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In chemistry, a reactive intermediate is a highly reactive, high-energy, short-lived molecule that, when produced in a chemical process, swiftly transforms into a stable molecule. They are segregated and kept in some cases. These chemicals can only be separated and preserved in extreme circumstances, such as low temperatures or matrix isolation.
A reactive intermediate is a high-energy, yet stable, product that exists only in one of the intermediate phases in most chemical processes. A response mechanism is made up of a succession of stages. A reactive intermediate is stable in the sense that it is formed by an elementary reaction and must be destroyed by an elementary reaction in the next step.
Features of Reactive Intermediates
Reactive intermediates share several common characteristics:
- To stabilize a reaction, conjugation or resonance are used.
- they have a low concentration in relation to the reaction substrate
- products are difficult to distinguish from a transition state
- They are frequently formed during the disintegration of a chemical compound
Detection of Reactive Intermediates
It’s difficult to tell the difference between a transition stage. It’s a state that has a reaction coordinate and correlates to potential energy at a higher level. It’s a form of chemical reaction in which a certain configuration exists along a reaction coordinate.
Chemical trapping can be used to demonstrate its presence. The chemical trap is just a chemical substance that detects a molecule in specific situations, as shown below:
- When a molecule is present in a mixture, the detection of that molecule is hampered by the presence of components.
- When the molecule’s concentration is below the detection limit.
- When a molecule is extremely reactive or cannot be determined by spectroscopic methods.
Reaction intermediate examples
- Free radicals
An atom or a collection of atoms with an odd or unpaired electron is known as a free radical. These are denoted by adding a dot (•) against the symbol of the atom or group of atoms and originate from homolytic fission of a covalent bond. Due to the existence of an odd electron, they are paramagnetic in nature and are usually generated in the presence of either UV/visible light or peroxides.
Free radicals are classified as Primary, Secondary, Tertiary free radicals. Its stability is in the order: Tertiary > Secondary > Primary > •CH3. Its stability is dependent upon the resonance and attachment of electron-donating groups.
- Carbocation
When a covalent connection between carbon and a more electronegative atom or group breaks down due to heterolytic fission, the more electronegative atoms take away the bonded pair of electrons, while carbon loses an electron and gains a positive charge. Carbocations or carbonium ions are organic ions with a positive charge on the carbon atom.
The charged carbon atom in a carbocation has a “sextet,” which means it only contains six electrons in its outer valence shell, rather than the eight required for optimal stability (octet rule). As a result, carbocations are frequently reactive, attempting to reclaim an octet of valence electrons.
- Carbanions
When a covalent connection between carbon and a less electronegative atom breaks down due to heterolysis, the atom leaves without carrying away the bonding pair of electrons, causing the carbon atom to receive a negative charge because of the additional electron.
Carbanions have a diamagnetic property. They are extremely reactive because the negatively-charged carbon in them is electron-rich, allowing it to offer its non-bonding pair of electrons to another group for sharing. Carbanions, as a result, act like nucleophiles and are easily attacked by electrophiles.
A carbanion is divided into three categories: primary, secondary, and tertiary.
- Carbenes
:CH2 is a highly reactive, neutral species. Carbenes are divalent neutral carbon intermediates. It is a form of carbon that has two unpaired electrons but no charge. It has a valence of two and two unshared valence electrons. The carbon atom in the chemical structure of carbenes has six electrons in its outer shell, two of which are lone pairs and two of which are shared (electron-deficient). As a result, Carbenes are neutral, transitory chemical intermediates having a carbon atom with two bonds and two electrons that are sp2 and sp-hybridized. It is short-lived and diagonal in geometry. The formula is expressed as: R-(C:)-R’ or R=C: (R is the substituents of hydrogen atoms).
- Nitrenes or Imidogenes
Organic substances with the general formula R-:N: are known as nitrenes. These are nitrogen intermediates that are comparable to carbenes and are neutral univalent nitrogen (with one bond and two non-bonded electron pairs). These are “electron-deficient species in which nitrogen has a sextet of electrons,” according to the definition (six electrons in the outermost shell).
Because they need a pair of electrons to complete the octet, they are extremely reactive and strong electrophiles. :N:-H is the parent species (known as nitrene or imidogene or azene or imine). Because it tends to polymerize to (NH)n , as soon as it is created, it is difficult to form. As a result, substituted nitrenes have got a lot of interest.
Nitrenes, like carbenes, can exist in singlet and triplet forms, with one of the covalent links replaced by a nitrogen lone pair. The ground state is the triplet state. Nitrenes have important synthetic applications particularly acyl nitrene has been proposed as possible intermediate in the Hofmann, Curtius, and Lossen rearrangements.
- Benzyne (1, 2-Dehydrobenzeile or Aryne)
Benzyne is a neutral, highly reactive chemical intermediate with a relatively unaltered aromatic property. It is generated after the nucleophilic substitution of aromatic compounds as an intermediate. It has a Carbon-Carbon triple bond in the benzene ring, which is formed by sideways overlapping of sp2 hybridized orbitals of two nearby carbon atoms, resulting in the development of a new weaker C-C bond.
Conclusion
“Reactive intermediates are molecular entities with lives significantly longer than a molecular vibration formed during the conversion of reactants to products for a chemical reaction.” When these intermediates are generated in a chemical reaction upon breaking of the covalent bonds in a substrate known as homolytic or heterolytic bond fission, they instantly convert into a more stable form of the molecule within a few seconds. Organic synthesis, photolithography, photocrosslinking, and photoaffinity labeling are just a few of the fields where these intermediates are used.
