The Hammond Postulate

The Hammond postulate suggests (as an unprovable assertion) that species that are sequential on the reaction coordinate and similar in energy are therefore similar in structure.  Conversely, changes in structure that stabilize or destabilize reactive intermediates will stabilize or destabilize transition states leading to them.  This can be illustrated by ionization of different bromobutanes and measured by the rate of reaction with water at 50°C.  Compare this to the energies of the reactants, and the ionized intermediates, as calculated at the AM1 level (with Cramer-Truhlar solvent correction for water).
 
 
 
 
 

We can compare the relative energies of the cations with the reaction rate, which reflects the energy of the transition state relative to the ground state:
 
 

The nature of the alkyl stabilization derives from the electron-donating ability of the alkyl group; look at the electrostatic potential maps for each ion (mapped on a common scale).

Blue represents positive electrostatic potential, and red represents negative potential.

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Last updated: 09/21/00
Comments to K. Gable