Thread: The Hammond Postulate

States - The structure of a transition state resembles the structure of the nearest stable species.
Transition states for endothermic steps structurally resemble products, and transition states for exothermic steps structurally resemble starting materials.

I can't get my head round the second paragraph. I learned that in an exothermic reaction the product is more stable than the reactant therefore the structure should surely resemble the product and vice versa for endothermic reactions.

am i missing something here? or did my book just get it wrong?

I learned that in an exothermic reaction the product is more stable than the reactant therefore the structure should surely resemble the product and vice versa for endothermic reactions.

But not every reaction has to out a product that tend to be more stable, other wise how can we have so many firece reactions, since after 4.6 billion years "reacting" of the earth everything should've been quite stable.........don't know if it's a agood point.b

But not every reaction has to out a product that tend to be more stable, other wise how can we have so many firece reactions, since after 4.6 billion years "reacting" of the earth everything should've been quite stable.........don't know if it's a agood point.b

it is an interesting point. However the reactions we get that our fierce are not necessarily spontaneous. If the reaction has a high activation energy it will still not happen after 4.6 billion years.

For a good example look at the desk your computer is currently on. If it is wooden then the most stable state would be ash so it should spontaneously burst into flames however the activation energy prevents it.

You are right that not every reaction has to out a product that tend to be more stable, these are endothermic reactions where energy is taken in during the reaction producing a less stable product however this is taken into account of my question.

unfortunately your point doesnt help answer the question which is related to structure of the transition states. Thanks anyway

Oh I see... But then I don;t get what your question is.

Vice versa

means that you think product of endothermic reactions should also be resemble to he intermediate, and near to the trasition state, as the product tend to be more stable, don't you? I got feel a bit obsecure.......

well i have already stated what the hammond postulate is.

The product of an endothermic reaction is less stable than the reactants as energy is taken in. thats pretty much the definition.


therefore by my understanding if the transition state resembles the structure of the nearest stable species. it should structually resemble starting products as this is more stable.

yet the definition goes onto say that the endothermic reaction transition states resemble products.

seems to be in direct contradiction to my understanding.

so either i'm wrong or the book is wrong.
books are usually right so i was hoping someone could help me out

....if the transition state resembles the structure of the nearest stable species. it ......

Is it what your book said, cuz' as I learned transition state resembles that of the species nearest to it in free energy. I think these two are different. Go check on wikipe...

definetly what the book says

im guna read it again

ah i see the higher the energy of the transition state. the later it will occur in a reaction.

because the endothermic reactions have a high energy transition state it will occur later in the reaction thus resembling a product

but have you check whether "the transition state resembles the structure of the nearest stable species." is wrong or not?

yeh i think when my book speaks of stability it is referring to the relative bond strengths in the transition state

in an exothermic reaction...the bond formed is going to be stonger than the bond broken. therefore the transition state will be formed earlier in the reaction as the new bond is more stable than the old one.

i think my book used a poor choice of words in stability as it was rather unclear