A radical is an uncharged atom or molecule that has an unpaired, or “free,” electron. Radicals are formed when a covalent bond in an atom or molecule is split apart and the remaining pieces retain one electron of the original shared pair. These reaction products, called free radicals, are highly reactive entities that can participate in a variety of reactions. In chemical notation, radicals are indicated by the chemical symbol of the parent compound followed by a dot (.).
Radicals are formed by the cleavage of an atom or molecule and can be grouped into three categories depending on their source. They can come from atoms, (e.g., H, F, Cl), inorganic molecules (e.g., such as OH, CN NO), or organic molecules (e.g., CH3 or C2 H5). In some areas of chemistry the term radical is used to indicate a reaction intermediate, which exists in nature for very short periods of time. However, the term is more commonly used to describe chemical species that persist long enough to react with other molecules to form more radicals in a cascading effect. This cascade effect can create sustained reactions in chemical and biological systems.
Avogadro and others postulated the existence of radicals early in the nineteenth century. Unfortunately, they did not fully understand how radicals could exist in nature and therefore they incorrectly proposed structures and mechanisms of formation. Due to this lack of understanding, at the end of the century it was fairly well established that radicals could not exist. Chemists did not have real evidence of the existence of radicals until the early twentieth century when Moses Gomberg discovered the triphenyl methyl radical. He proved this radical could exist with evidence based on reaction characteristics including color changes, molecular weight determination, and the specie’s reactivity toward iodine, oxygen and nitric oxide. Still, his discovery was initially met with skepticism from his peers. Additional evidence was uncovered by F. Paneth in 1929 when he found experimental proof that tetramethyllead (Pb(CH3)4) generates radicals as well. Eventually enough evidence was collected that convinced chemists that free radicals do exist and that they do participate in reactions.
Today it is known that radicals are formed when a stable molecule is disrupted and split into two portions, each with an unpaired electrons. A variety of effects can generate this disruption including thermal decomposition, electric or microwave discharge, photochemical decomposition, electrolysis, and gamma or x-ray exposure. The free radical process involves three steps: initiation where the free radical is formed; propagation in which the radicals react with other molecules to form additional free radicals; and termination where the radicals react with each other to form non-radical products.
Free radical reactions are useful in certain beneficial chemical processes, such as those used in the production of rubber and plastics. In these processes the free radicals react quickly to form long chains of chemicals known as polymers. However, in biological systems these reactions can cause harm. For example, radicals known as superoxides are formed when oxygen molecules are split apart. While these radicals can participate in the destruction of invading organisms by white blood cells, they can injure or kill cells when natural enzyme controls fail. Unchecked they can attack lipids, proteins, and nucleic acids. Therefore, free radicals in the body can contribute to cancer, heart attacks, strokes, and emphysema and may even play a role in arthritis and Alzheimer’s disease.
Free radicals were originally detected using simple analytical techniques that were based on the experiments by Paneth. Modern detection methods include a variety of spectral methods. For example, absorption spectroscopy is relatively simple way to detect radicals in the gas phase. A another method is based on spec-trometry. This technique works by measuring ionization energy of the free radicals and can be used to quantitatively measure radical concentration. The best technique is considered to be electron paramagnetic resonance spectroscopy which can characterize radicals in liquids, solids, or gases. Furthermore, this method is very sensitive and can be used to gain information on the structure of the detected radicals.
Free radicals can be quenched by antioxidants, chemicals that are capable of absorbing their extra electron. Antioxidants are free radical scavengers that can dampen the propagation reactions which create further radicals. They are important dietary supplement and find some use in topical skin care applications. Many of these radical scavengers, like vitamin E from green tea and polyphenols from red wine, are naturally occurring compounds.
Antioxidant— An organic chemical compound capable of retarding the deterioration of other chemicals that occurs because of contact with oxygen or other oxidizing agents.
Initiation— The chemical or physical event which causes the formation of free radicals.
Quenching— The process by which antioxidant materials can absorb free radicals, thus halting potentially damaging reactions.
Superoxide— A chemical compound containing the O2– ion.
Termination— The final step in a free radical chain reaction. Termination occurs when two radical combine to form a nonradical product.
Aruoma, O. I., M. Grootveld, and T. Bahorun, eds. Free Radicals in Biology and Medicine: From Inflammation to Biotechnology. Amsterdam: IOS Press, 2006.
Parsons, Andrew. An Introduction to Free-Radical Chemistry. Malden, MA: Blackwell Publishing, 2000.
“The Cellular Aging Process and Free Radicals,” Drug & Cosmetic Industry (February 1989): 22.