7-7) The Hammond Postulate Provides Insight into the Transition States of the SN1 Reaction
- The reaction of SN1 can be represented by an energy diagram
- Each step has a transition state with higher energy than the carbocation
- Unlike cationic intermediates, carbocations cannot be observed in its transient state: rather, its structure can be speculated--bond getting longer as leaving group approaches leaving point.
Hammond Postulate-- structure of a transient state should resemble the species to which it is closest in energy.
- For exergonic process (-delta G or spontaneous), the transition state will occur late in the reaction and its structure will look like that of product(s).
- For endergonic process (+ delta G value), the transition state will occur late in the reaction and its structure will look like that of the product(s)
- Link to picture slideshow
- Provides a way to understand the relative rates of SN1 Reactions
- the pathway leading to the more stable carbocation must also have a lower free energy of activation
*Ranking of reaction speed is the same order as carbocation stability*
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