Combustion torch showing excited molecular radical band emission and
Swan bands A familiar radical reaction is
combustion. The
oxygen molecule is a stable
diradical, best represented by •O–O•. Because
spins of the electrons are parallel, this molecule is stable. While the
ground state of oxygen is this unreactive spin-unpaired (
triplet) diradical, an extremely reactive spin-paired (
singlet) state is available. For combustion to occur, the
energy barrier between these must be overcome. This barrier can be overcome by heat, requiring high temperatures. The triplet-singlet transition is also "
forbidden". This presents an additional barrier to the reaction. It also means molecular oxygen is relatively unreactive at room temperature except in the presence of a catalytic heavy atom such as iron or copper. Combustion consists of various radical chain reactions that the singlet radical can initiate. The
flammability of a given material strongly depends on the concentration of radicals that must exist, or be obtained - as in laboratory conditions, before initiation and propagation reactions dominate leading to
combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out. As indicated, promotion of propagation or termination reactions alters flammability. For example, because lead itself deactivates radicals in the gasoline-air mixture,
tetraethyl lead was once commonly added to gasoline. This prevents the combustion from initiating in an uncontrolled manner or in unburnt residues (
engine knocking) or premature ignition (
preignition). When a hydrocarbon is burned, a large number of different oxygen radicals are involved. Initially,
hydroperoxyl radical (HOO•) are formed. These then react further to give
organic hydroperoxides that break up into
hydroxyl radicals (HO•).
Polymerization Many
polymerization reactions are initiated by radicals. Polymerization involves an initial radical adding to non-radical (usually an alkene) to give new radicals. This process is the basis of the
radical chain reaction. The art of polymerization entails the method by which the initiating radical is introduced. For example,
methyl methacrylate (MMA) can be polymerized to produce
Poly(methyl methacrylate) (PMMA – Plexiglas or Perspex) via a repeating series of
radical addition steps: Newer radical polymerization methods are known as
living radical polymerization. Variants include reversible addition-fragmentation chain transfer (
RAFT) and atom transfer radical polymerization (
ATRP). Being a prevalent radical, O2 reacts with many organic compounds to generate radicals together with the
hydroperoxide radical.
Drying oils and alkyd paints harden due to radical crosslinking initiated by oxygen from the atmosphere.
Atmospheric radicals The most common radical in the lower atmosphere is molecular dioxygen.
Photodissociation of source molecules produces other radicals. In the lower atmosphere, important radical are produced by the photodissociation of
nitrogen dioxide to an oxygen atom and
nitric oxide (see below), which plays a key role in
smog formation—and the photodissociation of ozone to give the excited oxygen atom O(1D) (see below). The net and return reactions are also shown ( and , respectively). In the upper atmosphere, the photodissociation of normally unreactive
chlorofluorocarbons (CFCs) by solar
ultraviolet radiation is an important source of radicals (see eq. 1 below). These reactions give the
chlorine radical, Cl•, which catalyzes the conversion of
ozone to O2, thus facilitating
ozone depletion (– below). {{NumBlk|:|Cl^\bullet {}+ O3 -> ClO^\bullet {}+ O2|}} {{NumBlk|:|O {}+ ClO^\bullet -> Cl^\bullet {}+ O2|}} Such reactions cause the depletion of the
ozone layer, especially since the chlorine radical is free to engage in another reaction chain; consequently, the use of chlorofluorocarbons as
refrigerants has been restricted.
In biology , a common biosynthetic intermediate , which constitutes about 30% of plant matter. It is formed by radical reactions. Radicals play important roles in biology. Many of these are necessary for life, such as the intracellular killing of bacteria by phagocytic cells such as
granulocytes and
macrophages. Radicals are involved in
cell signalling processes, known as
redox signaling. For example, radical attack of
linoleic acid produces a series of
13-hydroxyoctadecadienoic acids and
9-hydroxyoctadecadienoic acids, which may act to regulate localized tissue inflammatory and/or healing responses, pain perception, and the proliferation of malignant cells. Radical attacks on arachidonic acid and docosahexaenoic acid produce a similar but broader array of signaling products. 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 radicals underlie the
aging process itself. Similarly, the process of
mitohormesis suggests that repeated exposure to radicals may extend life span. Because radicals are necessary for life, the body has a number of mechanisms to minimize 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 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
gout.
Reactive oxygen species Reactive oxygen species or ROS are species such as
superoxide,
hydrogen peroxide, and
hydroxyl radical, commonly associated with cell damage. ROS form as a natural by-product of the normal metabolism of
oxygen and have important roles in cell signaling. Two important oxygen-centered radicals are
superoxide and
hydroxyl radical. They derive from molecular oxygen under reducing conditions. However, because of their reactivity, these same radicals can participate in unwanted side reactions resulting in cell damage. Excessive amounts of these radicals can lead to cell injury and
death, which may contribute to many diseases such as
cancer,
stroke,
myocardial infarction,
diabetes and major disorders. Many forms of
cancer are thought to be the result of reactions between radicals 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 radical induced oxidation of cholesterol to 7-ketocholesterol. In addition, radicals contribute to
alcohol-induced
liver damage. Radicals produced by
cigarette smoke are implicated in inactivation of
alpha 1-antitrypsin in the
lung. This process promotes the development of
emphysema.
Oxybenzone has been found to form radicals in sunlight, and therefore may be associated with cell damage as well. This only occurred when it was combined with other ingredients commonly found in sunscreens, like
titanium oxide and
octyl methoxycinnamate. ROS attack the
polyunsaturated fatty acid,
linoleic acid, to form a series of
13-hydroxyoctadecadienoic acid and
9-hydroxyoctadecadienoic acid products that serve as signaling molecules that may trigger responses that counter the tissue injury which caused their formation. ROS attacks other polyunsaturated fatty acids, e.g.
arachidonic acid and
docosahexaenoic acid, to produce a similar series of signaling products. Reactive oxygen species are also used in controlled reactions involving singlet dioxygen {}^{1}\mathrm{O}_2 known as type II
photooxygenation reactions after
Dexter energy transfer (
triplet-triplet annihilation) from natural triplet dioxygen {}^{3}\mathrm{O}_2 and triplet excited state of a photosensitizer. Typical chemical transformations with this singlet dioxygen species involve, among others, conversion of cellulosic biowaste into new
poylmethine dyes. ==Depiction in chemical reactions==