Radical (chemistry)
In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron.[1][2] With some exceptions, these unpaired electrons make radicals highly
A notable example of a radical is the
2) which have two unpaired electrons.
Radicals may be generated in a number of ways, but typical methods involve
Radicals are important in
Formation
Radicals are either (1) formed from spin-paired molecules or (2) from other radicals. Radicals are formed from spin-paired molecules through homolysis of weak bonds or electron transfer, also known as reduction. Radicals are formed from other radicals through substitution, addition, and elimination reactions.
Radical formation from spin-paired molecules
Homolysis
Homolysis makes two new radicals from a spin-paired molecule by breaking a covalent bond, leaving each of the fragments with one of the electrons in the bond.[3] Because breaking a chemical bond requires energy, homolysis occurs under the addition of heat or light. The bond dissociation energy associated with homolysis depends on the stability of a given compound, and some weak bonds are able to homolyze at relatively lower temperatures.
Some homolysis reactions are particularly important because they serve as an initiator for other radical reactions. One such example is the homolysis of halogens, which occurs under light and serves as the driving force for radical halogenation reactions.
Another notable reaction is the homolysis of dibenzoyl peroxide, which results in the formation of two benzoyloxy radicals and acts as an initiator for many radical reactions.[4]
Reduction
Classically radicals form by one-electron
.Radical formation from other radicals
Abstraction
Addition
In free-radical additions, a radical adds to a spin-paired substrate. When applied to organic compounds, the reaction usually entails addition to an alkene. This addition generates a new radical, which can add to yet another alkene, etc. This behavior underpins radical polymerization, technology that produces many plastics.[5][6]
Elimination
Radical elimination can be viewed as the reverse of radical addition. In radical elimination, an unstable radical compound breaks down into a spin-paired molecule and a new radical compound. Shown below is an example of a radical elimination reaction, where a benzoyloxy radical breaks down into a phenyl radical and a carbon dioxide molecule.[7]
Stability
Stability of organic radicals
Although organic radicals are generally stable intrinsically (in isolation), practically speaking their existence is only transient because they tend to dimerize. Some are quite long-lived. Generally organic radicals are stabilized by any or all of these factors: presence of electronegativity, delocalization, and steric hindrance.[8] The compound 2,2,6,6-tetramethylpiperidinyloxyl illustrates the combination of all three factors. It is a commercially available solid that, aside from being magnetic, behaves like a normal organic compound.
Electronegativity
Organic radicals are inherently electron deficient thus the greater the electronegativity of the atom on which the unpaired electron resides the less stable the radical.[9] Between carbon, nitrogen, and oxygen, for example, carbon is the most stable and oxygen the least stable.
Electronegativity also factors into the stability of carbon atoms of different hybridizations. Greater s-character correlates to higher electronegativity of the carbon atom (due to the close proximity of s orbitals to the nucleus), and the greater the electronegativity the less stable a radical.[9] sp-hybridized carbons (50% s-character) form the least stable radicals compared to sp3-hybridized carbons (25% s-character) which form the most stable radicals.
Delocalization
The delocalization of electrons across the structure of a radical, also known as its ability to form one or more resonance structures, allows for the electron-deficiency to be spread over several atoms, minimizing instability. Delocalization usually occurs in the presence of electron-donating groups, such as hydroxyl groups (−OH), ethers (−OR), adjacent alkenes, and amines (−NH2 or −NR), or electron-withdrawing groups, such as C=O or C≡N.[3]
Delocalization effects can also be understood using molecular orbital theory as a lens, more specifically, by examining the intramolecular interaction of the unpaired electron with a donating group's pair of electrons or the empty π* orbital of an electron-withdrawing group in the form of a molecular orbital diagram. The HOMO of a radical is singly-occupied hence the orbital is aptly referred to as the SOMO, or the Singly-Occupied Molecular Orbital. For an electron-donating group, the SOMO interacts with the lower energy lone pair to form a new lower-energy filled bonding-orbital and a singly-filled new SOMO, higher in energy than the original. While the energy of the unpaired electron has increased, the decrease in energy of the lone pair forming the new bonding orbital outweighs the increase in energy of the new SOMO, resulting in a net decrease of the energy of the molecule. Therefore, electron-donating groups help stabilize radicals.
