Hammond's postulate is a hypothesis that concerns the transition state of organic chemical reactions. Hammond’s postulate states that the transition state of a reaction resembles either the reactants or the products depending on which is closer in energy. It was proposed by George S. Hammond.
Hammond's postulate is also known as the Hammond–Leffler postulate. Hammond's postulate helps chemists by providing information about the structure of transition state, which usually cannot be directly characterized experimentally. The Hammond's postulate enables the accurate prediction of the shape of a reaction coordinate diagram and is used extensively for things such as explaining the effects of aromatic substituents in electrophilic aromatic substitution.