Chlorine is a chemical element that belongs to the halogen family of the periodic table of elements. It is available in huge concentrations on earth, but due to its high reactivity, it usually occurs in the compound form with other elements. The most familiar example of a chlorine compound is common salt or sodium chloride (NaCl). The elemental chlorine is highly electronegative, and therefore, highly reactive. This property accounts for its toxic nature, which poses many difficulties to work with it in its elemental form. Chlorine has a wide range of applications ranging from disinfectant to bomb manufacturing. The biggest repository of chlorine on planet earth is the oceans in which it is found in the form of a compound, i.e. sodium chloride, which makes it easily available and employable through several industries.
Discovery and Naming
Carl Wilhem Scheele
In 1774, Swedish pharmacist Carl Wilhem Scheele released a few drops of hydrochloric acid onto a piece of manganese dioxide and observed a greenish-yellow gas evolving from that chunk of manganese dioxide. At the time, he had no idea that he had just discovered chlorine. Until an English chemist, Sir Humphrey Davy, recognized that the gas produced in the reaction was elemental chlorine, people were convinced that the gas was a compound of oxygen. Davy gave the element its name based on the Greek word “khloros” in reference to its greenish-yellow color. In 1810, he suggested the name “chloric gas” or “chlorine.” Since then, it has become one of the most effective and economical germ-killers. Chlorine disinfectants destroy and deactivate a wide range of dangerous germs in homes, hospitals, swimming pools, hotels, restaurants, and other public places. Chlorine’s powerful disinfectant qualities come from its ability to bond with and destroy the outer surfaces of bacteria and viruses. In fact, it was first used in 1847 as a germicide to prevent the spread of “childbed fever” in the maternity wards of Vienna General Hospital in Austria.
Sir Humphrey Davy
Isotopes of Chlorine
Chlorine has 25 isotopes with mass numbers ranging from _{ }^{ 28 }{ Cl } to _{ }^{ 52 }{ Cl } out of which only two are stable, _{ }^{ 35 }{ Cl } and _{ }^{ 37 }{ Cl }. These isotopes are usually born in stars by the process of oxygen and silicon burning. Among all its radioactive isotopes, _{ }^{ 36 }{ Cl } has the longest half-life of 301000 years, while all other radioactive isotopes have a half-life of less than an hour. _{ }^{ 36 }{ Cl } decays further to a stable isotope of sulphur by the process of electron capture. It can be produced in the laboratory by neutron activation; however, _{ }^{ 36 }{ Cl } prepared in the laboratory can have a half-life ranging from {3.0 × 10 }^{ 5 }{ years} to 37.2 minutes.
Properties of Chlorine
Physical Properties
Chlorine Gas Leak
Chlorine belongs to the halogen group of p-block elements of the periodic table with atomic no. 17 and atomic mass 35.43 u. Diatomic chlorine ({ Cl }_{ 2 }) is a yellow-green poisonous gas at room temperature, which is 2.5 times heavier than air and has a pungent odor similar to bleach even at very low concentrations. It liquefies at the temperature of -34.05ºC (boiling point) and forms yellow-colored crystals on further lowering the temperature to -100ºC (melting point). It has a density of 0.0028985 gram per cubic centimeter at 20ºC. Chlorine is more soluble in water than other aqueous solutions. The yellow color of chlorine corresponds to the electron transition between the highest occupied antibonding molecular orbital and the lowest vacant antibonding molecular orbital. Therefore, when we lower the temperature up to -195ºC, chlorine appears almost colorless.
Chemical Properties
Chlorine has an electronic configuration of [Ne] { 3s }^{ 2 }{ 3p }^{ 5 }. It is the second most reactive non-metallic element after fluorine in the periodic table. Like other elements of the halogen family, chlorine also requires one electron to achieve its optimal electronic configuration, thereby forming chloride ion. Chlorine can take part in various compound formation by sharing one to seven electrons of its valence shell electrons. Therefore, it can exist in diverse oxidation states, for example, in the oxides of chlorine, hypochlorites have (+ 1) oxidation state, chlorates have (+ 5) oxidation state, and perchlorates have (+ 7) oxidation state. Chlorine prefers to make polar ionic bonds with most of the elements.
When it comes to the compound formation with other halogen compounds, chlorine shows some unusual characteristics. While forming a compound with fluorine, such as ClF and {ClF}_{3}, it shows ionic character by itself being anion. Whereas, while forming compounds with iodine, such as {ICl}_{3} and ICl, it shows some ionic character to the bond, acting as a cation. With other halogen elements, it takes parts in covalent bonding for compound formation. Chlorine is very reactive, combining directly with most elements but only indirectly with nitrogen, oxygen, and carbon. Excess chlorine in the presence of ammonia salts forms the very explosive nitrogen trichloride, {NCl}_{ 3 }. Hypochlorites react with ammonia to produce the chloramines {NH}_{ 2 }{ Cl } and {NHCl}_{ 2 }.
