Free Radicals



The triphenylmethyl radical is a persistent radical and the first radical ever described in organic chemistry, by Dr. Moses Gomberg.


Free radical is a chemistry term that describes an atomic or molecular species with unpaired electrons on an otherwise open shell configuration. Dr. Moses Gomberg (1866-1947) was the founder of radical chemistry


When exposed to 1) ___ the triphenylmethyl radical rapidly oxidizes to the peroxide (above, Scheme 2) and the color of the solution changes from yellow to colorless. Likewise, the radical reacts with iodine to triphenylmethyl iodide.




The radical was discovered by Moses Gomberg in 1900, when he tried to prepare hexaphenylethane from triphenylmethyl chloride and zinc in benzene in a Wurtz reaction and found that the product, based on its behavior towards iodine and oxygen, was far more reactive than anticipated.


A radical (more precisely, a free radical) that has unpaired valence electrons or an open electron shell, may, therefore, be seen as having one or more “dangling“ covalent bonds. With some exceptions, these “dangling“ 2) ___ make free radicals highly chemically reactive towards other substances, or even towards themselves: their molecules will often spontaneously dimerize or polymerize if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum. A notable example of a free radical is the hydroxyl radical (HO?), a molecule that is one hydrogen atom short of a water molecule and thus has one bond “dangling“ from the oxygen. Two other examples are the carbene molecule (:CH2), which has two dangling bonds; and the superoxide anion (?O-2), the oxygen molecule O2 with one extra electron, which has one dangling bond.


Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger 3) ___. The latter can be affected by any process that puts enough energy into the parent molecule, such as ionizing radiation, heat, electrical discharges, electrolysis, and chemical reactions. Indeed, radicals are intermediate stages in many chemical reactions. Free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes.


In living organisms, the free radicals superoxide and nitric oxide and their reaction products regulate many processes, such as control of vascular tone and thus blood 4) ___. 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. Until late in the 20th century the word “radical“ was used in chemistry to indicate any connected group of atoms, such as a methyl group or a carboxyl, whether it was part of a larger molecule or a molecule on its own. The qualifier “free“ was then needed to specify the unbound case. Following recent nomenclature revisions, a part of a larger molecule is now called a functional group or substituent, and “radical“ now implies “free“. However, the old nomenclature may still occur in the literature.


Free radicals play an important role in a number of biological processes. Many of these are necessary for life, such as the intracellular killing of 5) ___ by phagocytic cells such as granulocytes and macrophages. Researchers have also implicated free radicals in certain cell signalling processes, known as redox signaling. The two most important oxygen-centered free radicals are superoxide and hydroxyl radical. They derive from molecular 6) ___ under reducing conditions. However, because of their reactivity, these same free radicals can participate in unwanted side reactions resulting in cell damage. Excessive amounts of these free radicals can lead to cell injury and death, which may contribute to many diseases such as cancer, stroke, 7) ___ infarction, diabetes and major disorders.


Many forms of cancer are thought to be the result of reactions between free 8) ___ and DNA, potentially resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Some of the symptoms of aging such as atherosclerosis are also attributed to free-radical induced oxidation of cholesterol to 7-ketocholesterol. In addition free radicals contribute to alcohol-induced liver damage, perhaps more than 9) ___ itself. Free radicals produced by cigarette smoke are implicated in inactivation of alpha 1-antitrypsin in the lung. This process promotes the development of emphysema.


Free radicals may also be involved in Parkinson’s disease, senile and drug-induced deafness, schizophrenia, and Alzheimer’s. The classic free-radical syndrome, the iron-storage disease hemochromatosis, is typically associated with a constellation of free-radical-related symptoms including movement disorder, psychosis, skin pigmentary melanin abnormalities, deafness, arthritis, and diabetes mellitus. The free-radical theory of aging proposes that free radicals underlie the aging process itself. Similarly, the process of mitohormesis suggests that repeated exposure to free radicals may extend life span.


Because free radicals are necessary for life, the body has a number of mechanisms to minimize free-radical-induced damage and to repair damage that occurs, such as the enzymes superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. In addition, antioxidants play a key role in these defense mechanisms. These are often the three vitamins, vitamin A, vitamin C and vitamin E and polyphenol antioxidants. Furthermore, there is good evidence indicating that bilirubin and uric acid can act as antioxidants to help neutralize certain free radicals. Bilirubin comes from the breakdown of red blood cells’ contents, while uric acid is a breakdown product of purines. Too much bilirubin, though, can lead to jaundice, which could eventually damage the central nervous system, while too much uric acid causes 10) ___.


ANSWERS: 1) air; 2) bonds; 3) molecules; 4) pressure; 5) bacteria; 6) oxygen; 7) myocardial; 8) radicals; 9) alcohol; 10) gout



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