# chlorine

properties
[ Ne ] 3 s 2 3 p 5
17 cl
General
Name , symbol , atomic number Chlorine, Cl, 17
Element category Halogens
Group , period , block 17 , 3 , p
Appearance yellowish green
CAS number 7782-50-5
EC number 231-959-5
ECHA InfoCard 100.029.053
Mass fraction of the earth's envelope 0.19%
Atomic
Atomic mass 35.45 (35.446-35.457) u
Atomic radius (calculated) 100 (79) pm
Van der Waals radius 175 pm
Electron configuration [ Ne ] 3 s 2 3 p 5
1. Ionization energy 12.967 632 (Fig 16) eV 1 251.19 kJ / mol
2. Ionization energy 23.81364 (12) eV2 297.67 kJ / mol
3. Ionization energy 39.80 (11) eV3 840 kJ / mol
4. Ionization energy 53.24 (12) eV5 137 kJ / mol
5. Ionization energy 67.68 (10) eV6 530 kJ / mol
6. Ionization energy 96.94 (4) eV9 353 kJ / mol
7. Ionization energy 114.2013 (6) eV11 019 kJ / mol
Physically
Physical state gaseous (Cl 2 )
Crystal structure orthorhombic
density 3.215 kg m −3 at 273 K.
magnetism diamagnetic ( Χ m = −2.3 10 −8 )
Melting point 171.6 K (−101.5 ° C)
boiling point 238.5 K (−34.6 ° C)
Molar volume (solid) 17.39 10 −6 m 3 mol −1
Heat of evaporation 20.4 kJ mol −1
Heat of fusion 3.2 kJ mol −1
Vapor pressure 6.78 · 10 5 Pa at 293 K.
Speed ​​of sound 206 m · s −1
Specific heat capacity 480 J kg −1 K −1
Thermal conductivity 0.0089 W m −1 K −1
Chemically
Oxidation states ± 1 , 3, 4, 5, 6, 7
Normal potential 1.36 V (Cl + e - → Cl - )
Electronegativity 3.16 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
35 cl 75.77  % Stable
36 cl in traces 301,000 a β - 0.709 36 ares
ε 1.142 36 p
37 Cl 24.23% Stable
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
35 cl 3/2 0+2.624 · 10 7 4.72 · 10 -3 019.63
37 Cl 3/2 0+2.184 · 10 7 2.72 · 10 −3 016.34
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary

danger

H and P phrases H: 270-280-330-315-319-335-400
EUH: 071
P: 260-220-280-244-273-304 + 340-305 + 351 + 338 332 + 313-370 + 376-302 + 352-315-405-403
MAK
• DFG : 1.5 mg m −3
• Switzerland: 0.5 ml m −3 or 1.5 mg m −3
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Chlorine is a chemical element with the symbol Cl and the atomic number 17. In the periodic table of the elements it is in the 7th main group and, together with fluorine , bromine , iodine , astatine and Tenness, belongs to the 17th  IUPAC group , the halogens . Elemental chlorine is in gaseous form under normal conditions in the form of the diatomic molecule Cl 2 . It is one of the most reactive elements and reacts with almost all other elements and many compounds. The high reactivity also causes the toxicity of the elemental chlorine. The name of the element is derived from the ancient Greek χλωρός chlōrós , German 'light green' . This name was chosen after the typical yellow-green color of the chlorine gas.

In nature, chlorine does not occur in elemental form, but only in bound form in various compounds. The most important compounds are the chlorides , in which chlorine occurs in the form of the anion Cl - . The best-known chloride is sodium chloride , often referred to as table salt or salt for short . Chloride is a common component of sea ​​water and has important biological functions, especially in controlling the water balance in the body.

The chlorine, which is obtained almost exclusively through electrolysis , is mainly used for the synthesis of chlorine-containing compounds such as vinyl chloride , a starting product for the production of the plastic PVC .

