Radical reactions are an important process of making many of the common goods we use in the modern world (ever heard of Styrofoam?). So, how do they work?

First, we need to look at how bonds can break. We refer to this as bond cleavage. Bonds can be cleaved in two ways: heterolytically or homolytically. In a homolytic bond cleavage, each atom that comprises the molecule gets one electron. In the heterolytic cleavage, the electrons go with one atom completely. Since one atom is getting an extra electron, this one will have a negative charge. However, the other atom has lost an electron and therefore gains a positive charge. See the picture below for a better understanding.

Make sure you pay close attention to the different arrows. Some arrows have a full head and some have half. Those that have half of an arrow do so because they are only showing the movement of one electron and not 2! Now that we have deciphered how bonds are cleaved, let's look at how radical reactions progress.

In a radical reaction, there are 3 main steps: initiation, propagation, and termination. Below, we will look at an in-depth view of each of the steps. In brief, the initiation steps starts the entire process, this generates a radical. The propagation steps prolongs the process in that there must be a radical that is developed in the product. Finally, the termination steps end by terminating the radical-ending with no new radicals developed.

We can view this process by looking at the radical halogenation of methane below.

In the above picture, we can see the pka of a C-H bond is roughly 60. This is too high to allow facile deprotonation even with the strongest bases. However, if we react methane with a halogen (in this case bromine) in the presence of light, we see that there is indeed a reaction. So, how can this happen? A radical reaction is the only way to achieve the products.

Notice that in the termination steps above, there is even a reaction that is undesirable-the formation of ethane. Feel free to respond with questions and suggest more material for review and stay tuned for new posts and videos!

Good Luck!