In chemistry, a stepwise reaction (also called an overall reaction, complex reaction, and multistep reaction, among others) is a

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...

with one or more

reaction intermediate In chemistry, a reaction intermediate or an intermediate is a molecular entity that is formed from the reactants (or preceding intermediates) but is consumed in further reactions in stepwise chemical reactions that contain multiple elementary ...

s, which by definition involves at least two consecutive

elementary reaction An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state. In practice, a reaction is assumed to be elementary if no reaction ...

s. In a stepwise reaction, not all bonds are broken and formed at the same time. Hence, intermediates appear in the reaction pathway going from the reactants to the products. A stepwise reaction distinguishes itself from an elementary reaction in which the transformation is assumed to occur in a single step and to pass through a single

transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...

.IUPAC Gold Book definition
/ref> In contrast to elementary reactions which follow the law of mass action, the

rate law In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reac ...

of stepwise reactions is obtained by combining the rate laws of the multiple elementary steps, and can become rather complex. Moreover, when speaking about

catalytic reaction Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s, the diffusion may also limit the reaction. In general, however, there is one very slow step, which is the

rate-determining step In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...

, i.e. the reaction doesn't proceed any faster than the rate-determining step proceeds. Organic reactions, especially when involving catalysis, are often stepwise. For example, a typical enol reaction consists of at least these elementary steps: #Deprotonation next to (α to) the

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containin ...

: #Attack of enolate: R^δ+ is an electron acceptor, for example, the carbon of a

(C=O). A very strong base, usually an alkoxide, is needed for the first step. Reaction intermediates may be trapped in a ''trapping reaction''. This proves the stepwise nature of the reaction and the structure of the intermediate. For example, superacids were used to prove the existence of

carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encoun ...

External links

Chemical kinetics {{reaction-stub

See also

External links