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7-7) The Hammond Postulate Provides Insight into the Transition States of the SN1 Reaction

Page history last edited by daniel.s.brashear@... 14 years, 4 months ago

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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