## history

Carl Wilhelm Scheele

Elemental chlorine was first presented by Carl Wilhelm Scheele in 1774 . He made hydrochloric acid react with manganese dioxide. In doing so, he did not realize that the resulting product was a previously undiscovered element. Instead, most chemists like Antoine Laurent de Lavoisier assumed that the substance was "oxygenated muric acid ". The reason for this assumption was that the hydrochloric acid was believed to be an oxygenated acid of a hypothetical element, the murium . When it comes into contact with the manganese dioxide , it should then absorb more oxygen. This was apparently confirmed by Claude-Louis Berthollet , who observed that chlorine water gave off oxygen when exposed to light and therefore called it "oxidized hydrochloric acid".

After attempts to split oxygen from the compound , for example by heating with carbon , had failed , Humphry Davy recognized in 1808 that the substance was a new element and not an oxygen-containing compound. Because of its characteristic light green color, he named the new element “chlorine”, after the Greek χλωρός chlōrós , German “light green” , “fresh”. The Philosophical transactions of the Royal Society of London document his findings on February 21, 1811 .

Initially, chlorine was mainly obtained from hydrochloric acid and manganese dioxide using a process developed by Walter Weldon . Since this was not very effective, it was in 1866 by that of Henry Deacon developed Deacon process replaced. In this case, cheaper atmospheric oxygen served as an oxidizing agent and copper (II) chloride as a catalyst . Chlorine was first produced electrolytically as early as 1800 , but this did not play a major role until Werner von Siemens developed the necessary generators at the end of the 19th century. Since then, electrochemical manufacturing processes have been by far the most important production processes for chlorine.

### Bleach

The historically most important use of chlorine is as a bleaching agent . It could either be used in elemental form or processed into chlorinated lime by reacting with calcium hydroxide .

### Chlorine as poison gas (weapon)

In World War I chlorine gas was first as a chemical weapon used. The first major deployment took place on April 22, 1915 near the city of Ypres in Flanders by a German special unit advising Fritz Haber, who later won the Nobel Prize . Since it has a higher density than air, the gas mainly accumulated in the trenches where the enemy soldiers were. The result was many deaths and numerous injuries, some for life. While the German method of blowing the chlorine gas out of steel cylinders was only to be used when the wind was blowing in the right direction, the French side used grenades around the same time (e.g. on April 25, 1915 in the Mametz - Montauban area ) that could be shot precisely into the opposing positions. These grenades consisted of two layers, a yellow ( picric acid ) and a white, a mixture of potassium chlorate and a waxy organic substance. During the burn, chlorine gas - more precisely chloropicrin - developed, which after inhalation led to coughing, runny nose and stomach pain. From a military point of view, it wasn't particularly efficient. For example, in the attack of April 22, 1915, despite the use of 150 tons of chlorine gas, according to recent research, only 1200 French people were killed. In other words, under optimal conditions, when the enemy is huddled in deep trenches, 125 kilograms of chlorine gas were needed to kill a soldier. As a result, chlorine was soon replaced by more injurious toxic gases, such as phosgene .

In addition to frequent accidents with chlorine gas in swimming pools, chlorine is still used as a chemical warfare agent despite its unsatisfactory injury effectiveness, mainly because it is a widespread industrial chemical to which, in principle, every lifeguard has access. In 2014, Human Rights Watch spoke of “strong indications” that government forces in Syria dropped chlorine gas in “ barrel bombs ” in mid-May 2014 . The UN Commission to Investigate Human Rights Abuses in Syria reported that the government had used poison gas five times between 2017 and April. In January 2018 there were allegedly at least five incidents involving chlorine gas.

## Occurrence

Due to its high reactivity, chlorine only occurs in extremely small amounts in elemental form on earth, e.g. B. in volcanic gases or in the ozone layer. Here it is split off from chlorofluorocarbons and mainly contributes to the formation of the ozone hole . Its anion, chloride , is found relatively frequently on earth, especially in salt-like compounds. In the continental crust , it is in a content of 145 ppm in the frequency behind elements such as zirconium , carbon or sulfur 19th in.

