transition state

Meaning: The transition state is the highest-energy point along the reaction coordinate for one elementary step. The atoms are in a fleeting arrangement with old bonds partly broken and new bonds partly formed. It cannot be isolated. The energy gap from reactants to the transition state is the activation energy. A catalyst provides a pathway with a lower-energy transition state, so Ea falls, while ΔG and Keq stay unchanged.

Equation: k = Ae^(−Ea/RT), where k is the rate constant, A is the frequency factor (same units as k), Ea is the activation energy in J/mol, R is 8.314 J/(mol·K), and T is temperature in K. Here Ea = E(transition state) − E(reactants).

On the MCAT: A passage shows a reaction coordinate diagram. You label the peak as the transition state, read Ea from the reactants up to that peak, and compare catalyzed and uncatalyzed curves. In an SN2 reaction there is a single transition state, with the nucleophile and leaving group both partly bonded to carbon.

Common confusion: Students mix up the transition state and an intermediate. A peak is a transition state. A valley between two peaks is an intermediate, a real species such as the carbocation in SN1 that can sometimes be detected.