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 stabilised 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-hybridised 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 hybridised 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.