The Hammond-Postulate more accurately describes what transition states look like.
“Transition states most closely resemble the species with the highest energy”
Defining the Hammond Postulate.
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Now I want to talk about transition states a little bit more in depth, because earlier, when I mentioned them, I mentioned them in very vague terms. I just said that it has to do with bonds being broken and destroyed at the same time. Okay, but it turns out that there's actually a very famous rule, or postulate that was developed a while back to determine exactly what thes transition states will look like, depending on where they are in the free energy diagram. And that is called the Hammond Postulate. All right, so what's the Hammond posture? Let's say it has to do with transition states. And the paraphrased version of it, the one that I think makes the most sense is that transition states are gonna most closely resemble they're gonna look the most, like the species with the highest energy. Okay, so that means that remember that a transition state is always gonna be your highest energy point on the graph on the free energy diagram. It's always gonna relate some higher state of energy and some lower state of energy to each other. Okay, What your transition state is gonna look like is going to be like the species that has the highest energy, whether it's the beginning or the end. Okay. And I'm gonna show you guys what? That what I mean by that in a second. Okay, If a transition state more closely resembles the re agents, we call that in early Transition state. Okay, I'm just dyslexic today. Early. Okay? And if the transition state more resembles the products, then we call that a late transition state, okay?
Early transition state = Resembles reagents
Late transition state = Resembles products
Chlorination explains the Hammond Postulate.
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So in this case, notice that we have a two step reaction here. Okay, This is called radical chlorination. By the way, what I'm doing is I'm taking an Al cane and I'm adding die atomic, chlorine, and I'm breaking some bonds. What I get the end is an alcohol. Hey, lied. And a strong acid hcl. Okay, What we see is that overall, the delta g of this reaction is spontaneous. Okay, so we know this is gonna be a spontaneous reaction. Okay, but what we don't know is what is this transition state gonna look like up here? Okay. Is that gonna look more like the products, which I mean, more like the re agent, which would just be a regular al cane with a chlorine radical. Okay, which, by the way, you don't need to know this mechanism yet. I'm just explaining the steps, Okay. Or is it gonna look more like the intermediate here, which happens to be the radical on the on the Al cane and then the hcl together? Okay, well, the way we would determine this is by looking at the energy state of the Regent and the energy state of the Intermediate basically the two things on different sides of the transition state. And I would ask myself, which one has the higher energy, which everyone has. The higher energy is gonna be the one that is going to look more like the transition or the transition is gonna look more like. Okay, So what I'm gonna do is I'm gonna do this first example is a worked example together, and then I want you guys to do the next one on your own. All right, So we already said which one is the higher energy? Is it the re agents, or is it the intermediate? It looks like it's the re agents. Okay, so the one of the higher energy is Regent. Let's circle that. Okay, So since the transition state is gonna basically which one is the higher energy? The re agents? So the transition state is gonna look more like the species with the highest energy, so it's gonna look more like the re agents. Okay, Since it looks more like the re agents, that means I'm gonna have an early transition state. Okay, now, this is what the transition state would look like without Hammond's postulate. If I wasn't using him is postulate. I would just say Okay, I have an alcohol group that still is partially bonded to an H, but then that h is partially bonded toe a C l. And they're all perfectly breaking and perfectly making at the same time, so they'll have equal distances from each other. Okay, so that would be what my transition state, I would think would look like without Hammond's postulate. But we know that Hammond's postulate exists. Hammond's postulate tells me that it's actually not gonna look like this. Instead, what it's gonna look like is that it's gonna look more like the thing with the highest energy. The highest energy is this. So that means that notice that that if it was perfectly just starting off, is the re agent. What I would have is a CH two with a full bond to H and then the H having no bond to the seal. That's the Regent. Okay, If I was completely at the intermediate side, what I would have is a ch 32 with no bond to the H and then the H with a full bond to the C. L so see how this is kind of like an action sequence where my H is slowly going this way, and it's basically moving closer and closer to the chlorine until it gets here and it is fully possessed by the chlorine. Okay, this transition state shows that middle step of, Well, that's what the hydrogen would look like right in the middle when it's like at the highest point. Okay, But it turns out that, like I said, it's not gonna look like that. Since this is an early transition state, it's gonna look more like this and less like the intermediate. So what I would expect the transition state so look like is actually more like this where I have a dotted line to an H that's pretty close by. And then I have a really, really far dotted line to the seal. Why is that? Because this transition state should look more like this and less like that. Okay, so what that means is, it should look more like the H is still attached to the alcohol group and less like the H is attached the CEO, because the CL doesn't happen until later on. That's not the highest energy step does that kind of makes sense, guys. So what I'm trying to do is I'm trying to get you guys to draw transition states based on the Hammond postulate. And all you do is just say which everyone has the highest energy. That's the one that my transition states gonna look more like. Okay, so I hope that you guys can see now that the distance between my alcohol group of my H is way shorter than the distance between my age and the seal. Why? Because this is an early transition state. So it happens a lot closer to the alcohol group. All right, so now what I want you guys to do is go ahead and draw the transition state for the radical Brahma nation all on your own. By understanding and dissecting this this free energy diagram. So go for it.
Bromination explains the Hammond Postulate.
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all right. So as you guys can see, this reaction was very, very similar to the chlorination. What we have for our re agents is that the H is fully attached to the alcohol group. Okay, What we have for our intermediate is that the H is fully attached to the bro. Mean, so you know, this is gonna be another example where in our transition state, my h is gonna be somewhere between the alcohol group and the bro. Me? Okay, but we know that it's not gonna be perfectly in the middle because Hammond's postulate states that it's usually not gonna be right in the middle. It's always gonna look more like one or the other side. Okay, so now we just have to figure out which side does it look more like? So what I do is I compare the energy level of my re agents and the energy level of my intermediate. Which of these is higher? The energy level of my intermediate this time is higher. So that means that my transition state is gonna look more like the intermediate or more like the products. So this is gonna be ah, late transition state. Okay, Ah, late transition state means that it has to look more like this where the H is bonded to the BR and less like the H is born into the alcohol group. So the way this transition state should have been drawn is like this where I have a really, really, really far bond to the H. So it's almost completely gone. Okay, the age is almost completely gone, and then a really short bond to the BR. So what you can see is that the transition state looks almost completely like this. Okay, the only difference is that I still just have to use a dotted line to show that it's all happening in one step. All right, does that make sense? Guys, um, one more thing, because the fact that there's too many bonds here, I should have a negative charge in my transition state. So, for both of these, I forgot to include that. There should be a, like, partial negative here, Um, and partial negative here. So sometimes what happens is that they'll just draw partial negatives on all the species. Okay. And that's fine, too. Okay, that just shows that there's a negative charges being distributed throughout. Why would there be a negative charge? Do you guys want to think about that for a second? Because the fact that hydrogen doesn't like to have two bonds. Okay, so that means that this hydrogen right now has one more bond that it likes toe have because they're kind of both being formed in both being destroyed at the same time. So we have an extra bond that we need to distribute that negative charge for. So that's why I put those little Delta negatives. Okay, Lower case Delta. So I hope you guys can see the difference in these transition states, and hopefully, Hammond's postulate doesn't have to be really hard for you guys. I think if you just remember, it looks like the highest energy, Um, like species that's going to really help you guys be able to draw these accurately. All right, so let me know if you have any questions. Let's move on