Hey, everyone. In this video, I'm going to introduce you to an entirely new class of reactions that you haven't been exposed to yet called pericyclic reactions. So, guys, we're just going to read this first sentence together, and then I'm going to explain it. Conjugated polyenes have the ability to react in non-ionic concerted cyclization reactions that we refer to as pericyclic reactions. Okay? Now, pericyclic reactions are an umbrella term for many different reactions, but the crazy thing or the cool thing is that all those subtypes of reactions all have the same properties in common. So, what we're going to go through now is what those properties are that are shared by all pericyclic reactions.
The first one is non-ionic. What does that mean? That means that many times with reactions in organic chemistry, we're used to having some kind of acid and some kind of base, some kind of electron donor and electron acceptor, right? But pericyclic reactions are unique because there are no ions, no partial charges, no negative and positive nucleophiles; it doesn't exist. So solvents are going to have no effect on them since there are no partial charges. Usually, solvents might have actually influenced the rate of a reaction that has acid and base qualities because there are charges that need to be stabilized. But in this case, there are no charges, so solvents won't matter; they won't change the rate of your reaction.
Concerted: all bonds are created and destroyed simultaneously, there are no intermediates. You might have seen other concerted reactions by now, but these are all going to be concerted. Cyclizations: for a reaction to be a pericyclic reaction, it needs to involve a ring of electrons around a closed loop forming a cyclic transition state. Remember to keep this in mind: it has no intermediates, but it does have transition states. Transition states are what happen in concerted reactions and are reversible, so that means that we're always going to want to use our equilibrium arrows to describe them. This principle of reversibility is also referred to in your textbook as the principle of microscopic reversibility. That's just a complicated way to say that, under the right conditions, you can go from reactant to product and from product to reactant.
Finally, all of these pericyclic reactions either occur thermally, meaning they're heat-activated, or photochemically, meaning that they are light-activated. So you're always going to have those two options to start off your reaction.
Remember that I mentioned that there are several types of pericyclic reactions? What I'm going to do in this video is introduce you to the three most common types. There are more than three types of pericyclics, but the others, like numbers 4 and 5, are for much more advanced organic chemistry courses, and you're not going to be responsible for them. If you just know these three, that's probably more than sufficient. In fact, your professor or your textbook may not even teach you all three, so in the next video, I'll make sure only to include the ones that you need to know. Pericyclic reactions can be easily categorized by the number of pi bonds that are destroyed after the cyclic mechanism. So, what I mean by destroyed is going from reactants to products.
Let's talk about the first one, which is cycloadditions. Cycloadditions are pericyclic reactions in which two pi bonds are destroyed after a cyclic mechanism. Look at our reactants. How many pi bonds do we have in the set of reactants on the left? We have three total: two on the diene and then one on that alkene. So, I would write here three reactant pi bonds. Now, notice that I am using an equilibrium arrow and heat to show that through a heat-activated cyclic mechanism, I am going to form this product here. How many pi bonds does this product have? Just one. So, how many pi bonds were destroyed? Two. This would be a cycloaddition. Anytime that you're doing a cyclic heat or light activated mechanism to get rid of two pi bonds, you automatically know it's a cycloaddition.
The next category is electrocyclic reactions. Electrocyclic reactions are pericyclic reactions in which one pi bond is destroyed after a cyclic mechanism. In this typical electrocyclic reaction, I have three pi bonds to start off with. Then, after I react that with heat in a reversible mechanism, I would get only two pi bonds in my product. Once again, there was probably a cyclic type of mechanism because I'm forming a ring at the end, and this means that this is an electrocyclic reaction because one pi bond is missing.
Finally, we have sigmatropic shifts. Sigmatropic shifts are pericyclic reactions in which zero pi bonds are destroyed after a cyclic mechanism. Here, notice that we started with two pi bonds and in the product, we have how many pi bonds? Also two. The pi bonds did switch places, and we actually did get a cyclic mechanism, but all that happened was that the pi bonds changed their position. A sigmatropic shift would be a heat or light-activated mechanism in which you're not losing any pi bonds.
Just knowing these three facts, you would be able to distinguish what's a cycloaddition, what's electrocyclic, and what's sigmatropic. Also, something that you might have already noticed, but I just want to draw your attention to it so that you can memorize it easier, is that these happen to be in alphabetical order, which is nice. Two bonds breaking is a cycloaddition; then one bond breaking is an electrocyclic reaction, and then zero bonds would be a sigmatropic shift. Cool? So, they're in order C-E-S, and now you know in very general terms what the different categories of pericyclic reactions are, and once again, all the properties that we talked about at the beginning apply to these three categories of reactions.
Wonderful. So, we're done with this video. Let's move on to the next one.
