one.

Count the pi electrons present

The answer for this next one was 10 pi electrons because I've got 2, 2, 2, 2, 2. That equals 10 pi electrons. Too easy. Let's move on to the next one.

Count the pi electrons present

The answer for this next compound was 4 pi electrons because I had 2 electrons coming from my double bond and then I had another 2 coming from my anion, which we said contributed 2 pi electrons as well. That would give us 4 pi electrons total. Now, in order to answer the next three questions, we're going to have to get a handle on this tricky notation. If you see a hydrocarbon ring with just a charge in the middle, that means you've got a lazy organic chemist on your hands. But hey, this is an acceptable notation and you do need to learn what it means. Basically, if you just see a charge in the middle, that means that we're going to assume, so I'm just going to put here by definition, this means that we're going to assume alternating double bonds and then the space left over, or the carbon left over, the atom left over would possess that charge. That means that this molecule, that 5 of a ring is actually a shortcut or a shorthand for a 5 carbon diene with a positive charge on 1 carbon. That being said, go ahead and try to figure out how many pi electrons that has, and then use that information to then draw the correct structures of the following tubes so that you can get the right answers for those as well. You got this.

Count the pi electrons present

The correct answer for this one was 4 now that we know how to draw it because you had 2, 2 and 0 giving us a total of 4 pi electrons. Go ahead and try the next one.

Count the pi electrons present

The correct structure of this molecule was a triangle with simply one double bond and a negative charge in the space afterwards because there was only enough space for one double bond total. This means that this molecule would have a total of 4 pi electrons. Let's move on to the last question.

Count the pi electrons present

The answer for this last one was 8 pi electrons. How do we get the answer? Because the negative charge in the middle meant that we had to assume double bond, double bond, double bond with a negative charge here. I can just cross that out. We would get 8 pi electrons from the 3 double bonds and the anion which could contribute electrons 7 and 8. So this would be 8 pi electrons total. Now I know that you're not really completely sure how important this pi electron thing is. But it's a huge deal when we're talking about aromaticity. Now that you know how to count them, let's go ahead and figure out why they're going to be so important for aromaticity.

Here’s what students ask on this topic:

What are pi electrons and how do you count them in organic compounds?

Pi electrons are electrons found in unhybridized p orbitals. To count them in organic compounds, you need to consider the contributions from different types of bonds and species. Double bonds contribute 2 pi electrons, radicals contribute 1 pi electron, and cations contribute 0 pi electrons due to their empty orbitals. By identifying these features in a molecule, you can sum up the total number of pi electrons. This method is crucial for understanding resonance and stability in molecular structures.

How do double bonds, radicals, and cations contribute to pi electrons?

In organic compounds, double bonds, radicals, and cations contribute differently to the count of pi electrons. Double bonds contribute 2 pi electrons because they consist of one sigma bond and one pi bond. Radicals, which have an unpaired electron, contribute 1 pi electron. Cations, on the other hand, have empty orbitals and therefore contribute 0 pi electrons. Understanding these contributions helps in accurately counting the pi electrons in a molecule.

Why are pi electrons important in understanding resonance and stability in molecules?

Pi electrons play a crucial role in resonance and stability of molecules. Resonance involves the delocalization of pi electrons across multiple atoms, which can stabilize the molecule by distributing electron density more evenly. This delocalization lowers the overall energy of the molecule, making it more stable. Therefore, accurately counting pi electrons is essential for predicting and understanding the resonance structures and stability of organic compounds.

Can you explain the difference between sigma and pi bonds?

Sigma (σ) and pi (π) bonds are types of covalent bonds that differ in their formation and properties. A sigma bond is formed by the head-on overlap of atomic orbitals, resulting in a bond along the axis connecting the two nuclei. It is the strongest type of covalent bond and allows for free rotation around the bond axis. A pi bond, on the other hand, is formed by the side-to-side overlap of unhybridized p orbitals, above and below the plane of the atoms involved. Pi bonds are weaker than sigma bonds and restrict rotational freedom, contributing to the rigidity of double and triple bonds.

How do you determine the number of pi electrons in a benzene ring?

A benzene ring consists of six carbon atoms arranged in a hexagonal ring, with alternating double and single bonds. Each double bond contributes 2 pi electrons. Since there are three double bonds in benzene, the total number of pi electrons is 6. These pi electrons are delocalized over the entire ring, contributing to the stability and unique chemical properties of benzene.