In
chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
. When generated in 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 ...
, it will quickly convert into a more stable molecule. Only in exceptional cases can these compounds be isolated and stored, e.g. low temperatures,
matrix isolation. When their existence is indicated, reactive intermediates can help explain how a chemical reaction takes place.
Most chemical reactions take more than one
elementary step to complete, and a reactive intermediate is a high-energy, yet stable, product that exists only in one of the intermediate steps. The series of steps together make a
reaction mechanism. A reactive intermediate differs from a reactant or product or a simple
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 ...
only in that it cannot usually be isolated but is sometimes observable only through fast
spectroscopic methods. It is stable in the sense that an
elementary reaction forms the reactive intermediate and the elementary reaction in the next step is needed to destroy it.
When a reactive intermediate is not observable, its existence must be
inferred through experimentation. This usually involves changing reaction conditions such as temperature or concentration and applying the techniques of
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 ...
,
chemical thermodynamics, or
spectroscopy. Reactive intermediates based on carbon are
radicals
Radical may refer to:
Politics and ideology Politics
*Radical politics, the political intent of fundamental societal change
*Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
,
carbenes,
carbocations,
carbanions,
arynes, and
carbynes.
Common features
Reactive intermediates have several features in common:
* low
concentration with respect to reaction substrate and final reaction product
* with the exception of carbanions, these intermediates do not obey the
lewis octet rule, hence the high reactivity
* often generated on
chemical decomposition of a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...
* it is often possible to prove the existence of this species by
spectroscopic means
*
cage effects have to be taken into account
* often stabilisation by
conjugation
Conjugation or conjugate may refer to:
Linguistics
*Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
*Complex conjugation, the change ...
or
resonance
* often difficult to distinguish from a
transition state
* prove existence by means of
chemical trapping
Carbon
File:Radical metilo--methyl radical.svg, Radical
File:Carbene.svg, Carbene
File:Methyl cation.svg, Carbocation
File:碳负离子.svg, Carbanion
File:Carbyne quartet.svg, Carbyne
File:1,2-Didehydrobenzol.svg, Benzyne (an aryne)
Other reactive intermediates
*
Carbenoid
*
Ion-neutral complex
*
Keto anions
*
Nitrenes
*
Oxocarbenium ions
*
Phosphinidenes
*
Phosphoryl nitride
*
Tetrahedral intermediates in
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 ...
addition reactions
See also
*
Activated complex
*
Transition state
References
Extranol links
*
{{Reaction mechanisms
Reaction mechanisms