 As promised in the previous video, today we will be comparing the reactivities of an allylic and a vinylic substrate towards an SN1 reaction and also the reactivities of benzylic versus phenylic substrate towards an SN1 reaction. Just to recall what allylic, vinylic, benzylic and phenylic actually mean, I have written these down here. A carbon attached to a doubly bonded carbon is called an allylic carbon. While the carbon which is actually the one that has the double bond is called a vinylic carbon. A carbon adjacent to the benzene ring is called a benzylic carbon while the carbon atom that belongs to the ring that is a part of the benzene ring is called a phenylic carbon. So let's just take one example and let's ask ourselves which of the following would react the fastest via SN1. Since we have already studied the mechanism in the previous videos, we can see that the leaving group is same in each case that is I- so we must focus on what carbocation would form in each case and compare the stability of various carbocations to realize which of the following would react the fastest via the SN1 mechanism. So let's form the carbocations. Why don't you try comparing the stability of each before we do it together? If I look carefully at these cations, I can see how the double bonded carbon has a positive charge in this case while the carbon next to the double bonded carbon has a positive charge in the other two cases. So the first one is a vinylic cation. Vinylic cation means a positive charge lying on a double bonded carbon and the other two are allylic cations. The positive charge resides on a carbon atom adjacent to the double bond. Digging a little deeper, I see there's a positive charge on a carbon atom that already forms a double bond. There's no possibility of resonance, right? This is what we have studied in the previous units while if the positive charge is alternate to a pi bond, there is a possibility of resonance. So let's draw the resonating structures. So this is the resonating structure for the second one and we can see how the positive charge has spread over all these atoms and has stabilized. Let's draw the resonating structure for the last one and here also I can see how the positive charge has spread out. So there's a possibility of resonance in both the second and the third one. What extra factor can you see? Well, there's an attached methyl group to the carbocation in the third case. Three alpha hydrogens, right? So there's a possibility of hyper conjugation as well. So I can see the last one is stabilized via resonance as well as hyper conjugation while the second one is just stabilized via resonance. What about the first one? It's highly unstable. Do you remember how a doubly bonded carbon was sp2 hybridized and it was a pretty good electronegative thing? So an electronegative atom carrying a positive charge, nah, not so stable, right? So it was highly unstable. So would it form? I don't think it would want to form. If you're not stable doing something, why would you do it? So the carbocation would not form in the first case while the carbocation would be the most stable in the third case. And therefore the rate of reaction is as follows. And I must mention that a vinylic substrate would not react via SN1 mechanism. This is highly, highly improbable for it to react via SN1 mechanism because it wouldn't form a carbocation. That would be highly unstable. So let's move on to comparing vinylic and benzylic cations. So what if I'm asked which of the following would not react via SN1 mechanism and which of the following would be the fastest to react via SN1 mechanism? In each case, Cl- would be the leaving group, right? So why don't we form the carbocations and compare their stabilities and figure out which one would actually not react via SN1? These are the carbocations. Hey, try comparing yourself first and then we'll do it together. If I look carefully at the first one, there's a benzene ring and there's a positive charge on the ring. There's a doubly bonded carbon that has a positive charge and it's highly unstable. Would it form? Nah, it wouldn't. So vinylic substrate wouldn't want to react via SN1 mechanism, right? Let's compare the other two. Shall we? In both the cases, I can see how a positive charge is alternate to a pi bond. There is a possibility of resonance in each case and through this resonance, the positive charge would spread out in the entire ring in each case, right? What else is there? What extra factor? Well, I can see the third one has three alpha hydrogens as well. So there's a possibility of hyperconjugation here. So the third one is stabilized via hyperconjugation and via resonance. While the second one is just stabilized via resonance and the first one is highly unstable. So what is the order of stability? The first one would not, not, not react via SN1 mechanism because this carbocation is highly, highly unstable. So the substrate that reacts the fastest via SN1 would be this one while if I talk about a phenyl substrate, it would not want to react via SN1. So what did we learn in this video? We learned how allylic and benzylic substrates are super happy to undergo substitution via SN1 mechanism while the vinylic and phenylic ones, not so much. The carbocations in those cases won't form at all as they would be highly, highly unstable. In the next video, we'll be talking about how and why rearrangements of carbocations take place in an SN1 mechanism.