With a group that is instead electron-withdrawing, the SOMO then interacts with the empty π* orbital. There are no electrons occupying the higher energy orbital formed, while a new SOMO forms that is lower in energy. This results in a lower energy and higher stability of the radical species. Both donating groups and withdrawing groups stabilize radicals.
Another well-known albeit weaker form of delocalization is hyperconjugation. In radical chemistry, radicals are stabilized by hyperconjugation with adjacent alkyl groups. The donation of sigma (σ) C−H bonds into the partially empty radical orbitals helps to differentiate the stabilities of radicals on tertiary, secondary, and primary carbons. Tertiary carbon radicals have three σ C-H bonds that donate, secondary radicals only two, and primary radicals only one. Therefore, tertiary radicals are the most stable and primary radicals the least stable.
Steric hindrance
Most simply, the greater the steric hindrance the more difficult it is for reactions to take place, and the radical form is favored by default. For example, compare the hydrogen-abstracted form of N-hydroxypiperidine to the molecule TEMPO.[3] TEMPO, or (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, is too sterically hindered by the additional methyl groups to react making it stable enough to be sold commercially in its radical form. N-Hydroxypiperidine, however, does not have the four methyl groups to impede the way of a reacting molecule so the structure is unstable.[3]
Facile H-atom donors
The stability of many (or most) organic radicals is not indicated by their isolability but is manifested in their ability to function as donors of H•. This property reflects a weakened bond to hydrogen, usually O−H but sometimes N−H or C−H. This behavior is important because these H• donors serve as antioxidants in biology and in commerce. Illustrative is
Inorganic radicals
A large variety of inorganic radicals are stable and in fact isolable. Examples include most first-row transition metal complexes.
With regard to main group radicals, the most abundant radical in the universe is also the most abundant chemical in the universe, H•. Most main group radicals are not however isolable, despite their intrinsic stability. Hydrogen radicals for example combine eagerly to form H2.
Many radicals can be envisioned as the products of
Diradicals
Triplet carbenes and nitrenes are diradicals. Their chemical properties are distinct from the properties of their singlet analogues.
Occurrence of radicals
Combustion
A familiar radical reaction is
Combustion consists of various radical chain reactions that the singlet radical can initiate. The
When a hydrocarbon is burned, a large number of different oxygen radicals are involved. Initially,
Polymerization
Many polymerization reactions are initiated by radicals. Polymerization involves an initial radical adding to non-radical (usually an alkene) to give new radicals. This process is the basis of the radical chain reaction. The art of polymerization entails the method by which the initiating radical is introduced. For example, methyl methacrylate (MMA) can be polymerized to produce Poly(methyl methacrylate) (PMMA – Plexiglas or Perspex) via a repeating series of radical addition steps:
Newer radical polymerization methods are known as
Being a prevalent radical, O2 reacts with many organic compounds to generate radicals together with the hydroperoxide radical. Drying oils and alkyd paints harden due to radical crosslinking initiated by oxygen from the atmosphere.
Atmospheric radicals
The most common radical in the lower atmosphere is molecular dioxygen.
-
(eq. 1.1)
-
(eq. 1.2)
-
(eq. 1.3)
-
(eq. 1.4)
In the upper atmosphere, the photodissociation of normally unreactive
-
(eq. 2.1)
-
(eq. 2.2)
-
(eq. 2.3)
-
(eq. 2.4)
-
(eq. 2.5)
Such reactions cause the depletion of the ozone layer, especially since the chlorine radical is free to engage in another reaction chain; consequently, the use of chlorofluorocarbons as refrigerants has been restricted.