Many chlorides are readily soluble in water. Therefore, the seawater of the oceans contains a high concentration of chloride ions. With a content of 19.4 g Cl - / l, these are most common in seawater after oxygen and hydrogen in water molecules (for comparison: 1.4 mg F - , 68 mg Br - , 0.06 mg I - ). In addition, with 18.1 g Cl - / l , sodium chloride forms half of all the salts dissolved in it. Many lakes without drainage, such as the Dead Sea , have high levels of chloride, as the water supplied by the rivers evaporates and the salt remains behind.

The major chlorine-containing minerals are halite (main component: sodium chloride ), often referred to as rock-salt referred sylvite ( potassium chloride ), carnallite (KMgCl 3 · 6 H 2 O), Bischofit (MgCl 2 · 6 H 2 O) and kainite (KMgCl (SO 4 ) · 3H 2 O). There are large deposits that were formed when parts of the sea dried up. Since the less soluble sodium salts precipitate first and the potassium salts are deposited on top as the drying process progresses, the beds are often stratified. Larger deposits of halite are located in Germany, for example, in Bad Friedrichshall and Bad Reichenhall , and one in Austria is near Hallein . The category: Chlorine mineral provides an overview of chlorine minerals .

A large number of natural organochlorine compounds are known, in February 2002 there were 2200. The largest part is synthesized by marine life such as seaweed , sponges , tunicates or corals . Animals and plants living on the land form much less organic chlorine compounds. Organochlorine compounds are also formed during volcanic eruptions and the combustion of biomass .

Chlorine radicals arise from the decomposition of organic chlorine compounds in the stratosphere . Many of these organochlorine compounds, especially the chlorofluorocarbons (CFCs), are not, or only to a small extent, of natural origin, but were released by humans. Chlorine radicals can catalyze the breakdown of ozone and are responsible for the so-called ozone hole , which mainly occurs in the area of ​​the poles .

## Extraction and presentation

Chlorine is one of the most important basic chemicals and, with an amount of 58.9 million tons in 2006, is one of the most widely produced chemicals. Technically, chlorine is almost exclusively produced by various electrochemical processes; on a smaller scale, it can also be obtained chemically. It is a by-product of the electrochemical production of sodium and magnesium from the corresponding chlorides.

The starting material for the chlor-alkali electrolysis is an aqueous sodium chloride solution. From this, in various processes that differ in the structure of the electrolysis cell , caustic soda and, as by-products, chlorine and hydrogen are generated.

${\ displaystyle \ mathrm {2 \, NaCl + 2 \, H_ {2} O \ rightarrow 2 \, NaOH + Cl_ {2} + H_ {2}}}$
Reaction equation for chlor-alkali electrolysis

It is important in all processes for chlorine production that the anode , on which the chlorine is produced, is separated from the cathode , on which hydrogen and hydroxide ions are produced. If these were combined in a vessel, the explosive chlorine-hydrogen mixture would form chlorine detonating gas and the chlorine would react with the hydroxide ions to form hypochlorite .

${\ displaystyle \ mathrm {Cl_ {2} +2 \ OH ^ {-} \ longrightarrow OCl ^ {-} + Cl ^ {-} + H_ {2} O}}$

The most frequently used process at present is the diaphragm process (2001: 49% market share). The electrode spaces are separated by an asbestos diaphragm through which sodium ions can diffuse , but not chloride and hydroxide ions . However, this process can only produce a low concentration and not pure sodium hydroxide solution as well as chlorine contaminated with oxygen. The use of the carcinogenic asbestos is also problematic. That is why it is being replaced by the membrane process for new production plants (2001: 28% market share). Due to the use of a plastic membrane made of Nafion instead of the asbestos diaphragm, this is cheaper in terms of health protection and offers several technical advantages. The membrane provides a better separation of the anode and cathode compartments and thus enables the production of a purer and more highly concentrated sodium hydroxide solution. However, as in the diaphragm process, the chlorine is contaminated by oxygen, which is produced in a side reaction at the anode. Disadvantages of the process are the high costs for the membranes and the high purities required for the starting substances.

