Reactive intermediate
Encyclopedia
In chemistry
a reactive intermediate is a short-lived, high energy, highly reactive molecule
. When generated in a chemical reaction
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 only exists in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product 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.
An example of a reaction mechanism would be as follows.
Given the overall observable chemical equation
: A + 2B → C + D + E,
the proposed elementary steps might be:
Here X and Y are the reactive intermediates. They are formed in one elementary step and destroyed in a subsequent step. Note that the algebraic sum of the proposed elementary steps must equal the observed chemical equation
.
When a reactive intermediate is not an 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 thermodynamics
, or spectroscopy
.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
a reactive intermediate is a short-lived, high energy, highly reactive molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
. When generated in a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
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
Matrix isolation is an experimental technique used in chemistry and physics which generally involves a material being trapped within an unreactive matrix. A host matrix is a continuous solid phase in which guest particles are embedded. The guest is said to be isolated within the host matrix...
. When their existence is indicated, reactive intermediates can help explain how a chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
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 only exists in one of the intermediate steps. The series of steps together make a reaction mechanism. A reactive intermediate differs from a reactant or product 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
Elementary reaction
An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
forms the reactive intermediate and the elementary reaction
Elementary reaction
An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
in the next step is needed to destroy it.
An example of a reaction mechanism would be as follows.
Given the overall observable chemical equation
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
: A + 2B → C + D + E,
the proposed elementary steps might be:
- A + B → C + X
- X → D + Y
- B + Y → E.
Here X and Y are the reactive intermediates. They are formed in one elementary step and destroyed in a subsequent step. Note that the algebraic sum of the proposed elementary steps must equal the observed chemical equation
Chemical equation
A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...
.
When a reactive intermediate is not an 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
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
, chemical thermodynamics
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...
, or spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
.
The main carbon reactive intermediates.
based on carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
- carbocationCarbocationA carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
s - carbanionCarbanionA carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.where B...
s - free radicals
- carbeneCarbeneIn 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 RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
s
Common features
- low concentrationConcentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
with respect to reaction substrate and final reaction product - with the exception of carbanions, these intermediates do not obey the lewis octet ruleOctet ruleThe octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
hence the high reactivity - often generated on chemical decompositionChemical decompositionChemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
of a chemical compoundChemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together... - it is often possible to prove the existence of this species by spectroscopicSpectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
means - cage effectsCage effect (chemistry)The cage effect in chemistry describes how properties of a molecule are affected by its surroundings.In a solvent a molecule is often more accurately described existing in a cage of solvent molecules, the so-called solvent cage. Reactions occur when a molecule occasionally "jumps out" and meets...
have to be taken into account - often stabilisation by conjugationConjugated systemIn chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
or resonance - often difficult to distinguish from a transition stateTransition stateThe transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate. At this point, assuming a perfectly irreversible reaction, colliding reactant molecules will always...
- prove existence by means of chemical trapChemical trapIn chemistry, a chemical trap is a chemical compound that is used to detect a certain molecule when* The concentration of this molecule is very small and below detection limit...
ping
Other reactive intermediates
- nitreneNitreneIn chemistry, a nitrene is the nitrogen analogue of a carbene. The nitrogen atom has only 6 valence electrons and is therefore considered an electrophile...
s - Phosphinidenes
- carbyneCarbyneIn chemistry, a carbyne is a monovalent carbon radical species containing an electrically neutral univalent carbon atom with three non-bonded electrons.- Gas phase/reactive intermediate :...
- carbenoidCarbenoidIn chemistry a carbenoid is a reactive intermediate that shares reaction characteristics with a carbene. In the Simmons-Smith reaction the carbenoid intermediate is a zinc / iodine complex that takes the form of...
- aryneAryneIn chemistry, an aryne is an uncharged reactive intermediate derived from an aromatic system by removal of two ortho substituents, leaving two orbitals with two electrons distributed between them....
s - Keto anions
- tetrahedral intermediates in carbonylCarbonylIn 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....
addition reactions