Hammond’s Postulate

by James

in Organic Chemistry 1, Thermodynamics and Kinetics

Last week Diana left this comment:

I recently had my first orgo exam and one of the concepts that I have trouble visceralizing is Hammond’s Postulate. I can follow the explanation in my textbook (L.G. Wade, Jr. 7th edition), but I don’t really grasp it. Is there an alternative intuitive explanation?

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. the transition state) closest to the species 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. Let’s look at an exothermic reaction. In an exothermic reaction, the reactants are highest in energy. Therefore the transition state should be drawn closer to the reactant side. See how it doesn’t quite look right in the drawing on the left?

 

Similarly, in an endothermic reaction, the product is the high-energy species. So the transition state should be drawn closer to the product side, not the reactant side.

Let’s look at a concrete example: the conversion of an alkene to a carbocation by HCl. What Hammond’s postulate says is that the transition state will more closely resemble the product higher in energy. In this case, that’s the carbocation. So the structure of the transition state more closely resembles the carbocation than the alkene.

The official version of Hammond’s postulate is this:

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 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. This also means that the transition state will resemble the species higher in energy. Here’s a different way of looking at it.

Assume that the transition states for reactions involving unstable intermediates can be closely approximated by the intermediates themselves.
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.

 


One last question is what “the reaction coordinate” means. It’s meant to convey the progress of a reaction. 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. Think of it as “reaction progress”.

I hope this answers your question, Diana!


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{ 7 comments… read them below or add one }

axaybanyal

could you please explain which kind of reactions do not follow hammond’s postulates..

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Mercedes

Thank you man!!!! This explanation was of great help to me! I wish i had discovered this site when I was doing organic chemistry I.

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james

Thanks! Glad you found it useful!

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chandan

will you please explain the applicability of Hammett Equation.

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james

That’s more of an advanced topic, for now I’d refer you to Ansyln’s Modern Physical Organic Chemistry for an excellent treatment of this topic. Bottom line: donor groups stabilize positively charged transition states (and accelerate rates) – acceptor groups stabilize negatively charged transition states.

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jetharamjaipal(jodhpur)

BIG THANKS OF YOU FOR MY help

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shristy tiwary

will you plz..explain..How Hammond’s postulate ableto explain the similarity of structure with the substrate and product?

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