A process that is only used to a limited extent is the amalgam process (2001: 18% market share). In this case, the anode and cathode compartments are completely separated. A mercury cathode is used for this purpose, which, due to the high overvoltage, enables sodium to be formed first instead of hydrogen, which is present as sodium amalgam . The amalgam is reacted with water in a second cell on graphite contacts . Mercury, caustic soda and hydrogen are formed. This spatial separation enables very pure products. The biggest disadvantage is the use of the highly toxic and environmentally hazardous mercury, which makes complex and expensive protective measures necessary.

Various processes are known with which chlorine can be produced from hydrogen chloride by chemical oxidation ( Weldon process and Deacon process ). These only play a minor role in chlorine production. Another example is the KEL chlorine process, in which the hydrogen chloride is reacted with sulfuric acid and nitrosylsulfuric acid and which was developed by DuPont in 1975 . The decisive reaction step here is the oxidation of hydrogen chloride with nitrogen dioxide , which is released from the nitrosylsulfuric acid in several partial reactions. After testing in a test facility, however, the process was discontinued because of its low economic efficiency and material problems. Other processes are based on copper (II) chloride - or chromium (III) oxide - catalysts .

On a laboratory scale, elemental chlorine can be represented by acidifying chlorinated lime , for example with sulfuric acid.

${\ displaystyle \ mathrm {CaCl (OCl) + H_ {2} SO_ {4} \ longrightarrow CaSO_ {4} + H_ {2} O + Cl_ {2}}}$

## properties

### Physical Properties

Chlorine molecule
Ampoule with chlorine gas; Pressure about 600-800  hPa
liquid chlorine in quartz ampoule, encased in acrylic glass for safety ; Pressure about 6700 hPa
Solid chlorine at −150 ° C

At room temperature, chlorine is a yellow-green gas that, with a density of 3.214 g / l at 0 ° C, is about 2.5 times as heavy as air . It condenses to a yellow liquid at −34.6 ° C and solidifies at −101 ° C. Since the critical point is relatively high at 143.9 ° C, 77.1 bar and 0.67 g / cm³, chlorine can easily be liquefied under pressure. It is liquid at a pressure of 6.7 bar at 20 ° C and can be transported in steel cylinders or tank wagons . The intensity of the color decreases at lower temperatures; at −195 ° C, chlorine is almost colorless.

Like the other halogens, chlorine is also a diatomic molecule . The distance between the chlorine atoms is 199 pm. At 242 kJ / mol, chlorine has the highest enthalpy of dissociation of all halogens. Another indication of this is the temperature at which 1% of all halogen molecules are dissociated and which is 975 ° C for chlorine, 775 ° C for bromine and 575 ° C for iodine . Also fluorine has 765 ° C to a lower temperature. The fact that chlorine and not, as expected, fluorine, is the halogen with the highest enthalpy of dissociation is due to the particularly short bond of fluorine, in which there is repulsion between the lone pairs of electrons and thus the bond is weakened. On the other hand, there is no such effect between the more distant chlorine atoms and therefore a stronger bond despite the greater distance of the atoms.

Chlorine crystallizes in the orthorhombic crystal system with the lattice constants a = 624  pm , b = 448 pm and c = 826 pm. The chlorine molecules, like those of iodine and bromine, are arranged in layers. Each atom of a Cl 2 molecule is weakly associated with two further atoms of other molecules at a distance of 334 pm. In contrast, the distances between the layers are larger with a minimum distance of 369 pm. This layer structure causes the platelet shape and the easy cleavage of chlorine crystals.

The solubility is different in different solvents . It is moderately soluble in water with partial dissociation; about 2.3 liters of chlorine can be dissolved in one liter of water. The resulting solution is called chlorinated water. In contrast, it dissolves well in liquid chlorine-containing compounds such as disulfur dichloride , silicon tetrachloride and organic chlorine compounds such as chloroform . Large amounts of chlorine also dissolve in some organic solvents such as benzene , acetic acid and dimethylformamide .

