Early and Late Transition States: Reading Energy Proximity

A reaction-coordinate profile helps predict which bonds are mostly intact, partly broken or partly formed at a transition state. The key comparison is with the two stable states immediately adjacent to that transition state.

Two energy differences answer different questions

The reaction free-energy change, $\Delta G^\circ=G_P-G_R$, compares products with reactants. The forward activation free energy, $\Delta G^\ddagger=G_{TS}-G_R$, compares the transition state with reactants. A downhill reaction can still have an appreciable barrier. The reaction coordinate describes molecular rearrangement; it is not a clock.

Translate energy proximity into a structural prediction

Hammond reasoning predicts that neighboring states close in energy tend to be similar in structure. An exergonic elementary step is commonly described by an early, reactant-like transition state; an endergonic step by a late, product-like one. “Early” and “late” refer to structural progress, not a measured time or a numerical fraction of a bond.

Consider a hypothetical one-step process with reactants at 0, products at +40 and the transition state at +55 kJ/mol on the same free-energy scale. The transition state is 15 kJ/mol above the product but 55 above the reactants. A product-like structure is therefore the qualitative prediction. The forward barrier is 55 kJ/mol, whereas the reaction free-energy change is +40 kJ/mol. These are illustrative values, not experimental data.

Turn the prediction into a useful sketch

For a hypothetical bond-cleavage step $\mathrm{A{-}B\rightarrow A^+ + B^-}$ with that profile, draw a strongly elongated A–B contact and substantial developing opposite charges. Do not write an exact bond length or assign exact partial charges from energy proximity alone.

In a multistep mechanism, repeat the comparison for each peak and its neighboring minima. The overall product may be far removed from the step being analyzed. A useful final check is to label both adjacent states, the barrier and the reaction energy before making the structural prediction.