reaction mechanism

When you bake a cake, the overall recipe says flour plus eggs plus sugar become cake. But that line hides everything that actually happens — creaming the butter, folding in flour, the batter rising in the oven. A reaction mechanism is the step-by-step story of how a chemical reaction really proceeds: the sequence of simple molecular events that, added up, give the overall equation you write on paper.

More precisely, a reaction mechanism is a proposed sequence of elementary reactions — individual collisions, bond breakings, and bond formings — that converts reactants into products. Each step involves only a few molecules at a time, and the steps may create short-lived intermediates that appear partway through and disappear before the end. Chemists deduce mechanisms indirectly, mainly by measuring how the reaction rate changes when concentrations or temperature change, since the overall balanced equation by itself says nothing about the route taken.

Why it matters: the same reactants can reach the same products by very different paths, and the path controls how fast the reaction goes and which by-products form. The honest caveat is that a mechanism is always a model — a best current explanation consistent with the evidence, not something we can watch directly. A single new experiment can overturn a long-accepted mechanism, so they are proposed, tested, and sometimes replaced.