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My textbook states," According to the Hammond-Leffler postulate, the transition- state structure for a step that is uphill in energy should show a strong resemblance to the structure of the product of that step."
Also stating,"One way that the postulate can be stated is to say that structure of a transition state resembles the stable species that is nearest to it in free energy. For example, in a highly endergonic step the transition state lies close to the products in free energy,and we assume,therefore,that it resembles the products of that step in structure as well. " (Source:Solomons' Organic Chemistry Book by Craig B. Fryhle, Scott A. Snyder, and T. W. Graham Solomons)

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  • $\begingroup$ If you quote from a book, please cite it. $\endgroup$ Jun 3, 2020 at 9:59
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    $\begingroup$ That is quite literally what the Hammond postulate says (Wikipedia). So the direct answer to why we say that is because that's what the postulate says. Are you asking about the justification for the postulate? $\endgroup$ Jun 3, 2020 at 10:41
  • $\begingroup$ Yes , I meant is there any logical explanation for it?Or should we just accept it just because it's a postulate.. $\endgroup$
    – Fallon
    Jun 3, 2020 at 11:16
  • $\begingroup$ Because it seems quite reasonable at first. Of course there might very different structures with similar energy, but it is unlikely. And even more so if the structures lie along a reaction path. $\endgroup$
    – Alchimista
    Jun 3, 2020 at 13:12
  • $\begingroup$ Fallon, it is clear to you that the transition state is higher in energy than any other species involved (reactants, metastable intermediates that can be eventually isolated, products) involved? $\endgroup$
    – Alchimista
    Jun 5, 2020 at 11:49

1 Answer 1

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EDIT for a different approach:

The energy of a quantum mechanical system of nuclei and electrons is governed by the positions of the pieces. So, we're looking at the energy associated with the wave functions of all the pieces.

For two species that are similar in structure, the idea is that those two structures lead to relatively similar solutions of the wave functions. If the wave functions are similar, the energies of the particles described by those wave functions must be similar as well.

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  • $\begingroup$ The use of early and late makes this answer very obscure. At least to me. $\endgroup$
    – Alchimista
    Jun 3, 2020 at 13:14
  • $\begingroup$ Early and late are standard terms when discussing the Hammond Postulate. I tried to relate early to "early on the reaction coordinate." Do you have any specific suggestions for improving the answer? $\endgroup$
    – Zhe
    Jun 3, 2020 at 20:15
  • $\begingroup$ Because isn't clear to which logic you refer to, at the very start. Then what you said reiterate the postulate. Perhaps I am wrong but the "answer" should be that in the comment by @orthocresol, integrated by my comment. The problem is how to explain something intuitive to OP. As I said one can imagine exceptions or violations, of course. But it should be natural to relate similar energy to similar structures, and this is what the postulate does. $\endgroup$
    – Alchimista
    Jun 4, 2020 at 15:17
  • $\begingroup$ I see what you mean. Let me try again. $\endgroup$
    – Zhe
    Jun 4, 2020 at 15:25
  • $\begingroup$ @Alchimista Better? $\endgroup$
    – Zhe
    Jun 4, 2020 at 15:29

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