Hammond's Postulate, also referred to as the Hammond-Leffler postulate, deals with the transition state of a chemical reaction.
If two states, as 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.
Hammond, G. S. A Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334-338.
This is my interpretation of Hammond's Postulate.
consider an exothermic elementary reaction of A -> B
1. the curve from A to B in energy diagram must be in the shape of an inverted U.
2. being in the shape of the inverted U, the energy level of the TS must be higher than the reactant (A).
3. the energy level of A is closer to the energy level of the TS, compared to that of B.
4. interconversion from reactant to TS requires a small molecular reorganisation
5. hence, the structure of the TS somehow resembles the reactant.
Hammond's Postulate is built on the basis of energy level, not time. It is a thermodynamic arguement.
There are 2 conflicting factors that decide the outcome of a chemical reaction. One is thermodynamics and the other is kinetics. The Arrhenius equation is a kinetics equation. Hammond's postulate is especially important when looking at the rate limiting step of a reaction. However, one must be cautious when examining a multistep reaction or one with the possibility of rearrangements during an intermediate stage. In some cases, the final products appear in skewed ratios in favor of a more unstable product (called the kinetic product) rather than the more stable product (the thermodynamic product). In this case one must examine the rate limiting step and the intermediates. Often times, the rate limiting step is the initial formation of an unstable species such as a carbocation. Then, once the carbocation is formed, subsequent rearrangements can occur. In these kinds of reactions, especially when run in cooler temperatures, the reactants simply react before the rearrangements necessary to form a more stable intermediate have time to occur. At higher temperatures when microscopic reversal is easier, the more stable thermodynamic product is favored because these intermediates have time to rearrange. Whether run in high or low temperatures, the mixture of the kinetic and thermodynamic products will eventually reach the same ratio, one in favor of the more stable thermodynamic product, when given time to equilibrate due to microreversal.