In biology
Radicals play important roles in biology. Many of these are necessary for life, such as the intracellular killing of bacteria by phagocytic cells such as
Radicals may also be involved in
Because radicals are necessary for life, the body has a number of mechanisms to minimize radical-induced damage and to repair damage that occurs, such as the
Reactive oxygen species
Reactive oxygen species or ROS are species such as superoxide, hydrogen peroxide, and hydroxyl radical, commonly associated with cell damage. ROS form as a natural by-product of the normal metabolism of oxygen and have important roles in cell signaling. Two important oxygen-centered radicals are
.Oxybenzone has been found to form radicals in sunlight, and therefore may be associated with cell damage as well. This only occurred when it was combined with other ingredients commonly found in sunscreens, like titanium oxide and octyl methoxycinnamate.[22]
ROS attack the
Reactive oxygen species are also used in controlled reactions involving singlet dioxygen known as type II photooxygenation reactions after Dexter energy transfer (triplet-triplet annihilation) from natural triplet dioxygen and triplet excited state of a photosensitizer. Typical chemical transformations with this singlet dioxygen species involve, among others, conversion of cellulosic biowaste into new poylmethine dyes.[24]
History and nomenclature
Until late in the 20th century the word "radical" was used in chemistry to indicate any connected group of atoms, such as a
The term radical was already in use when the now obsolete radical theory was developed. Louis-Bernard Guyton de Morveau introduced the phrase "radical" in 1785 and the phrase was employed by Antoine Lavoisier in 1789 in his Traité Élémentaire de Chimie. A radical was then identified as the root base of certain acids (the Latin word "radix" meaning "root"). Historically, the term radical in radical theory was also used for bound parts of the molecule, especially when they remain unchanged in reactions. These are now called functional groups. For example, methyl alcohol was described as consisting of a methyl "radical" and a hydroxyl "radical". Neither are radicals in the modern chemical sense, as they are permanently bound to each other, and have no unpaired, reactive electrons; however, they can be observed as radicals in mass spectrometry when broken apart by irradiation with energetic electrons.
In a modern context the first
In most fields of chemistry, the historical definition of radicals contends that the molecules have nonzero electron spin. However, in fields including spectroscopy and astrochemistry, the definition is slightly different. Gerhard Herzberg, who won the Nobel prize for his research into the electron structure and geometry of radicals, suggested a looser definition of free radicals: "any transient (chemically unstable) species (atom, molecule, or ion)".[27] The main point of his suggestion is that there are many chemically unstable molecules that have zero spin, such as C2, C3, CH2 and so on. This definition is more convenient for discussions of transient chemical processes and astrochemistry; therefore researchers in these fields prefer to use this loose definition.[28]
Depiction in chemical reactions
In chemical equations, radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:
Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons:
The homolytic cleavage of the breaking bond is drawn with a "fish-hook" arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. The second electron of the breaking bond also moves to pair up with the attacking radical electron.
Radicals also take part in
involving radicals can usually be divided into three distinct processes. These are initiation, propagation, and termination.- Initiation reactions are those that result in a net increase in the number of radicals. They may involve the formation of radicals from stable species as in Reaction 1 above or they may involve reactions of radicals with stable species to form more radicals.
- Propagation reactions are those reactions involving radicals in which the total number of radicals remains the same.
- Termination reactions are those reactions resulting in a net decrease in the number of radicals. Typically two radicals combine to form a more stable species, for example:
- 2 Cl• → Cl2
See also
- Electron pair
- Globally Harmonized System of Classification and Labelling of Chemicals
- Hofmann–Löffler reaction
- Free radical research
References
- ^ IUPAC Gold Book radical (free radical) PDF Archived 2017-03-02 at the Wayback Machine
- PMID 26875845.
- ^ OCLC 761379371.
- ^ "Diacyl Peroxides". polymerdatabase.com. Retrieved 2020-12-08.
- PMID 11740917.
- .
- ISBN 978-3-642-38730-2
- .
- ^ a b Forrester, A.R. (1968). Organic Chemistry of Stable Free Radicals. London: Academic Press. pp. 1–6.
- ISBN 978-0-470-16637-6. Archived from the original(PDF) on 2015-09-23. Retrieved 2011-03-31.
- ISBN 978-0-12-020762-6.
- ^ However, paramagnetism does not necessarily imply radical character.
- .
- PMID 24476342.
- PMID 17237348.
- PMID 24379787.
- PMID 10381209.
- ISBN 978-0-7484-0916-7.
- S2CID 27537163.
- PMID 15152078.
- PMID 10224662.
- S2CID 27248506.
- PMID 24379787.
- S2CID 248165785.
- .
- PMID 27631602.
- ISBN 0-486-65821-X.
- ^ 28th International Symposium on Free Radicals Archived 2007-07-16 at the Wayback Machine.