Topic overview

Start with the big picture

A reaction energy profile traces reactants through transition states and, in multistep reactions, possible intermediates to products. A transition state is a momentary, highest-energy configuration and cannot be isolated; an intermediate has a finite lifetime in a local energy minimum and may sometimes be detected or trapped. The highest transition-state barrier helps determine the rate, while the Hammond postulate relates transition-state structure to a nearby stable species. Mechanistic pathways also explain why carbocations can rearrange, whereas cyclic bromonium or mercurinium intermediates restrict rearrangement and influence stereochemistry. Concerted pathways such as hydroboration–oxidation avoid discrete carbocations. The full lesson connects these ideas to stabilization, energy diagrams, and examples including radical additions and catalytic hydrogenation.