A free radical is an atom or specie that possesses one or more unpaired electrons which react easily with themselves in bond forming reactions (Bruice, 1998, p. 156). Elements such as halogen atoms and alkali metals are free radicals. They have unpaired electrons. Oxygen (O2) is commonly classed as a diradical. The reason for this is, is that two of its ten valence electrons possess identical spins and not all of its valence electrons are paired.
Free radicals, commonly known just radicals, have a rather interesting history in the field of organic chemistry (Giese, 1986, pp. 1-86). Radical chemistry dates back as far back as the 1900 when Gomberg investigated the formation and reactions of the triphenylmethyl radical (Pine, Hendrickson, Cram, & Hammond, 1980, pp. 864-880). When triphenylmethyl chloride was treated with silver metal, a yellow solution developed. Oxygen and iodine, very good reagents for reactions involving carbon free radicals, decolorized the solution very rapidly. Gomberg wanted to prepare hexaphenylethane by his method, and a material was recovered known as the hexaphenylethane structure.
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In the 1920's Paneth provided rather interesting experimental evidence for smaller organic free radicals. He showed that less stabilized alkyl radicals exist and measured the lifetime expectancy of these radicals in the gas phase. Paneths experiment involved passing tetramethyllead vapor through a glass tube which had a thin film of lead metal deposited at one point. Methyl free radicals were generated on heating the tetramethyllead, and re-formed the same compound with the lead deposited.
Organic synthesis with radicals began in 1937 when Hey and Waters described the phenylation of aromatic compounds by benzoyl peroxide as a radical reaction.
Kharasch explained that the anti-Markovnikov addition of hydrogen bromide to alkenes proceeds via a radical chain process.
In the following years to come, three chemists, Mayo, Walling and Lewis, discovered the rules of radical copolymerization reactions. Free radicals are now considered to be one of the common types of carbon-centered reactive intermediates along with carbanions and carbocations.
Formation of radicals
Free radicals are formed in two general ways:
Through homolytic cleavage of bonds: p 867 top
By reaction of molecules with other free radicals: p867 top
A free radical is easily formed when a covalent bond between entities is broken and one electron remains with each newly formed atom. This process requires a significant amount of energy. Splitting H2 into 2H has a Î”H0 of +435kJ/mol, and Cl2 into 2Cl has a Î”H0 of +243kJ/mol. This is known as the homolytic bond dissociation energy. Homolytic cleavage of sigma bonds can be made to occur with any compound if the temperature is sufficiently high. This thermal method is most effective when selective bonds within a molecule dissociate at temperatures below 200 0C. Free radicals formed through this method are most often the precursors for generation of other free radicals.
A common reaction pathway for radical formation is the abstraction of a hydrogen atom from a molecule by a free radical. A bromine atom adds to the alkene in the first step of the addition reaction. The initial adduct is a reactive alkyl free radical which abstracts a hydrogen atom from another molecule of HBr and generates a new bromine atom. This distinct process is known as propagation. A propagation reaction involves free radicals in which the total number of free radicals remains the same. It is a very exothermic reaction.
Free radicals may also be generated by various oxidation-reduction processes.
Reactivity and Stability of radicals
Most free radicals are very reactive and usually cannot be isolated. In some cases indirect methods must be used to be able to detect a radical. Activation energies for reactions between two free radicals are very low, often near zero, so that an actual reaction rate may depend on how rapidly the two species come together. Such reactions are diffusion controlled. The most common method for determining relative reactivities of free radicals is to measure the rates of hydrogen atom abstraction by the radicals from non-radical molecules. The position of a hydrogen atom in a molecule may influence the rate of radical abstraction. A hydrogen atom on a tertiary carbon is abstracted more rapidly than one on a secondary carbon, which in turn is abstracted more rapidly than one on a primary carbon.
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The size of a free radical may affect reactivity and consequently influence selectivity. Steric effects can play a significant role in the chemistry of free radicals. The contribution of steric factors to the reactivity of free radicals provides an interesting example of thermodynamic and kinetic considerations.
Solvent and substituent effects on free- radical reactivity are usually small because reactions commonly involve electrically neutral reactants and intermediates.
The stability of radicals depends on their structure. Higher substituted radicals are more stable than lower substituted radicals. The stability of radicals can be determined by the dissociation energies of the C-H bonds that must be homolytically cleaved to be able to obtain the radical. The radical stability is significantly lower in hydrocarbons than in carbocations.
