Hammond Postulate - Online Tutor, Practice Problems & Exam Prep

The Hammond postulate explains that transition states in chemical reactions resemble the species with the highest energy, whether reactants or products. A transition state is the peak on a free energy diagram, linking higher and lower energy states. If it resembles the reactants, it's termed an early transition state; if it resembles the products, it's a late transition state. Understanding these concepts is crucial for grasping reaction mechanisms and energy profiles in organic chemistry.

Defining the Hammond Postulate.

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 these transition states will look like depending on where they are in the free energy diagram. That is called the Hammond postulate. Alright? So what does the Hammond postulate 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 going to most closely resemble, they're going to look the most like the species with the highest energy. Okay? So that means that remember that a transition state is always going to be your highest energy point on the graph, on the free energy diagram. And it's always going to relate some higher state of energy and some lower state of energy to each other. Okay? What your transition state is going to look like is going to be like the species that has the highest energy, whether that's the beginning or the end. Okay? And I'm going to show you guys what I mean by that in a second. Okay? If a transition state more closely resembles the reagents, we call that an 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?

Chlorination explains the Hammond Postulate.

So in this case, notice that we have a 2 step reaction here. Okay? This is called radical chlorination by the way. What I'm doing is I'm taking an alkane and I'm adding diatomic chlorine and I'm breaking some bonds. What I get at the end is an alkyl halide and a strong acid, HCl. What we see is that overall, the delta g of this reaction is spontaneous. Okay? So we know this is going to be a spontaneous reaction. Okay? But what we don't know is what this transition state is going to look like up here? Okay? Is that going to look more like the products, which I mean more like the reagents, which would just be a regular alkane with a chlorine radical? Okay? Which by the way, you don't need to know this mechanism yet. I'm just explaining the steps. Or is it going to look more like the intermediate here, which happens to be the radical on the alkane and then the HCl together? Okay? Well, the way we would determine this is by looking at the energy state of the reagent and the energy state of the intermediate. Basically, the 2 things on different sides of the transition state. And I would ask myself which one has the higher energy. Whichever one has the higher energy is going to be the one that is going to look more like the transition state or the transition state is going to look more alike. Okay? So what I'm going to do is I'm going to do this first example as a worked example together and then I want you guys to do the next one on your own. Alright? So we already said which one has the higher energy? Is it the reagents or is it the intermediate? It looks like it's the reagents. Okay? So the one with the higher energy is reagents. Let's circle that. Okay? So since the transition state is gonna basically Which one has the higher energy? The reagents. So the transition state is going to look more like the species with the highest energy. So it's going to look more like the reagents. Okay? Since it looks more like the reagents, that means I'm going to 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 Hammond's postulate, I would just say, okay, I have an alkyl group that still is partially bonded to an H, but then that H is partially bonded to a Cl. And they're all perfectly breaking and perfectly making at the same time, so they 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 going to look like this. Instead, what it's going to 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 if it was perfectly just starting off as the reagent, what I would have is a CH₂ with a full bond to H and then the H having no bond to the Cl. That's the reagent. Okay? If I was completely at the intermediate side, what I would have is a CH₃ with no bond to the H and then the H with a full bond to the Cl. 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 going to look like that since this is an early transition state, it's going to look more like this and less like the intermediate. So what I would expect the transition state to 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 Cl. Why is that? Because this transition Okay? So what that means is it should look more like the H is still attached to the alkyl group and less like the H is attached to the Cl because the Cl doesn't happen until later and that's not the highest energy step. Does that kind of make 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 whichever one has the highest energy, that's the one that my transition state's going to look more like. Okay? So I hope that you guys can see now that the distance between my alkyl group and my H is way shorter than the distance between my H and Cl. Why? Because this is an early transition state, so it happens a lot closer to the alkyl group. So now what I want you guys to do is go ahead and draw the transition state for the radical bromination all on your own by understanding and dissecting this free energy diagram. So go for it.

Bromination explains the Hammond Postulate.

All right. So as you guys can see, this reaction is very similar to the chlorination. What we have for our reagents is that the H is fully attached to the alkyl group. Okay? What we have for our intermediate is that the H is fully attached to the bromine. So we know this is going to be another example where in our transition state, my H is going to be somewhere between the alkyl group and the bromine. Okay? But we know that it's not going to be perfectly in the middle because Hammond's postulate states that it's usually not going to be right in the middle. It's always going to 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 reagents 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 going to look more like the intermediate or more like the products. So this is going to be a late transition state. Okay? A 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 bonded to the alkyl 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. The H 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. The only difference is that I still just have to use a dotted line to show that it's all happening in one step. Alright? Does that make sense, guys? One more thing, because of 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 partial negative here 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. That just shows that there's a negative charge that's being distributed throughout. Why would there be a negative charge? Do you guys want to think about that for a second? Because of the fact that hydrogen doesn't like to have 2 bonds. Okay? So that means that this hydrogen right now has one more bond than it likes to have because they're kind of both being formed and 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? Lowercase 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-like species that's going to really help you guys be able to draw these accurately. Alright? So let me know if you have any questions. Let's move on.

Here’s what students ask on this topic:

What is the Hammond postulate in organic chemistry?

The Hammond postulate states that the transition state of a chemical reaction resembles the species (reactants or products) with the highest energy. This means that if the transition state is closer in energy to the reactants, it will resemble the reactants more closely, and if it is closer in energy to the products, it will resemble the products more closely. This concept helps in understanding the nature of the transition state and predicting the reaction mechanism. The transition state is the highest energy point on a free energy diagram, linking higher and lower energy states.

How does the Hammond postulate help in understanding reaction mechanisms?

The Hammond postulate aids in understanding reaction mechanisms by providing insight into the nature of the transition state. By knowing whether the transition state resembles the reactants or products, chemists can infer details about the reaction pathway. For example, an early transition state (resembling reactants) suggests that the reaction proceeds quickly from reactants to transition state, while a late transition state (resembling products) indicates that the reaction has a higher activation energy and proceeds more slowly. This information is crucial for predicting reaction rates and designing efficient chemical processes.

What is the difference between an early and a late transition state according to the Hammond postulate?

According to the Hammond postulate, an early transition state resembles the reactants more closely and occurs when the transition state is closer in energy to the reactants. This typically happens in exothermic reactions where the energy barrier is lower. Conversely, a late transition state resembles the products more closely and occurs when the transition state is closer in energy to the products. This is common in endothermic reactions where the energy barrier is higher. Understanding whether a transition state is early or late helps in predicting the reaction's energy profile and mechanism.

Why is the transition state the highest energy point on a free energy diagram?

The transition state is the highest energy point on a free energy diagram because it represents the point at which the reactants are in the process of being converted into products. At this stage, bonds are being broken and formed, requiring a significant amount of energy. This energy peak is necessary to overcome the activation energy barrier of the reaction. Once the transition state is reached, the system can proceed to form the products, which are at a lower energy level. The height of this peak determines the reaction rate and is a critical factor in reaction kinetics.

How can the Hammond postulate be applied to predict the outcome of a chemical reaction?

The Hammond postulate can be applied to predict the outcome of a chemical reaction by analyzing the energy levels of the reactants and products. By determining whether the transition state is early or late, chemists can infer the reaction pathway and the relative stability of intermediates. For instance, in an exothermic reaction with an early transition state, the products are likely to form quickly and be more stable. In an endothermic reaction with a late transition state, the reaction may proceed more slowly, and the products may be less stable. This predictive power is valuable for designing reactions and optimizing conditions.