What is a Radical in Chemistry Definition Types and Examples
All radicals in chemistry are also referred to as free radicals because the radical in chemistry is an atom that consists of at least one unpaired valence electron with them. These unpaired electrons make the radicals highly reactive elements with few exceptions. Most of these radicals have a short life span, and they tend to easily dimerize instantly with other atoms of the same kind. One of the most notable examples of radicals is the hydroxyl radical, which has one unpaired electron situated on an oxygen atom. The two other major examples of radicals in chemistry are triple oxygen and triplet carbene (꞉CH2), which have two unpaired electrons.
The radicals may generate in a number of ways but the most prominent and easy methods for the formation of the free radicals are ionisation radiation, redox reaction heat, electrical discharges, and electrolysis which results in the production of free radicals. Radicals form an intermediate product in many of the chemical reactions that are very much evident from the balanced equations. In chemistry like combustion, atmospheric chemistry, polymerisation, plasma chemistry, biochemistry, and many other chemical processes, radicals play an important role.
The radical generating enzymes are the prime element for the production of many of the naturally occurring products. The radicals such as nitric oxide and superoxide and their products in living organisms play a very crucial role in regulating many important processes such as controlling vascular tone and thus blood pressure. They are also known to regulate many intermediary metabolisms of various biological compounds. For a process dubbed redox signalling, such radicals can even be a messenger. It can be bound or trapped in a solvent cage.
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Properties of Free Radical
All the structure of the free radicals has a common property that they have at least one unpaired electron present in their atomic orbital and still can exist independently. Generally, the molecules possess the bonding pair or the lone pair of electrons that are unbonded, also referred to as unshared pair of electrons. The electrons present in each bonding and the non-bonding pairs have opposite spin orientation, +1/2 and -1/2 in the orbital depending on Paul’s Exclusion Principle. But an unpaired electron in the radical is just one single electron that is present in the orbital. Thus, the atom or ion containing the single electron is known as a free radical and is a paramagnetic species. Therefore, some of their properties are as follows:
They are rare and unique species that are present in very special conditions that are very limited in nature. However, we are certain of some of the free radicals in our daily lives.
Molecular oxygen is a biradical molecule that is one kind of free radical. The stable molecule of oxygen is in a triple molecular state where the two unpaired electrons present in each oxygen atom have the same spin orientation in different orbitals. They also have the same orbital energy based on Hund’s rule.
Nitrogen monoxide and nitrogen dioxide are the two species that are considered stable free radical species. Moreover, oxygen radical is the reactive species that are involved in immunity that consists of singlet molecular oxygen and superoxide anion radical.
Free radicals thus are very familiar to us and are an important part of our immunology, and support major biological activities.
Free radicals are very unstable and thus are highly reactive in nature. They are the unique species that has the ability to both donate the electrons or accept the electrons and, therefore, acts as reactants or oxidants.
Examples of Radicals
The examples of free radicals are methyl radicals. Three reactive species are considered, which are methyl anion and methyl cation along with methyl radical. These free radicals have been illustrated below.
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Ethane is composed of two methyl groups that are highly stable in nature and are connected to each other through covalent bonds. The methyl cation and the methyl anion have a positive charge and lone pair of electrons that make the methyl and negatively charged where the carbon is connected to the counter iron with an ionic bond. It is a rather moisture-sensitive species but not particularly unstable in nature.
But the methyl radical is extremely unstable in nature and therefore is very reactive. It is because the octane of carbon is not completely filled. The carbon in the methyl cation is obtained for SP2 hybridization which gives it a structure that is triangular and planar. The carbon atom in the methyl anion comprises SP3 hybridisation, which gives it a tetrahedral structure. But the carbon atom in the methyl radical adopts a middle structure that lies between methyl cation and methyl anion. Therefore, we get a structure that is a pyramid and the rapid inversion occurs at extremely low temperatures. Some of the other examples of such free radicals contain Oxygen and hydroxyl radicals.
Hypochlorite
Hydrogen peroxide
Nitric oxide radical
Superoxide anion radical
Peroxynitrite radical
Types of Free Radicals
Most of the organic radicals are unstable in nature and therefore become highly reactive. Therefore, there are two types of free radicals, namely e neutral radicals and charged radicals, as shown below.
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There are also two kinds of different radicals known as Sigma radicals and the pi radicals. The unpaired electron in the Sigma radical is in the Sigma orbital, whereas the unpaired electron in the pi radical is in the PI orbital, respectively. Therefore, the radicals that are illustrated are the PI radicals where le t-butyl radical is also considered as Pi radical as it is stabilised by the hyperconjugation, where the Sigma radicals have phenyl radicals and vinyl radicals as examples.
Generally, the resonance effect on the hyperconjugation effect stabilises the PI radicals but since there is no such stabilising effect for the Sigma radicals, therefore, they are very reactive in nature.
Uses and Source of Free Radicals
The sources of free radicals that are generated internally are as follows:-
Inflammation
Mitochondria
Phagocytosis
Exercise
Peroxisomes
Externally the free radicals are found in the following sources.
