Radical reactions require an initial first step to get going. We call this reagent the radical initiator.
Concept: Heterolytic vs. Homolytic Bond Cleavage .5m
Starting from the very beginning, radicals are these very high-energy intermediates that in the natural world exist for only very short periods of time. If you ever hear about free radicals in nature or in your body, these are things that last for tiny amounts of time and they're gone. They terminate. When we're dealing with reactions with radicals, the first thing we need to ask is how do we even get these radicals in the first place.
That brings us to radical initiators. Every radical reaction that we talk about is always going to start off with a radical initiator. First of all, let's just talk about how radicals break off differently than regular single bonds. Basically, it turns out that single bonds can be broken in two different ways. They can be broken heterolytically or homolytically. Let me show you the difference between that.
A normal heterolytic cleavage, that means that I'm breaking this bond and I'm going to get different charges on both sides, would mean that two electrons, both of the electrons from that bond, are moving to one atom. That means that one atom is going to have a negative charge and one's going to have a positive.
Let's look at this example bond right here. I have a carbon and some kind of halogen. How could we predict which of the species would get the negative charge or would get the lone pair on it? Do you guys know how to predict that? The way that we would predict is that we would say the one that's the most electronegative is the one that gets the electrons when the bond breaks.
So there's actually a pretty powerful dipole going towards the X depending on which halogen we're using. What we would say is that, if we were to break this bond, the way we would break it is towards the X. Notice that I'm using a full arrow and that's showing that both electrons kind of pick up, pack their bags, and move to the X. What I wind up getting is ions. I wind up getting a C+ and an X-. This is, like I said, this would be heterolytic cleavage.
Now the reason that we call it heterolytic is because hetero stands for the word different. You're getting different amounts of electrons on both. Now notice what this creates is ions. Your products are different ions, a cation and an anion. That's one way to would break bonds.
But another way to break them is that I could break them just taking one electron from each side. So I could take one electron and give it to that X. I could take another electron, give it to that X. Notice that one thing that was different about this bond than the other one was that there was really no dipole. I couldn't tell which one was more electronegative or not because they both had the same electronegativity. That's actually going to be important.
What that's going to do is it's going to give me instead of a negative and a positive, that's going to give me two of the same thing, hence the homolytic cleavage. In this case, homo meaning same. That you're getting basically the same electrons on both. Notice that our product here would be radicals. Cool so far? Awesome.
Basically, I want to show you guys the difference between the arrows that I just drew. When we want to draw that two electrons are moving to an atom, we say that full curved arrows are used to indicate the movement of two electrons. That means it's a full-headed arrow. It has both sides of that arrowhead. When we want to only show that one electron is moving, we would use a half-headed arrow or what is sometimes called a fish-hook arrow because it's only got half of the arrowhead on it, like I use on the X's.
It turns out that homolytic dissociation is usually much higher, dissociation energy, is typically higher than corresponding heterolytic dissociation energies. What that means is that most of the time when we're breaking bonds in organic chemistry, we're actually going to be using the blue method, the one that's heterolytic. And you're going got see that whether you get into other types of reaction or just later on the course, you're going to see that we're going to use a lot of heterolytic cleavage.
Homolytic cleavage is really reserved just for radical reactions. These are reactions that are favorable for just a small set of reasons. It only starts off with initiator.
Chemical bonds can be cleaved in two ways: Heterolytically (ionic cleavage) and homolytically (radical cleavage).
Homolytic dissociation energy is much higher than a corresponding heterolytic dissociation energy.
Concept: What are Radical Initiators?4m
Radical initiators have relatively weak bonds that can be more easily cleaved by hemolysis.
Counting all stereoisomers (but not conformations), how many different monochlorinated products from the free-radical chlorination of (R)-2-bromobutane are possible?
DRAW the structures of the transition states (geometry unnecessary) for the two propagation steps (in either order) in the chlorination of chloromethane to dichloromethane, with dotted lines to represent partial bonds. You may omit δ·.
Use single-barbed curved arrows to show a homolytic bond cleavage of ethane to produce two methyl radicals.