A layer of ozone (O3) surrounding Earth shields it from harmful solar radiation. Ozone is formed in the atmosphere as a result of the interaction of molecular oxygen with very short wavelength ultraviolet light. In the stratosphere we find the most abundance of ozone. The stratospheric ozone layer acts as a filter to protect life on earth's surface from harmful ultraviolet radiation. This high energy, short-wavelength ultraviolet light can damage DNA in skin cells, causing pigmentation on the skin, mutations that can trigger skin cancer and various diseases. The ozone layer protects all life on earth, and without it, life would not have been possible.
Scientists have picked up, in earlier years, a precipitous drop in stratospheric ozone over Antarctica (see Figure ).
Total Ozone Mapping Spectrometer (TOMS)
Figure -Figure - Total Ozone Mapping Spectrometer (TOMS) image showing ozone depletion over Antarctic taken from NASA's NIMBUS-7 SATELLITE.
Strong circumstantial evidence implicates synthetic chlorofluorocarbons (CFCs) as a major cause of ozone depletion. CFCs are alkanes in which all the hydrogens have been replaced by fluorine and chlorine e.g. CFCl3 and CF2Cl2. These gases, commonly known as Freons, have been used extensively as cooling fluids in various home appliances, e.g. refrigerators and air conditioners, and in plastic foams, and in industrial cleaning solvents. They were once widely used as propellant in aerosol spray cans because of their nontoxic and nonflammable properties. Chlorofluorocarbons (CFCs) remain very stable in the atmosphere, but it is when they come into contact with the stratosphere that they generate chlorine radicals (see reaction below). A wavelength of ultraviolet light is what causes a chlorofluorocarbon its homolytic cleavage.
The chlorine radicals are the ozone-removing agent. They react with ozone to form chlorine monoxide radicals and molecular oxygen. The chlorine monoxide radicals react with ozone to regenerate chlorine radicals. By propagating these two steps repeatedly over and over, it destroys a molecule of ozone in each step (see reaction below). Scientists have calculated that each chlorine atom destroys 100Â 000 ozone molecules.
Chlorofluorocarbons have half-lives of 70 to 120 years, because of their stability. A frightening consequence of such long half-lives is that, if all the CFC production stops, it will take more than a century to recover from the damage caused by the CFCs already in the atmosphere.
Ultraviolet (UV) radiation
Ultraviolet light is electromagnetic radiation with wavelengths ranging from 180 to 400 nm (nanometers). Visible light has wavelengths ranging from 400 to 780 nm.
Ultraviolet radiation is defined as that portion of the electromagnetic spectrum between x rays and visible light, between 40 and 400 nm. http://students.umf.maine.edu/~delanonh/images/Protecting-Your-Eves-From-U.jpg
The UV spectrum is divided in different UV wavelength bands (see Figure ):
Vacuum UV (40-190 nm)
Far UV (190-220 nm)
UVC (220-290 nm)
UVB (290-320 nm)
UVA (320-400 nm)
Figure -Different UV wavelength bandsThe sun is the primary natural source of UV radiation. Unique hazards apply to different artificial sources, e.g. tanning booths and fluorescent sources etc., depending on the wavelength range emitted.
UVC is completely absorbed in the atmosphere and there for can't be observed in nature, meaning that UVC rays never reaches the earth's surface. Far UV and Vacuum UV can also not be observed in nature. UVC radiation has the ability to kill bacteria there for the Germicidal lamp was designed to emit this UVC radiation. In humans, UVC is absorbed in the outer layers of the epidermis.
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Figure -Sunburn is the damage that a UV-B photon can induce in DNAUVB, lower energy level than UVC, is the most destructive form of UV radiation because it carries enough energy through the atmosphere to cause photochemical damage to cellular DNA (see Figure ). UVB is crucial for human beings as it provides vitamin D, but however, it can cause harmful effects e.g. skin cancer. Individuals working outdoors are at great risk of UVB effects. Our ozone layer is slowly deteriorating which means that less and less UVB is blocked, causing an increase in the prevalence of skin cancer.
UVA is the most common encountered type of UV light, UVA rays make up 90% of UV light that reaches earth. UVA rays don't have enough energy to break apart the bonds of the ozone, so UVA radiation passes the earth's atmosphere unfiltered. The dark pigment effect (tanning) we see on humans is the UVA that moved almost unfiltered from the sun to the earth's surface. UVA is also needed by humans for the synthesis of vitamin D. When a human is overexposed to UVA it can be associated with suppression of the immune system. UVA is a major contributor to skin damage. UVA light is commonly referred to as black light. Most phototherapy and tanning booths use UVA lamps.