Ozone layer
Drugs and pesticides
Radiation
Environmental pollution
Smoking cigarette
Following are the uses of free radicals, as mentioned below.
The membrane of the cell that is damaging biologically relevant molecules such as DNA lipids, proteins, carbohydrates etc., contains the highly reactive structures of free radicals.
Mini micro molecules that lead to cell damage and homeostatic disruption, such as proteins and nucleic acids, has been attacked by the free radicals.
The radical precursors for R or Ar generally use up the alkyl halides or aryl halides, but what are the sugars and the nucleus eyes have many age groups and other delicate functional groups; therefore, the halogenation of these groups is rather difficult.
For the radical reactions in sugars nucleosides in peptides, the Barton Mccombie reaction is very useful.
Along with phenoxy thiocarbonate chloride, the other thiocarbonyl derivatives that are formed from alcohol can be put to use in place of methyl xanthate.
Some of the major radicals are given in the form of radical chat, which is as follows:
Radical Chat in Chemistry
Name of the radical | Representation | valency |
Ammonia | NH4+ | 1 |
Nitrate | NO3- | 1 |
Nitrite | NO2- | 1 |
Bisulphate | HSO4- | 1 |
bisulphite | HSO3- | 1 |
Bicarbonate | HCO3- | 1 |
Hydroxide | OH- | 1 |
Acetate | CH3COO- | 1 |
Sulphate | SO42- | 2 |
Sulphite | SO32- | 2 |
Carbonate | CO32- | 2 |
Dichromate | Cr2O7- | 2 |
Phosphate | PO43- | 2 |
FAQs on Radicals in Chemistry Explained with Types and Formation
1. What is a radical in chemistry?
A radical in chemistry is an atom, molecule, or ion that contains one or more unpaired electrons, making it highly reactive. Radicals are often called free radicals because they can exist independently and readily participate in chemical reactions. For example, the methyl radical (CH3•) has one unpaired electron on carbon, which makes it unstable and reactive in organic reactions.
2. What is a free radical?
A free radical is a neutral species that contains at least one unpaired electron and can exist independently. Free radicals are formed by homolytic bond cleavage, where each bonded atom takes one electron. For example, under UV light: Cl2(g) → 2Cl•(g). The chlorine atoms formed are chlorine free radicals.
3. How are free radicals formed?
Free radicals are formed mainly by homolytic fission of covalent bonds. Common methods of radical formation include:
- Photolysis: Bond breaking by UV light (e.g., Cl2 → 2Cl•).
- Thermolysis: Bond breaking by heat.
- Redox reactions: Electron transfer processes that generate radicals.
These processes produce highly reactive intermediates in organic and inorganic chemistry.
4. Why are radicals highly reactive?
Radicals are highly reactive because they contain an unpaired electron, which makes them energetically unstable. Atoms and molecules tend to achieve stable electron configurations, so radicals quickly react with other substances to pair their unpaired electron. This high reactivity is why radicals are important in chain reactions and combustion processes.
5. What is the difference between a free radical and an ion?
The main difference is that a free radical has an unpaired electron but no charge, while an ion has a net electric charge due to loss or gain of electrons. For example:
- CH3• is a neutral methyl radical.
- Na+ is a positively charged sodium ion.
Radicals are defined by unpaired electrons, whereas ions are defined by their charge.
6. What are the types of free radicals in organic chemistry?
The main types of free radicals in organic chemistry are alkyl, aryl, and allylic radicals. These include:
- Alkyl radicals: e.g., CH3• (methyl radical)
- Aryl radicals: derived from aromatic rings
- Allylic radicals: unpaired electron adjacent to a double bond
Radical stability generally follows: tertiary > secondary > primary > methyl.
7. What is a radical chain reaction?
A radical chain reaction is a reaction that proceeds through a series of steps involving free radicals. It typically has three stages:
- Initiation: Formation of radicals (Cl2 → 2Cl•).
- Propagation: Radicals react to form products and new radicals.
- Termination: Two radicals combine to form a stable molecule.
An example is the chlorination of methane under UV light.
8. Can you give an example of a free radical reaction?
A common example of a free radical reaction is the chlorination of methane under UV light. The steps include:
- Initiation: Cl2(g) → 2Cl•(g)
- Propagation: Cl• + CH4 → HCl + CH3•
- Propagation: CH3• + Cl2 → CH3Cl + Cl•
The overall balanced reaction is CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g).
9. What is the stability order of free radicals?
The stability order of alkyl free radicals is tertiary (3°) > secondary (2°) > primary (1°) > methyl. This order is due to:
- Hyperconjugation from adjacent C–H bonds.
- Inductive effects from alkyl groups donating electron density.
Greater substitution increases radical stability in organic chemistry.
10. What are the uses and effects of free radicals?
Free radicals are important in both industrial chemistry and biological systems because they participate in chain reactions and oxidation processes. Key roles include:
- Polymerization reactions in plastics manufacturing.
- Combustion reactions in fuels.
- Biological oxidation, where excess radicals can cause oxidative stress.
While essential in many chemical processes, uncontrolled free radicals can damage cells and materials.
