Formation and Stability of Radicals
- Radicals are formed from spin-paired molecules through homolysis of weak bonds or electron transfer.
- Homolysis occurs under the addition of heat or light and can serve as an initiator for other radical reactions.
- Examples of homolysis reactions include the homolysis of halogens and dibenzoyl peroxide.
- Radicals can also be formed from other radicals through substitution, addition, and elimination reactions.
- Hydrogen abstraction and radical addition are two common processes for radical formation from other radicals.
- Organic radicals tend to dimerize, but some are stabilized by electronegativity, delocalization, and steric hindrance.
- The compound 2,2,6,6-tetramethylpiperidinyloxyl exemplifies stability due to these factors.
- Carbon is more stable than nitrogen and oxygen in terms of radical stability.
- The hybridization of carbon atoms also affects radical stability, with sp3-hybridized carbons forming the most stable radicals.
- Delocalization of electrons through resonance structures helps minimize instability in radicals.
- Electronegativity influences radical stability, with carbon being more stable than nitrogen and oxygen.
- Greater s-character in hybridized carbons correlates to higher electronegativity and less stable radicals.
- Delocalization of electrons across the structure of a radical helps spread electron deficiency, minimizing instability.
- Electron-donating groups, such as hydroxyl groups and ethers, stabilize radicals through delocalization effects.
- Molecular orbital theory can explain delocalization effects, with electron-donating groups interacting with the unpaired electron to form lower-energy bonding orbitals.
Radical Formation from Spin-Paired Molecules
- Homolysis breaks a covalent bond in a spin-paired molecule, producing two new radicals.
- Homolysis occurs under the addition of heat or light and depends on the stability of the compound.
- Homolysis of halogens serves as the driving force for radical halogenation reactions.
- Dibenzoyl peroxide can undergo homolysis to form benzoyloxy radicals, initiating many radical reactions.
Radical Formation from Other Radicals
- Hydrogen abstraction generates radicals, particularly from allylic and doubly allylic C-H bonds.
- Radical addition involves a radical adding to a spin-paired substrate, often an alkene.
- This addition generates a new radical, which can further react with other alkenes.
- Radical addition is the basis for radical polymerization, used in the production of plastics.
- Radical elimination is the reverse of radical addition, breaking down unstable radical compounds into spin-paired molecules and new radicals.
Occurrence and Roles of Radicals
- Radicals play a role in combustion reactions.
- Oxygen molecules are stable diradicals, but combustion requires overcoming the energy barrier between the spin-unpaired and spin-paired states.
- Combustion consists of radical chain reactions initiated by singlet radicals.
- Polymerization reactions are often initiated by radicals, leading to the formation of new radicals.
- Molecular dioxygen is the most common radical in the lower atmosphere, with photodissociation of source molecules producing other radicals.
- Radicals are involved in the intracellular killing of bacteria by phagocytic cells.
- Radicals play a role in cell signaling processes known as redox signaling.
- Radical attack of linoleic acid produces 13-hydroxyoctadecadienoic acids and 9-hydroxyoctadecadienoic acids.
- Radical attacks on arachidonic acid and docosahexaenoic acid produce a range of signaling products.
- Radicals may be involved in various diseases such as Parkinson's disease, deafness, schizophrenia, and Alzheimer's.
Reactive Oxygen Species and Cell Damage
- Reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radical are associated with cell damage.
- ROS form as by-products of normal oxygen metabolism and have roles in cell signaling.
- Excessive amounts of ROS can lead to cell injury and contribute to diseases like cancer, stroke, and diabetes.
- Reactions between radicals and DNA can result in mutations that affect the cell cycle and potentially lead to cancer.
- Radical-induced oxidation of cholesterol contributes to atherosclerosis.
In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.
A notable example of a radical is the hydroxyl radical (HO·), a molecule that has one unpaired electron on the oxygen atom. Two other examples are triplet oxygen and triplet carbene (꞉CH
2) which have two unpaired electrons.
Radicals may be generated in a number of ways, but typical methods involve redox reactions, Ionizing radiation, heat, electrical discharges, and electrolysis are known to produce radicals. Radicals are intermediates in many chemical reactions, more so than is apparent from the balanced equations.
Radicals are important in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes. A majority of natural products are generated by radical-generating enzymes. In living organisms, the radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood pressure. They also play a key role in the intermediary metabolism of various biological compounds. Such radicals can even be messengers in a process dubbed redox signaling. A radical may be trapped within a solvent cage or be otherwise bound.
