Halides are chemical compounds between elements of the seventh main group (more precisely the 17th group) of the periodic table , the so-called halogens , and elements from other groups. The outdated term is haloids . In addition, the simply negatively charged ions of the halogens ( F - , Cl - , Br - , I - , At - ) are referred to as halide ions (also halides for short). Since the chemical properties of Tenness are completely unknown, the Tennessids are still unknown.
A distinction is made between the connections (depending on the type of chemical bond ):
- Salt-like halides
- Ionic compounds ( salts ) which, due to the large electronegativity difference between the elements involved, consist of anions and cations and are held together by electrostatic interactions. Examples are sodium chloride (NaCl) and potassium bromide (KBr).
- Covalent halides
- Covalent compounds in which the difference in electronegativity is not as great as in the above ionic compounds, but the bonds have a charge polarity . Examples are hydrogen halides such as hydrogen chloride (HCl) and interhalogen compounds . Also halogenated hydrocarbons such as methylene chloride (dichloromethane CH 2 Cl 2 ), and other organic compounds containing halogens, are often referred to as halides, which usually but not the current IUPAC corresponding nomenclature.
- Complex halides
- Complexes with halide ions as ligands , for example the tetrachloridoplatinate ion [PtCl 4 ] 2− .
Oxidation of halides
The halides oxidize in stages according to the electrochemical series to form elementary halogen.
- Fluorine oxidizes chloride to chlorine.
- Chlorine oxidizes bromide to bromine.
- Bromine oxidizes iodide to iodine.
Halides and halogen-containing compounds are often found in the chemical industry. So are z. B. chloroform and dichloromethane are good organic solvents. However, due to their low boiling point, some of them get into the environment and atmosphere. When halides are exposed to sunlight, halogen radicals are then formed, which in turn attack the ozone layer (see: ozone hole ).
The aim is therefore to keep the use of halogen-containing solvents as low as possible. Therefore, CFC- containing substances were banned from spray cans and refrigerators in the 1980s and 1990s.
In some countries, salt-like halides such as sodium fluoride and sodium iodide are added in small amounts to foods, table salt, dental care products or drinking water for the purpose of caries prophylaxis. However, these substances are often dangerous to the environment and, above certain concentrations, harmful to health.
Detection with silver nitrate and ammonia
For example, with a saline solution:
The silver halide precipitate is then examined more closely with ammonia water:
- Silver (I) fluoride (AgF) is the only silver halide that is readily soluble in water.
- Silver chloride (AgCl) forms a white, cheesy precipitate which, when ammonia is added , dissolves again in a complex formation reaction to form the colorless diammine silver (I) :
- Silver bromide (AgBr) precipitates out as a light yellow precipitate that is only soluble in concentrated ammonia.
- Silver iodide (AgI) appears as a yellow-greenish precipitate which does not dissolve even in concentrated ammonia.
Evidence as elemental bromine and iodine
- Iodide and chlorine react to form purple iodine and chloride
- Bromide and chlorine react to form brown bromine and chloride
By extraction in an organic solvent, the dyeings are particularly well visible. In oxygen-free solvents such as dichloromethane or n-hexane , iodine is pink-violet, in oxygen-containing solvents such as diethyl ether it is brown. Bromine turns the solution brown. The subsequent reaction to form bromine chloride turns the solution wine-yellow.
- Bromine and chlorine react to form wine-yellow bromine chloride
Three titration methods are used for the quantitative detection of halide ions , which are also based on the poor solubility of the silver halides:
- Dirk Häfner: Workbook qualitative inorganic analysis , 2nd revised edition, Govi-Verlag, Eschborn 2003, p. 124, ISBN 3-7741-0997-4 .