transition state
/ tran-ZISH-un state /
Picture a ball rolling up and over a hill. There is one instant at the very crest where it is balanced on the peak, about to tip down the other side. A transition state is that crest for a chemical step: the single highest-energy arrangement of atoms on the way from reactants to products, the fleeting configuration right at the top of the energy hill.
At the transition state, old bonds are partly broken and new bonds are partly formed, all at once. In an SN2 reaction, for instance, the transition state has the carbon halfway between its starting and ending shape, with the incoming nucleophile and the departing leaving group each half-bonded to it, the carbon momentarily flat with five things loosely attached. This arrangement exists for less than the time of a single molecular vibration and can never be isolated or bottled; it is not a molecule but a knife-edge moment of passage. Chemists denote it with a double-dagger symbol.
The transition state matters because its energy, relative to the reactants, is the activation energy, and that controls the reaction rate. Anything that stabilizes the transition state (the right solvent, a catalyst, a stabilizing substituent) lowers the barrier and speeds the reaction up. The Hammond postulate tells us what the transition state roughly looks like, resembling whichever neighbor (reactant or product) is closer in energy. Because the transition state determines the rate and often the stereochemical outcome, it is arguably the most important point in the whole mechanism.
In the SN2 transition state for OH- attacking CH3Br, the central carbon is flat with OH coming in on one side and Br leaving on the other, both only partly bonded, like an umbrella caught mid-flip in the wind.
Old bond half-broken, new bond half-made: the highest-energy instant of the step.
A transition state can never be isolated, unlike an intermediate; it is a peak on the energy curve, not a valley, and exists for only an instant of molecular motion.