![]() ![]() In the comments, Richard points out that I should have written “exergonic” not “exothermic”. I hope this answers your question, Diana! In simple examples, the “reaction coordinate” can refer to a bond length, although as multiple molecular entities can get involved, the application is quite abstract. It’s meant to convey the progress of a reaction. One last question is what “the reaction coordinate” means. This means that if you have a reaction that forms an unstable intermediate (such as a carbocation), the transition state for its formation will closely resemble that intermediate. Assume that the transition states for reactions involving unstable intermediates can be closely approximated by the intermediates themselves. This also means that the transition state will resemble the species higher in energy. ![]() The converse of this is that if two species occur consecutively and have a very different energy content, their interconversion will involve a large reorganization of their molecular structures. If two states, for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures. The official version of Hammond’s postulate is this: So the structure of the transition state more closely resembles the carbocation than the alkene. What Hammond’s postulate says is that the transition state will more closely resemble the product higher in energy. Let’s look at a concrete example: the conversion of an alkene to a carbocation by HCl. So the transition state should be drawn closer to the product side, not the reactant side. Similarly, in an endothermic endergonic reaction, the product is the high-energy species. See how it doesn’t quite look right in the drawing on the left? Therefore the transition state should be drawn closer to the reactant side. In an exothermic exergonic reaction, the reactants are highest in energy. You probably already do this, but it’s worth drawing out the “wrong” kind of transition state diagram to make it more clear. the transition state) closest to the species highest in energy. The official explanation of Hammond’s Postulate is below, but here’s my one-sentence quick intuitive explanation of Hammond’s postulate: When drawing transition state diagrams, put the “hump” (i.e. Is there an alternative intuitive explanation? 7th edition), but I don’t really grasp it. I can follow the explanation in my textbook (L.G. I recently had my first orgo exam and one of the concepts that I have trouble visceralizing is Hammond’s Postulate. ![]()
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