In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, 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 breaking ...
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 ...
s, which by definition involves at least two consecutive
elementary reactions.
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 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, i.e. the reaction doesn't proceed any faster than the rate-determining step proceeds.
Organic reactions
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemi ...
, especially when involving catalysis, are often stepwise. For example, a typical enol
In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). T ...
reaction consists of at least these elementary steps:
#Deprotonation next to (α to) the carbonyl:
#Attack of enolate:
Rδ+ is an electron acceptor, for example, the carbon of a carbonyl (C=O). A very strong base, usually an alkoxide
In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
, 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, superacid
In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superaci ...
s 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 encount ...
s.
See also
* 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 breaking ...
* Elementary reaction
* Rate equation
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 ...
* Rate-determining step
* Steady state approximation
* Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...
* Lindemann mechanism
External links
Chemical kinetics
{{reaction-stub