Definition of Hammond Postulate
Hammond’s postulate is a hypothesis which explains the geometrical structure of the instantaneous product (transition state) that is formed during an organic chemical reaction. It is stated as if two states, for instance, a transition and an unstable product exist at the same time during the chemical reaction and they constitute almost the same amount of energy, then they can be interconverted into one another by a small reorganization of the molecular structures.
The transition state has more similarity with the reactant in an exothermic reaction. on the other hand, in an endothermic reaction, the transition state and the product are geometrically similar.