### Chemical properties

Along with fluorine, chlorine is one of the most reactive elements and reacts with almost all elements. There is no direct reaction only with oxygen , nitrogen and the noble gases . However, many metals, such as manganese , zinc or the precious metals gold , silver and platinum only react with chlorine at elevated temperatures. The presence of water sometimes plays an important role; copper and iron only react with completely dry chlorine at temperatures above 200 ° C, while with moist chlorine they react at significantly lower temperatures.

The tendency of chlorine to react with hydrogen is particularly strong. After a necessary initiation by splitting a first chlorine molecule, which can be triggered, for example, by short-wave blue light, the elements react in an explosive chain reaction , the so-called chlorine detonation gas reaction . Due to the strong tendency to form hydrogen chloride, chlorine also reacts with other hydrogen-containing compounds such as ammonia , ethyne , hydrogen sulfide or water.

Chlorine reacts with alkanes via the reaction mechanism of radical substitution . Initially, individual chlorine radicals are formed through heat or irradiation , which can break the CH bond of an alkane with the formation of hydrogen chloride. The resulting radical then reacts with more chlorine and another chain reaction. Due to its high reactivity, chlorine is only weakly regioselective in the reaction with alkanes , and multiple chlorinations also occur. A radical reaction path is not possible with aromatic hydrocarbons; chlorination takes place here via electrophilic aromatic substitution with catalysis by a Lewis acid such as aluminum chloride .

## Isotopes

Characteristic 35 Cl / 37 Cl isotope pattern

A total of 23 isotopes and two further core isomers between 28 Cl and 51 Cl are known. Of these, two, the isotopes 35 Cl and 37 Cl, are stable. Natural chlorine consists of 75.77% of 35 Cl and 24.23% of 37 Cl. This typical relationship can always be observed in the mass spectra of organic and inorganic substances.

### 36 cl

With a half-life of 301,300 years, 36 Cl is the longest-lived of the unstable isotopes that otherwise decay within minutes or even shorter times, which is why it is used for marking.

36 Cl is produced in small amounts by spallation reactions of 40 Ar and 36 Ar with cosmic rays in the atmosphere. 36 Cl can also arise on the earth's surface through neutron adsorption , reactions with muons or spallation. The ratio of 36 Cl to 37 Cl is about 700 · 10 −15 : 1. Due to the long half-life and constant atmospheric concentration, the concentration of 36 Cl can be used to determine the age of groundwater from up to a million years.

The concentration of 36 Cl was increased between 1954 and 1963 as a result of nuclear weapon tests in the sea , in which 35 Cl contained in the seawater absorbed neutron radiation and reacted to 36 Cl. Since a treaty banning this type of test, the concentration in the atmosphere has steadily decreased and reached the natural ratio from around 1980, but increased concentrations of the isotope can still be found in seawater. The 36 Cl method is also used for palaeontological and prehistoric dating .

### 38 Cl and 37 Cl

38 Cl is a short-lived isotope with a half-life of 37 minutes and can arise, for example, through neutron adsorption from 37 Cl contained in seawater .

## use

Chlorine is mainly used to make other chemicals. With 33% in 1997, vinyl chloride , the starting substance for the manufacture of the plastic polyvinyl chloride , is the most important product. Other simple organochlorine compounds are also produced by reacting chlorine and corresponding hydrocarbons, for example by means of photochlorination . These are primarily used as an intermediate product, for example in the manufacture of plastics , pharmaceuticals or pesticides . In 1995, 85% of all pharmaceuticals were manufactured using chlorine. The chlorine is often split off again in the course of a manufacturing process in order to obtain chlorine-free end products. Examples of this are the production of glycerol via allyl chloride and epichlorohydrin or the chlorohydrin process for the production of propylene oxide .

