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 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 b ...
. 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 breaking ...
, 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
Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, i ...
. 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
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.
A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage o ...
. 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 s ...
only in that it cannot usually be isolated but is sometimes observable only through fast
spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
methods. It is stable in the sense that an
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 ...
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
Inferences are steps in reasoning, moving from premises to logical consequences; etymologically, the word ''infer'' means to "carry forward". Inference is theoretically traditionally divided into deduction and induction, a distinction that i ...
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
Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics. Chemical thermodynamics involves not only laboratory measurem ...
, or
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
. 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 ...
,
carbene
In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms.
The term "carbene" m ...
s,
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,
carbanion
In organic chemistry, a carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge (in at least one significant resonance form).
Formally, a carbanion is the conjugate base of a carbon acid:
:R3 ...
s,
arynes, and
carbynes.
Common features
Reactive intermediates have several features in common:
* low
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number 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
Chemical decomposition, or chemical breakdown, is the process or effect of simplifying a single chemical entity (normal molecule, reaction intermediate, etc.) into two or more fragments. Chemical decomposition is usually regarded and defined as th ...
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 element ...
* it is often possible to prove the existence of this species by
spectroscopic
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
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
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscil ...
* often difficult to distinguish from a
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 ...
* prove existence by means of
chemical trap In chemistry, a chemical trap is a chemical compound that is used to detect unstable compounds. The method relies on efficiency of bimolecular reactions with reagents to produce a more easily characterize trapped product. In some cases, the trappi ...
ping
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
Phosphoryl nitride in chemistry is a reactive intermediate with the structure OPN. It is a linear oxygen phosphorus nitrogen molecule with a formal double bond between P and O (phosphoryl) and a triple bond (nitride) between P and N. It can be obt ...
*
Tetrahedral intermediate
A tetrahedral intermediate is a reaction intermediate in which the bond arrangement around an initially double-bonded carbon atom has been transformed from trigonal to tetrahedral. Tetrahedral intermediates result from nucleophilic addition to a ...
s 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 containi ...
addition reactions
See also
*
Activated complex In chemistry an activated complex is defined by the International Union of Pure and Applied Chemistry (IUPAC) as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential ...
*
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 ...
References
Extranol links
*
{{Reaction mechanisms
Reaction mechanisms