D28.1 Elementary Reactions

A balanced equation for a chemical reaction indicates which substances react, which substances are produced, and how the amounts of reactants and products are related. But it does not necessarily show what is happening on the molecular level as the reaction takes place. For example, the overall chemical equation for combustion of octane in an automobile’s internal combustion engine is this:

2 C₈H₁₈(g)  +  25 O₂(g)  →  16 CO₂(g)  +  18 H₂O(g)

At the molecular scale, if the reaction took place as this equation is written, then two octane molecules would simultaneously collide with 25 oxygen molecules, electron density would rearrange as the C–C, C–H, and O=O bonds broke and C=O and O–H bonds formed, and all 34 product molecules would form at once. This is highly unlikely.

Although it is not obvious on the laboratory scale, most chemical reactions occur as a series of atomic-scale steps, that is, as a sequence of simpler reactions, each of which involves collisions between molecules or dissociation of a single molecule. The step-by-step sequence of simple reactions by which an overall reaction occurs is called a reaction mechanism.

Each step in a reaction mechanism is called an elementary reaction, a chemical reaction that has only a single transition state. An elementary reaction shows which molecules (and atoms) collide, break apart, or rearrange their structures to form other molecules (and atoms). A mechanism, a sequence of such elementary steps, then shows exactly which molecular interactions and in which order eventually lead to conversion of reactants to products.