3 {Cl}_{ 2 }+4 {NH}_{ 3 }→ {NCl}_{ 3 }+3 {NH}_{ 4 }{ Cl }
Compounds of Chlorine
Chemists began experimenting with chlorine and chlorine compounds in the 18th century. They learned that chlorine has an extraordinary ability to extend a chemical bridge between various elements and compounds that would not otherwise react with each other. Chlorine is one of the most abundant, naturally occurring chemical elements. It plays a significant role in the manufacture of thousands of products that we depend upon every day. Let’s discuss a few of its compounds.
1. Hydrochloric acid
Hydrochloric acid (HCl) is a colorless, corrosive, and strong mineral acid with many industrial uses. When it reacts with an organic base, it forms a hydrochloride salt. It was historically produced from rock salt and green vitriol, and later from treating the chemically similar common salt (NaCl) with concentrated sulfuric acid( { H }_{ 2 }{ SO }_{ 4 }).
NaCl + { H }_{ 2 }{ SO }_{ 4 } ⟶ { NaHSO }_{ 4 } + HCl
The above reaction takes place at 150ºC. On increasing the temperature further up to 500ºC-600ºC, sodium chloride and sodium bisulphate also react to form hydrochloric acid.
NaCl + { NaHSO }_{ 4 } ⟶ { Na }_{ 2 }{SO }_{ 4 } + HCl
Hydrochloric acid is also produced naturally by our stomach to digest food. It is used in the metal industry to process steel, the material of choice for suspension bridges, cars, and trucks. Hydrochloric acid is also used in the production of batteries, photoflash bulbs, and fireworks. It is also used to process sugar and make gelatin. However, hydrochloric acid is more acidic than hydrofluoric acid due to the presence of stronger hydrogen bonding in HF that does not break easily, and hence, does not form ions.
2. Common Salt (NaCl)
Common salt is the most abundant repository of chlorine on earth. A large amount of chlorine is produced with the help of mineral salts found in the ocean water. Electricity is used to electro-chemically split NaCl, releasing Cl for its many chemical uses. Chemical engineers design systems to make chlorine gas bubble out of salty-electrified water. The gas is captured and cooled down so that it can be liquefied. NaCl is essential to life on earth. It is a necessary ingredient in the diets of people and animals. It plays a key role in the manufacture of thousands of products that we depend on every day, including volleyballs, computers, cars, pool chemicals, medicines, and cosmetics.
3. Chlorine organic compounds
Chlorine usually forms covalent bonds with carbon. It generally goes under substitution or addition reactions with hydrocarbons. In saturated hydrocarbons, chlorine replaces hydrogen, either entirely or partially, to form chlorinated hydrocarbons and hydrogen chloride. Chlorine goes under free-radical electrophilic substitution with alkanes and aryl alkanes. Many of chloro-organic compounds can be found in biomolecules, and the majority of chloro-hydrocarbons such as chloromethane can be found in the atmosphere due to the biological decomposition of other organic molecules.
Friedel Crafts Mechanism
Uses of Chlorine
1. Water Treatment
Chlorine is widely used to keep the swimming pools clean. As it can not be used in its elemental form for this purpose, people make use of two of its compounds, sodium hypochlorite (bleach) and calcium hypochlorite. Either of these two chemicals, when mixed with water, forms a weak acid called hypochlorous acid that kills bacteria like salmonella and E. coli; however, too much exposure to hypochlorous acid brings harm to the human body; therefore, it is necessary to frequently monitor the pH levels of swimming pools.
Swimming Pool Cleaning Kit
Chorine does not only help in keeping the water clean, but it also helps in wastewater treatment. Wastewater disinfection is applied to protect humans against exposure to waterborne pathogenic microorganisms. It also helps in preventing aquatic pollution by preventing the accumulation of toxic microorganisms in fish, shellfish, and other aquatic organisms. Chlorine kills bacteria by attacking the lipids in their cell walls and destroying the pathogen’s structure. When this happens, the chemicals can get inside the bacteria and kill their enzymes, ultimately destroying the bacteria’s mechanisms to function properly. However, it can also cause the formation of mutagenic/carcinogenic and toxic by-products that are potentially harmful to human and aquatic organisms.