Inorganic chlorine compounds are often produced by reacting with chlorine. For example, the synthesis of high purity hydrogen chloride by the reaction of chlorine and hydrogen or the synthesis of titanium tetrachloride are technically important . This is either further processed into elemental titanium using the Kroll process or serves as an intermediate product in the purification of the white pigment titanium (IV) oxide . Other important chlorides, which are represented by the reaction of the element with chlorine, are aluminum trichloride and silicon tetrachloride .

If chlorine is passed into water, it slowly disproportionates with the formation of hypochlorous acid and hydrochloric acid . The former has a strong oxidizing effect and thus has a bleaching and disinfecting effect . The bleaching effect of chlorine was mainly used for the production of white paper. The chlorine is able to replace or oxidize the aromatic rings of the lignin . This destroys possible chromophores and the paper appears lighter. However, since chlorine bleaching sometimes produces carcinogenic organochlorine compounds such as polychlorinated dibenzodioxins and dibenzofurans or chlorophenols , chlorine bleaching has often been replaced by less dangerous methods such as bleaching with sodium dithionite .

The disinfecting effect of the hypochlorite produced by the reaction of chlorine and water is used in water treatment in what is known as chlorination . In addition to drinking water, swimming pool water in particular is freed of bacteria in this way . Since the reaction with other components of the water can also produce undesirable and sometimes toxic or carcinogenic substances such as trihalomethanes , chlorine is increasingly being replaced by chlorine dioxide or ozone for the disinfection of drinking water .

Due to the environmental pollution and toxicity of chlorine and many chlorine-containing compounds, demands are made and, in some cases, attempts are made to avoid these and replace them with chlorine-free compounds and processes. The recycling of chlorine-containing waste materials is also an alternative, as it means that no new products of this type have to be manufactured. The burning of organochlorine compounds, which can produce slightly toxic combustion products, can thus be avoided. However, higher prices and poorer properties of substitutes often speak against the use of chlorine-free products and processes, and chlorine is still used in large quantities in industry.

## Biological importance

Elemental chlorine has an oxidizing effect and can react with plant and animal tissue. Accordingly, it is toxic and has no biological significance. Chlorine compounds in high oxidation states, such as chlorine oxides and chloro-oxygen acids, also have a strong oxidizing effect and therefore have no biological functions .

The element is of biological importance in the form of the chloride anion. Chloride is essential and one of the more common components of the body. An average human body weighing around 70 kg contains 95 g of chloride. Most of the chloride is dissolved in the extracellular space as a counterion to sodium , so blood plasma has a chloride concentration of 100–107 mmol / l. Chloride has a major influence on the osmotic pressure and thus the body's water balance. Chloride also serves to balance the charge when ions are exchanged into and out of cells. This plays a role , for example, in the transport of carbon dioxide as hydrogen carbonate . For this balance and the restoration of the resting membrane potential , chloride channels are used , through which chloride ions can pass through the cell membranes.

The gastric juice contains a particularly high concentration of chloride because, in addition to the chloride ions, mainly oxonium ions are present there, the gastric acid is a hydrochloric acid with a concentration of about 0.1 mol / l.

The chloride is mainly absorbed as sodium chloride in table salt . The recommended daily intake for chloride is 3.2 g for adults and 0.5 g for infants.

## proof

Detection reaction for chlorine by introducing into NaI solution and NaBr solution, each with hexane

Chlorine has a typical green-yellow color and also a characteristic odor, but these do not allow a more precise determination. The oxidizing effect is mostly used to detect chlorine. For example, chlorine can oxidize iodides and bromides to the elements, whereby a solution containing bromide turns brown or a solution containing iodide turns purple. To make this color easier to see, the bromine or iodine is extracted with hexane . Reactions with other substances, such as the discoloration of methyl orange, can also be used as evidence of chlorine. However, these are not specific as other oxidizing agents are also able to react in the same way.

The reaction with tolidine provides evidence specific to chlorine, which is used in test tubes for gases , for example . A yellow dye is formed which can be detected by colorimetric methods.

