reaction mechanism

When you write a chemical equation, you only see the starting materials on the left and the products on the right, like a before-and-after photo. But how did the atoms actually get from one picture to the other? A reaction mechanism is the slow-motion movie that fills in every frame in between: which bonds break, which bonds form, in what order, and how the electrons move at each instant.

Concretely, a mechanism breaks a reaction down into one or more elementary steps. Each step is a single, simple event, often involving just one or two molecules colliding and electrons shifting. Chemists draw this with curved arrows, where each arrow shows the movement of a pair of electrons, not the movement of an atom. Along the way the molecules may pass through short-lived, unstable species called reactive intermediates (such as carbocations or free radicals), which are real molecules but exist for only a fleeting moment before reacting on.

Mechanisms matter because they let you predict and control. If you understand why a reaction happens the way it does, you can predict the product, choose conditions to favor it, and explain odd results. A mechanism is a proposed model, supported by evidence like reaction rates, stereochemistry, and isotope effects; it is never seen directly, so it can be refined or overturned when new data appear. Two reactions with the same overall equation can run by completely different mechanisms.