2. Automotive Industry
The major demand for the chlorine market around the world is driven by the automotive industry. It is used in nylon manufacturing, which is further used in manufacturing several parts of a car, such as safety belts, airbags, bumpers, mats, dashboards, coolants, and anti-freezers. Chlorine is also used in boat hulls, nets, and fishing lines as well as corrosion-resistant paint. Even the high-grade lubrication oils, gasoline additives, electrical components, bus windows, and brake fluids are made up using chlorine.
3. Health Care
The tendency of chlorine to act as a disinfectant, and being present in such abundant volumes, makes it a preferable choice for many industries including health care. Chlorine products are effective in reducing many of today’s infection risks. Both liquid (sodium hypochlorite solution, e.g., chlorine bleach) and solid (calcium hypochlorite) are available in hospitals, and when used according to product specifications, they do not stain or leave toxic residues. They are inexpensive and non-toxic chemicals at concentrations that disinfect. In hospitals, chlorine is primarily used in making sterile packaging, surgical instruments, laboratory reagents, and even prescription eyewear.
Chlorine chemistry plays a supporting role in drug synthesis. For example, a hydrochloric acid solution may be used to control pH during the manufacturing process, or chlorinated solvents may be used as separating or purification agents. Frequently, large and complex drug molecules containing chlorine are synthesized from smaller “intermediate” molecular fragments. It is widely used in the manufacture of several drugs, such as antibiotics, antihistamines, pain relievers, aspirin, and even used in cancer treatments.
4. Electronics Industry
Chlorine’s tendency to form polyvinyl chloride, which has high ignition resistance, makes it useful in making wire coatings for various electrical equipment, such as in computer cables. It is also used in the manufacture of computer disks and other semiconductor devices.
5. Metal Industry
Chlorine is a major element used in various metallurgical processes for the extraction of nonferrous elements from their ores and concentrations. The use of chlorine in metal extraction processes through pyro and hydrometallurgy routes has gained considerable attention in recent years. Chlorine compounds such as hypochlorous acid, chlorine gas, and several alkyls and alkaline compounds of chlorine have proved very useful in the extraction of many silicate ores, oxides, and several sulfides. Because of their high reactivity at a moderate temperature, selectivity in the chlorination of the desired metal values, and their easy availability with low cost, these chlorinating agents are being widely used in the ores and minerals for preparation of pure metal chlorides and the recovery of metal values from the scrap and other wastes.
Chlorination Plants in Various Metal Industries
6. Construction
Chlorine also plays an essential role in the construction of buildings and making our homes beautiful. From the very fundamental necessities, such as nuts and bolts to fancy decors, it plays an important role in their manufacturing. Chlorine can be found in most of the household items, such as carpets, PVC pipes, floorings, paints, coatings, wire insulations, etc. It helps to protect you from the elements with durable siding, and lets you enjoy the outdoors with products like garden hoses and lawn furniture.
7. Defence
Chlorine plays a significant role in the safety of soldiers and security personnel as various protective equipment such as bullet-resistant vests and tools for intelligence including surveillance cameras, all use chlorine chemistry for their manufacture. Also, medical equipment such as blood bags and tubing are made with chlorine and are crucial in protecting the lives of soldiers and police officers. It is also used in the manufacture of other protective gears, such as helmets, parachutes, water repellant fibers, and shatter-resistant glasses. Chlorine also helps in guarding our skies by playing an important role in the manufacture of titanium aircraft, jet engines, and missiles.
8. Miscellaneous Uses
Chlorine is also used in the manufacture of various equipment that makes extreme sports possible. Various types of equipment used for such recreational activities make use of chlorine for their reliability. For instance, nylon ropes made for rock climbing and raft manufacturing are made up of chlorine-based polymers. It is also used in the manufacture of neoprene wetsuits, inflatable rafts and even jet skis. Most essential equipment required for mountaineering such as sleeping bags and jackets has chlorine as a major manufacturing ingredient.
Health Aspects
Although chlorine can be found almost anywhere around us, its uncontrolled exposure can result in severe health conditions. For instance, low concentrations of chlorine can cause itching and burning of the eyes, nose, throat, and respiratory tract. At high concentrations, chlorine is a respiratory poison. Irritant effects become severe and may be accompanied by tearing of the eyes, headache, coughing, choking, chest pain, shortness of breath, dizziness, nausea, vomiting, unconsciousness, and even death. Bronchitis and accumulation of fluid in the lungs (chemical pneumonia) may occur hours after exposure to high levels of chlorine. Chlorine in its liquid, as well as vapor form, may cause irritation, burns, and blisters when we come in its contact. Ingestion can cause nausea and severe burns of the mouth, esophagus, and stomach. Prolonged or repeated overexposure may result in any or all of the effects reported for acute exposure (including pulmonary function effects).