Effects of excessive exposure of sunlight
The effects of sun exposure are cumulative, meaning that over time the damage from the sun accumulates. Too much sun over time can result in the following:
Skin cancer is a dark/pigmented growth on the skin which can have disastrous causes for a human being. Excessive sun exposure over a period of time appears to be the most significant environmental factor in the development of skin cancer. This disease affects the areas that received the most sun exposure, including the face, mouth, ears, neck and hands. There are two types of skin cancer, Non-melanoma and melanoma skin cancer.
Sunburn is a burn to living tissue such as skin produced by overexposure to UV radiation. The consequence of this burn is inflammation of the skin. Excessive acute sun exposure may result in sunburn, which is characterized by redness of the skin that may be accompanied by dryness, swelling, headaches, blistering, nausea and fever.
Photosensitivity (sun sensitivity)
Too much exposure to the sun can sometimes result in an allergic reaction. A bump on the skin can be a sign of a sun allergy. Certain cosmetics, e.g. in some cases sunscreens, can make individuals sensitive to sun exposure.
Excessive exposure to UV radiation may burn the cornea and increase the chance of developing a cataract.
Figure -Mechanism for the effect of ultraviolet radiation on skin.Photoaginghttp://img.medscape.com/fullsize/migrated/576/956/dn576956.fig1.gif
Over time, excessive sun exposure can lead to skin changes known as premature aging, or better known as photoaging (see Figure ). Individuals who sunbathe a lot can often show signs of photoaging prior to the age of 30 years. Photoaging symptoms may include the following: Rough skin, deep wrinkles, freckles on the facial area, very red veins on the body particularly the nose and uneven pigmentation.
Sunscreens can work in multiple ways. It either reflect oncoming UV rays away (physical sunscreen), or it absorbs the UV rays (chemical sunscreen) (see Error: Reference source not found). Both methods work well, but old chemical sunscreens have a tendency to only block UVB rays, but these days new chemical filters protects against both UVA and UVB rays.
Table -Comparison of physical sunscreens vs. chemical sunscreens
Physical sunscreens, commonly known as sunblock, contain ingredients like titanium dioxide and or zinc oxide which physically block the sun rays, UVR. Sunblocks provides a broad protection against both UVB and UVA light. Zinc oxide blocks both UVA and UVB rays, covering the whole spectrum, while titanium dioxide only gives protection to UVB rays. These ingredients work very well, but they have a tendency to leave a white cast behind on the skin.
Chemical sunscreens contain synthetic chemicals which absorb UV rays, there for absorbing radiation. These chemicals mostly only absorb UVB rays but usually contain small amounts of ingredients to block UVA rays. Chemical filters include a wide range of different ingredients which includes the following: avobenzone, oxybenzone, Tinosorb M, Tinosorb S, Mexoryl SX and Mexoryl XL.
Mexoryl and Tinosorb are new filters that deliver UVA protection. Avobenzone can also block UVA rays, but it degrades when exposed to sun light.
Protection against electromagnetic radiation
Electromagnetic radiation is radiant energy that displays both the properties of particles and the properties of waves. The different types of electromagnetic radiation constitute the electromagnetic spectrum. Electromagnetic radiation with the greatest energy is known as gamma rays. These rays are emitted from the nuclei of certain radioactive elements, and can severely damage biological organisms because of their high energy state.
Electromagnetic radiation is present in various electrical appliances, but radiates the strongest electromagnetic wave from:
Prolonged exposure to this electromagnetic radiation could cause:
Immune system disorders
There are a number of new products that can protect human beings from the harmful radiation produced by the sun, or different appliances mentioned above. These products range from:
Verandah - to protect our homes from sunlight http://www.sunsafe.com.au/images/image_07.jpg
(See Figure )
Sunscreens - use sunscreens with SPF of 15 or higher
Clothing which confers an SPF of 30
Figure - Verandah
All these products can protect you from harmful radiation, but the moral is that one must not encounter the sunlight for long periods. Although sunlight produces vitamin D, which is necessary for the body, a person has to limit the time of staying outside in the sun. These products work, but it depends on the user to utilize them in the correct manner.