 We have already studied about how carbocations are formed during an SN1 reaction. It's time that we talk about why and how do carbocations re-arrange during the reaction. What do I mean by that? If a carbocation formed during a particular reaction has a tendency to re-arrange itself and form a better carbocation, a more stable one, it is going to do that. Let's see how that happens. For example, we have this reaction. I know HCl is going to break and form H plus and Cl minus ions which makes me think that Cl minus is the attacking nucleophile. It should replace OH minus. But hey, OH minus is less stable than Cl minus. Why would it be replaced? So this direct replacement cannot happen. Let's think about something else. The oxygen atom has a lone pair of electrons and there is a proton. It will attack and form three bonds. But oxygen forming three bonds? Positive charge should be there, right? Hey, an electronegative atom having a positive charge? Not so stable. So the water molecule would want to leave. And that is what it does. Water leaves and we get this carbocation. This makes me think that okay, I have the carbocation, Cl minus, why don't you come and attack? And this should be my product. Yay, I am done. Not so quickly. Whenever a carbocation formed in a reaction gets a chance to rearrange itself and form a better carbocation, it does form one. So every time we form a carbocation, we have to take a moment and think, can something be done to make it a better one? So let's try and do that. Here, this carbocation is a one degree carbocation. If by any chance I am able to bring this positive charge to the middle carbon, it would be a two degree carbocation adjacent to the benzene ring. Hey, much more stable, right? Can I do that? Let's see. So if I try moving this hydrogen atom to the right most carbon, I might be able to get a carbocation. And that's exactly what happens. This hydrogen moves to the adjacent carbon in the form of a hydride ion. It takes the bond electrons and goes as H minus to the adjacent carbon. And this leads to the formation of another carbocation. This is called one-two hydride shift. And we have already studied that the rate of an SN1 reaction is directly proportional to the stability of the carbocation. The more stable the carbocation, the more are the chances of it to be formed, and the reaction is driven in that direction. So this carbocation would form the major product. What do I mean when I say major product? I mean that particular product would be in higher amount. So the major product would be Cl minus attaching to this carbocation. Hey, the previous carbocation wouldn't form any product. It would, it would, but not in a higher amount. So the reaction looks something like this. The rearranged carbocation forms the major product. Let's try another problem, shall we? We take an alcohol and we add aqueous HBr to it. The moment I see HBr, I think that it is going to break down as H plus and BL minus. Again, H minus is not such a good leaving group. But it can totally, totally attack the proton and leave as a water molecule. Let's just quickly do that and form the carbocation. Every time we form a carbocation, we have to think, is this the best possible carbocation in this mechanism? Or can it rearrange and form a better one? Let's find out. This carbocation here is a two-degree carbocation. By two-degree, I mean it's attached to a carbon on either side. If I try and move it to this cup, it would become a one-degree carbocation. I don't want that. I don't want it to become less stable. So what should I be doing? I see a carbon here, but I don't see any hydrogen attached to it. There's no possibility of hydride shift. Well, there's a possibility of alkyl shift or methyl shift to be precise. We have this methyl group that can take up its bond electrons and go to the adjacent carbon. What would it lead to? It would lead to the formation of a carbocation that is three-degree, much more stable. So my major product will be formed by the more stable carbocation. So there are two types of carbocations that can be formed. And this one is the most stable one and therefore it will form the major product. We are minus would simply attach itself to this carbocation to give the major product. And the other one would be forming the minor product. So let's go for another set of problems. Again, there is an alcohol, there is HCl that's added to it and we have to find out the major product. Why don't you try the left one yourself and then we'll do it together? OH- is not a good leaving group but oxygen has a lone pair of electrons. It's going to take up the proton and leave as a water molecule forming this carbocation. Let's just rewrite it so that we get a better image of what we are going to do. This carbocation here so formed is a one-degree carbocation. Every time I try to form a carbocation in a reaction I have to think whether I can rearrange it to get a better carbocation and here there's a possibility of a hydride shift that would give me a three-degree carbocation. So I'll do that and get this as the most stable rearranged carbocation. So what would my major product look like? Cl- would simply come and attach itself to this carbocation forming the major product that would look something like this. Why don't you try the next one and then we'll do it together? Again, the lone pair on the oxygen atom attack the proton and the water molecule leaves. We get the carbocation I just wrote it in a better way to understand what to do next. This here is a two-degree carbocation. Can I rearrange and get a better one? If I shift this hydrogen as hydride I would again get a two-degree carbocation. But but but if I then shift the methyl from the next carbon I would eventually get a three-degree carbocation. Would this happen? No. Why? Because the reaction doesn't know what's going to happen two steps later. See, if there is a two-degree carbocation that's formed the only way it's going to rearrange if it becomes better. If it does not become better right away it wouldn't do that. The reaction doesn't know what's going to happen next. It just knows the immediate next step. So this rearrangement cannot happen so the product would be this. What I'm trying to tell you folks here is that a carbocation will undergo rearrangement if and only if it has a possibility to become a better carbocation. A two-degree carbocation is not going to rearrange and form another two-degree carbocation hoping of getting something better later.