Introduction

A chemical reaction is, at its core, a process in which some of the bonds already present in the reacting molecules break while new bonds form -- the reactant is converted into the product, and this conversion may take one step or several. In general terms, an organic reaction can be written as

The substrate is the organic molecule that undergoes the chemical change. The reagent is whatever brings that change about -- it may itself be an organic or inorganic species, or even a physical agent such as heat or light (photons).

Most real reactions happen in one or more simple steps rather than in a single leap. Each step must cross its own energy barrier, and in doing so it usually passes through a short-lived intermediate or a fleeting, unisolable transition state. The complete sequence of these simple steps, taken together, is called the mechanism of the reaction -- and because a multi-step reaction can only go as fast as its slowest link, the single slowest step in the mechanism sets the overall rate of the reaction (the rate-determining step). …