Chlorides can be detected in aqueous solutions through the reaction with silver ions and the formation of the sparingly soluble silver chloride . This is present as a white precipitate and thus differs from the similarly sparingly soluble silver bromide and silver iodide , which have a yellow color. Using argentometry , quantitative measurements of chloride contents can also be carried out.

## safety instructions

As a gas, chlorine mainly affects the respiratory tract. When inhaled, it reacts with the moisture in the mucous membranes to form hypochlorous and hydrochloric acids . This leads to strong irritation of the mucous membranes, with prolonged exposure to coughing up blood and shortness of breath , as well as symptoms of suffocation. At higher concentrations, pulmonary edema and severe lung damage occur. A content of 0.5–1% chlorine in the breath has a lethal effect if breathing stops. The lethal doses over one hour (LC 50 ) are 293 ppm for rats and 137 ppm for mice. Liquid chlorine is very corrosive to the skin. Chronic exposure to chlorine can lead to chronic bronchitis , and higher concentrations can cause cardiovascular damage and stomach problems.

Chlorine is not flammable ( chlorine dioxide is produced in another way), but it can react strongly with many substances. If chlorine comes into contact with hydrogen , hydrocarbons , ammonia , amines , diethyl ether and some other substances, there is a risk of explosion .

A Spanish study found that the disinfection by-products resulting from the chlorination of the water and the reaction with organic contaminants ( urine , sweat , flakes of skin ) increase the risk of bladder cancer . This risk can be significantly reduced by adopting appropriate hygienic behavior on the part of the bathers (shower before entering the pool, do not urinate in the pool).

Chlorine forms compounds in various oxidation states from −1 to +7. The most stable and most frequent oxidation state is −1, the higher ones are only formed in compounds with the more electronegative elements oxygen and fluorine . The odd oxidation states +1, +3, +5 and +7 are more stable than the even ones. The category: Chlorine compounds provides an overview of the chlorine compounds

### Hydrogen chloride and chlorides

Inorganic compounds in which the chlorine is in the −1 oxidation state and thus as an anion are called chlorides . These are derived from the gaseous hydrogen compound hydrogen chloride (HCl). This is a strong acid and easily releases the proton in aqueous solutions . This aqueous solution is called hydrochloric acid . Hydrochloric acid is one of the technically most important acids and is used in large quantities. Chlorides are usually readily soluble in water, exceptions are silver chloride, mercury (I) chloride and lead (II) chloride.

The chlorides of the alkali metals , especially sodium chloride , are particularly well known . This is the main component of table salt and therefore an important part of the diet. At the same time, sodium chloride , which occurs in large quantities as halite, is the starting compound for the production of most other chlorine compounds. Also, potassium chloride is used in large quantities, mainly as a fertilizer and for the production of other potassium compounds.

### Chlorine oxides

A large number of compounds of chlorine and oxygen are known. These are based on the general formulas ClO x ( x  = 1–4) and Cl 2 O x ( x  = 1–7). Chlorine oxides are very reactive and explode into the elements. Only two of the chlorine oxides, dichloroxide (Cl 2 O) and chlorine dioxide (ClO 2 ) are of technical importance . The latter is gaseous under normal conditions and is one of the few compounds with a radical structure. When it solidifies, it dimerizes and changes the magnetization from para to diamagnetism .

### Chloro-oxygen acids

In addition to the chlorine oxides, chlorine and oxygen - analogous to the halogens bromine and iodine - also form several acids in which one chlorine atom is surrounded by one to four oxygen atoms. These compounds, which belong to the halogenated oxygen acids, are hypochlorous acid , chlorous acid , chloric acid and perchloric acid . The only one of these acids that is stable as a pure substance is perchloric acid, the others are only known in aqueous solution or in the form of their salts. The pK s value of these acids decreases with the increasing number of oxygen atoms in the molecule. While hypochlorous acid is only a weak acid, perchloric acid is one of the super acids , the strongest known acids.

