The Hammond Postulate

The Hammond-Postulate more accurately describes what transition states look like. Paraphrased version:

         “Transition states most closely resemble the species with the highest energy”

Concept: Defining the Hammond Postulate.

2m
Video Transcript

Now I'm going 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. 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.
What's the Hammond Postulate say? It has to do with transition states. 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.
That means that remember that a transition state is always going to be your highest energy point on the free-energy diagram. It's always going to relate some higher state of energy and some lower state of energy to each other. 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. I'm going to show you guys what I mean by that in a second.
If a transition state more closely resembles the reagents, we call that an early transition state. I'm just dyslexic today. Early. If the transition state more resembles the products, then we call that a late transition state. 

  • Early transition state = Resembles reagents
  • Late transition state = Resembles products 

Example: Using radical chlorination to explain the Hammond Postulate. 

6m

Example: Using radical bromination to explain the Hammond Postulate. 

3m