What is a free radical

As radical denotes atoms or molecules with at least one unpaired electron, which are usually particularly reactive. Radicals are represented with a 'point', e.g. B. Nitric oxide (NO), which symbolizes the free electron. If a radical contains several unpaired electrons, one speaks of Diradical (also Biradical), Tri-radical etc.

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Radicals play an important role in certain oxidation processes, in polymerizations and in some substitution reactions. Radicals from halogen molecules (e.g. chlorine, Cl-Cl) or di-tert.Obtain butyl peroxide by exposure to UV light.

Well-known examples

  • Dioxygen O2 - the oxygen molecule contains two unpaired electrons (biradical O-O; the Lewis formula O = O is incorrect) and is formed as a paramagnetic triplet in the magnetic field. However, the reactivity of this biradical is limited because the two unpaired electrons occur in different orbitals and do not have the same parallel spin (see also Pauli principle)
  • Nitric oxide NO - a molecule recognized as a messenger substance with an unpaired electron
  • Hydroxyl radicalOH - the most reactive and most important radical in the atmosphere (important for breaking down air pollutants)
  • Chlorine radicals Cl- are released from chlorofluorocarbons through exposure to light and are involved in the destruction of the ozone layer


Radicals arise in the body through overloading the combustion processes in mitochondria or through extreme external influences:

Radicals can enter the body when one of the above external influences breaks down molecules in the body into free radicals. Direct exposure to extreme heat is unimportant here because the other damage caused by fire is much greater. Radicals enter the body from eating or drinking food that has been exposed to ionizing radiation or from inhaling cigarette smoke. But some types of lymphocytes also produce radicals for defense against germs.

Radicals in Biology

Radicals, such as reactive oxygen species, play an important role in a large number of biological processes, but can also cause cell damage, which among other things can contribute to the development of cancer. The oxidation of various substances mediated by free radicals is also ascribed an important role in the development of arteriosclerosis, Alzheimer's disease, liver damage from alcohol and pulmonary emphysema from cigarette smoke. Among the intracellular signaling pathways that are activated by free radicals, the NF-κB signaling pathway is one of the most important.

Since protection against the effects of radicals is vital, the body has effective defense and repair mechanisms in the form of enzymes, hormones or other classes of substances that minimize damage. Antioxidants such as epigallocatechin gallate, superoxide dismutase, glutathione peroxidase, vitamin A, vitamin C, vitamin E, coenzyme Q10 and anthocyanins are involved in these defense mechanisms. Bilirubin and uric acid are also said to be able to neutralize certain free radicals. The hormone melatonin is also considered a radical scavenger against oxidative stress. The strongest known anti-oxidant, the hydride ion H-, plays an important role e.g. in the citric acid cycle and in many redox reactions of the metabolism.

Radicals play a role in the so-called “wear and tear” theories of the aging processes in the body, so that active substances against oxidative stress are being discussed as agents against aging. It is known that the cells of birds are far better able to withstand free radicals. In this context, however, it has also been shown that radicals are necessary to increase the body's defense capacity against free radicals; this process is called mitohormesis and is prevented by antioxidants, as recently shown by Michael Ristow and co-workers.

Historical meaning

When the theory prevailed at the beginning of the 19th century that all matter is made up of atoms (see John Dalton), the term was used by important chemists such as Lavoisier and Wöhler radical used to designate polyatomic molecules that behaved like single atoms in chemical reactions.[1] For example, the cyanate ion, which is made up of three atoms, often behaves like a chloride ion. On the other hand, an ammonium ion made up of five atoms often behaves like an ion of an alkali metal. This is why cyanate and ammonium ions, for example, were called radicals. See also:Radical theory.


  • Christoph Rüchardt: Radical. A chemical theory in a historical perspective. In: Meeting reports of the Heidelberg Academy of Sciences, Mathematical and Natural Science Class, year 1992, pp. 319–345 (full text)


  1. John Buckingham: Chasing the molecule. Stroud: Sutton, 2004., ISBN 0-7509-3345-3

Category: Chemical Bond