### Interhalogen compounds

Chlorine forms a number of interhalogen compounds mainly with fluorine , and in some cases with the other halogens as well . Chlorofluorides such as chlorofluoride and chlorotrifluoride have a strong oxidizing and fluorinating effect. While chlorine is the more electropositive element in the chlorine pentafluoride in oxidation states of up to +5 in the fluorine-chlorine compounds , it is the more electronegative component in compounds with bromine and iodine . With these elements only three compounds, bromine chloride , iodine chloride and iodine trichloride are known.

### Organic chlorine compounds

A large number of organic chlorine compounds (also organochlorine compounds ) are produced synthetically. In the group of halogenated hydrocarbons, the chloroalkanes, the chloroalkenes and the chloroaromatics are important. They are used, among other things, as solvents , refrigerants , hydraulic oils , pesticides or pharmaceuticals .

The organochlorine compounds also include some highly toxic, persistent and bioaccumulative substances, such as the polychlorinated dibenzodioxins and dibenzofurans . The first twelve compounds or groups of substances included in the Stockholm Convention for pollutant control , the so-called Dirty Dozen , are all organic chlorine compounds.

In addition, there are a large number of natural organic chlorine compounds in the biosphere that are produced by organisms such as B. soil bacteria , molds , seaweed and lichens can be synthesized. The compounds include biogenic halogenated hydrocarbons , such as methyl chloride , 70% of which comes from marine organisms, and chlorinated aromatics , but also chlorine-containing amino acids such as L -2-amino-4-chloro-4-pentenoic acid, which is found in certain mushrooms . The proportion of chlorinated humic substances in certain moors is also noticeably high .

The synthesis of these compounds takes place via haloperoxidases in the presence of hydrogen peroxide , via direct chlorination with enzymatically released chlorine or hypochlorite , via chlorine radicals or by nucleophilic ring opening of epoxides with chloride ions. Since chloride ions occur frequently in nature, they are the only source of chlorine for the biogenic organic chlorine compounds. The proportion of these compounds in the environment compared to the industrially caused proportion of organic chlorine compounds is not insignificant.

## literature

Wiktionary: Chlorine  - explanations of meanings, word origins, synonyms, translations
Commons : Chlorine  - Collection of pictures, videos and audio files

## Individual evidence

1. ^ Harry H. Binder: Lexicon of the chemical elements. S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
2. The values ​​for the properties (info box) are taken from www.webelements.com (chlorine) , unless otherwise stated .
3. The standard value recommended by IUPAC is given, since the isotopic composition of this element can vary locally, the mass range given in brackets results for the mean atomic weight. See: Michael E. Wieser, Tyler B. Coplen: Atomic weights of the elements 2009 (IUPAC Technical Report). In: Pure Appl. Chem. 2010, p. 1, doi: 10.1351 / PAC-REP-10-09-14 .
4. ^
5. Entry on chlorine in Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2019): NIST Atomic Spectra Database (ver. 5.7.1) . Ed .: NIST , Gaithersburg, MD. doi : 10.18434 / T4W30F ( https://physics.nist.gov/asd ). Retrieved June 11, 2020.
6. Entry on chlorine at WebElements, https://www.webelements.com , accessed on June 11, 2020.
7. Entry on chlorine in the GESTIS substance database of the IFA , accessed on August 9, 2016(JavaScript required) .
8. Robert C. Weast (Ed.): CRC Handbook of Chemistry and Physics . CRC (Chemical Rubber Publishing Company), Boca Raton 1990, ISBN 0-8493-0470-9 , pp. E-129 to E-145. Values ​​there are based on g / mol and given in cgs units. The value specified here is the SI value calculated from it, without a unit of measure. The note liquid is likely to be a misprint.
9. a b Yiming Zhang, Julian RG Evans, Shoufeng Yang: Corrected Values ​​for Boiling Points and Enthalpies of Vaporization of Elements in Handbooks. In: Journal of Chemical & Engineering Data . 56, 2011, pp. 328-337, doi: 10.1021 / je1011086 .
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 This version was added to the list of articles worth reading on June 26, 2009 .