Hammond postulate

We can never photograph a transition state, since it lasts less than a vibration, so how do we guess what it looks like? The Hammond postulate gives a wonderfully simple rule of thumb: a transition state resembles whichever species, reactant or product of that step, it is closest to in energy. The peak of a hill leans toward whichever valley it sits nearer to.

Put concretely, in a strongly exothermic (downhill) step the transition state comes early, when the molecules still look much like the reactants; we call this an 'early transition state'. In a strongly endothermic (uphill) step the transition state comes late, after the molecules have already changed to look much like the products; this is a 'late transition state'. The reasoning is that going from a structure to a nearby structure of similar energy is a small change, so the peak between them resembles both, while a peak between two very different energies leans toward the closer one.

This idea is powerful because it lets us reason about the unseeable transition state by looking at the much more familiar reactants and products. For example, carbocation-forming steps are endothermic, so their transition states are late and resemble the carbocation; that is why the stability order of carbocations carries over directly to the rates of the reactions that form them, and ultimately underpins Markovnikov's rule. It is a postulate, an approximation rather than a law, but it is one of the most useful reasoning tools in mechanistic chemistry.