Sigma and Pi Bonds

Bond Properties Summary

There’s some simple facts about different types of bonds that could show up on your exam. Here’s a really nice summary for you: 

Concept: Single bonds, double bonds, and triple bonds.

Video Transcript

So basically we have three types of bonds. You have single, double and triple. And what we need to know is just a few facts about each.
The first thing is composition. Composition deals with are they made out of sigma bonds, pi bonds, a combination. We need to know this. I already told you guys what single bonds were. Do you guys remember? It's just made out of one sigma. So whenever I say sigma, it means single. Single means sigma. They're the same thing if it's just one sigma, that's a single.
Now, double and triple get a little bit more complicated because remember that I said that it's actually more than one orbital combining. So for a double bond, it's actually going to be one sigmas always because there's still one region of overlap in the middle from the s orbitals. But, there are also these p orbitals. And the p orbitals have two regions of overlap, so that means that a double bond is made out of a pi and a sigma or a sigma and a pi. Overall that kind of makes sense because a double bond has two orbitals that are overlapping.
Then finally, a triple bond you could guess, now this one is going to have three orbitals overlapping. What's that going to be is still that one sigma from the s's, but for a triple bond now you have two sets of p orbitals on both sides that are overlapping, one up and down and one side to side. What that means is that a triple bond is made out of one sigma and two pi. Cool. That's the first thing.
Now I want to talk about free rotation. Why? Because this has a lot to do with composition. So remember that I said that a bond is just a region of shared space with electrons. So if I just have one interaction in the middle, where they're overlapping, is it possible to rotate one of the atoms like I'm doing right now and keep the bond intact? The answer is yes. If I rotate this atom, I can still keep this bond together. It turns out that for a single bond, since they have one region of overlap, we're just going to write a big yes right here. Yes, I have free rotation there because I can rotate one of the atoms as much as I want. It's always going to have that bond present.
Now let's look at double bonds. Double bonds, they do have that region of overlap in the middle, that one area, but they also have the region at the top and the region at the bottom from the pi bonds. So it's going to look something like this, where you have basically electrons overlapping at the top, electrons overlapping at the bottom.
If I try to rotate one of the atoms, is that going to break or change my bond at all? Actually, yeah. It will. What's going to happen is this top orbital or this top region is going to break and this bottom region is going to break. That's really unfavorable and really unlikely to happen. The reason is because remember that I said that when you form bonds, that saves a ton of energy, so what that means is that I'm going to have to spend a lot of energy in order to rotate and break this bond. That's energy that these atoms don't have most of the time.
What that means is that these double bonds are not going to be able to break. Actually, that's what I want you guys to write for double and triple. They're both not going to be able to rotate because of the fact that if you rotate them, you're going to have to break them and it's very difficult to break the bonds just to rotate. Good. Now you guys understand the only ones that rotate are the single bonds.
Now let's talk about length. Hopefully, you guys should remember what I talked about with sigma versus pi, which one was the longer one? Do you guys remember? Sigma is the longest. So single bond would be my longest. It actually turns out that because the triple bond is made out of one sigma and two pi, this one's going to be the shortest. Then you could guess that the double bond is going to be like the middle child, so just right in the middle. Going to be in the middle, not quite as long, but not quite as short.
Now let's go onto strength. So I already talked to you guys about this earlier, which of these is going to be the weakest overall, in terms of single, double or triple? The answer is that the single bond is the weakest. The reason for that is that there's only one region of overlap that is saving energy. Now the triple bond is actually going to be the strongest. The reason for that is that it has three orbitals that are overlapping and each one is saving a little bit more energy. That means that once again, my double bond is going to be in the middle.
Now I want to ask you guys a question though, which of the orbitals is the strongest? Is the sigma orbital the strongest or is the pi orbital the strongest? The answer is that the sigma bond, not orbital, the sigma bond is the strongest because the sigma bond saves the most energy of the three bonds for the triple. Then the next pi bond would save a little bit less energy. Then the last pi bond would not save that much energy at all, but it would still be a little bit more.
You know what, I can tell that this is a little bit confusing, so I'm just going to give you numbers like before. Imagine that your single bond is saving 436 and then your double bond is saving 599 and then imagine that your triple bond is saving like 650. By the way, I'm just making that number up, but it's pretty accurate. So what that means is that the single bond is saving a ton of energy, but as I go up and make these bonds a little stronger, each pi bond is going to get weaker and weaker and weaker. So by the end, I'm only saving like 50 kJ per mole by adding another pi bond. Does that make sense? I hope that that distinction is clear to you guys. Professors love to trick you on that. 

A single bond is made out of a single sigma (σ)-bond; add π-bonds to make double and triple bonds. 

Counting Total Sigma and Pi Bonds

This kind of question shows up on exams often, and it’s an easy one to get right. How many sigma and pi bonds are there in the following molecules?

Example: Determine the number of σ-bonds and π-bonds in the following molecule.


Example: Determine the number of σ-bonds and π-bonds in the following molecule.


Rank the following bonds from shortest to longest:


Problem: Rank the following bonds from shortest to longest


Now you should be a pro at bond lengths and bond strengths. Let’s move on to the next topic.