Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical reaction occurs.

A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see next section) or other quantitative and qualitative characteristics of the reaction. It also describes each reactive intermediate, activated complex, and transition state, which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also explain the reason for the reactants and catalyst used, the stereochemistry observed in reactants and products, all products formed and the amount of each.

The electron or arrow pushing method is often used in illustrating a reaction mechanism; for example, see the illustrations of the mechanisms for Michael addition and benzoin condensation in the following examples section.

Mechanisms also are of interest in inorganic chemistry. A often quoted mechanistic experiment involved the reaction of the labile hexaaquo chromous reductant with the exchange inert pentammine cobalt(III) chloride.

Reaction intermediates are chemical species, often unstable and short-lived. They can, however, sometimes be isolated. They are neither reactants nor products of the overall chemical reaction, but temporary products and/or reactants in the mechanism's reaction steps. Reaction intermediates are often confused with the transition state. The transition states are, in contrast, fleeting, high-energy species that cannot be isolated. The kinetics (relative rates of the reaction steps and the rate equation for the overall reaction) are discussed in terms of the energy required for the conversion of the reactants to the proposed transition states (molecular states that correspond to maxima on the reaction coordinates, and to saddle points on the potential energy surface for the reaction).

Information about the mechanism of a reaction is often provided by analyzing chemical kinetics to determine the reaction order in each reactant.

Illustrative is the oxidation of carbon monoxide by nitrogen dioxide:

The rate law for this reaction is: ${\displaystyle r=k[NO_{2}]^{2}}$ This form shows that the rate-determining step does not involve CO. Instead, the slow step involves two molecules of NO₂. A possible mechanism for the overall reaction that explains the rate law is: