Hammond Postulate (Polanyi-Hammond Postulate)
The Hammond Postulate (Polanyi-Hammond Postulate)
The reaction rate of chemical reactions is directly determined by the activation energy EA (or rather by the free activation energy) of the slowest reaction step, which is called the rate-determining step. In order to estimate the reaction rate, the quantity of activation energy has to be known as exact as possible. The activation energy EA is the energy difference between the starting products and the transition state of the respective reaction step. As the transition state cannot be directly investigated in an experiment, because of its (theoretically) infinitesimal lifetime, its structure can only be assumed. Therefore, the influence of different substituents on the energy of the transition state can never really be predicted.
At this point, the Polanyi-Hammond postulate can serve as a guide in many cases. This postulate is broadly explained below:
- Polanyi-Hammond postulate
- The mutual conversion between two species that are adjacent to each other on the reaction coordinate requires merely a minor change in the molecular structure if they have approximately almost the same energy.
- Consequences of the Polanyi-Hammond postulate for the practice
- What does this mean in practice?
- If after the transition state an energy-rich intermediate stage directly follows on the reaction coordinate, this intermediate stage has just some trivial structural differences to the transition state. The influence of different substituents on the energy of the transition state can therefore be estimated by the influence of these substituents on the energy of this intermediate stage. In practice, these estimations are actually possible, because the structures of such intermediate stages are experimentally accessible and are therefore known for many reaction types. Examples of such energy-rich intermediate stages are the σ complex in the electrophilic aromatic substitution and the carbenium ion in the addition of hydrogen halides to substituted alkenes (see also Markovnikov rule).
The Polanyi-Hammond postulate enables not only the estimation of the influence of substituents on the activation energy EA, but to a certain degree the estimation of the structure of the transition state, as well. This is because this should be very similar to the structure of the energy-rich intermediate stage, as they are near each other on the reaction coordinate. The structure of the transition state can be deduced even from the structure of the starting products or products, respectively, if the reaction is sufficiently exo- or endothermic (or more exactly: exer- or endergonic).
• In the case of (very) exothermic reactions, the energy of the transition state is namely much closer to the energy of the starting products than to that of the products. As a result, the structure of the transition state obviously resembles the structure of the starting products much more than the structure of the products (reactions with an early transition state).
• However, in the case of (very) endothermic reactions, the energy of the transition state is much more similar to the energy of the products than that of the starting products. As a result, the structure of the transition state obviously resembles the structure of the products to a larger degree than the structure of the starting products (reactions with a late transition state).
- "Naming": Polanyi-Hammond postulate or Hammond postulate
- Usually, particularly in the Anglo-Saxon literature the shorter term Hammond postulate is more common than the term Polanyi-Hammond postulate. However, the Hungarian physical chemist Polanyi had already made the fundamental considerations with regard to this postulate in the nineteen-thirties.