# Halides

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 The outdated term is haloids .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.

${\ displaystyle \ mathrm {F_ {2} +2 \ Cl ^ {-} \ longrightarrow Cl_ {2} +2 \ F ^ {-}}}$
Fluorine oxidizes chloride to chlorine.
${\ displaystyle \ mathrm {Cl_ {2} +2 \ Br ^ {-} \ longrightarrow Br_ {2} +2 \ Cl ^ {-}}}$
Chlorine oxidizes bromide to bromine.
${\ displaystyle \ mathrm {Br_ {2} +2 \ I ^ {-} \ longrightarrow I_ {2} +2 \ Br ^ {-}}}$
Bromine oxidizes iodide to iodine.

## Problem

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 reactions

### Detection with silver nitrate and ammonia

Precipitates of the silver halide before (left) and after the addition of ammonia water (right next to it), AgI on the left, AgBr in the middle, AgCl on the right

Chloride, bromide and iodide can be precipitated in a detection reaction from aqueous solution after acidification with nitric acid with silver nitrate .

For example, with a saline solution:

${\ displaystyle \ mathrm {NaCl _ {\ (aq)} + AgNO_ {3 \ (aq)} \ longrightarrow Na _ {\ (aq)} ^ {+} + NO _ {\ 3 (aq)} ^ {-} + AgCl_ {\ (s)} \ downarrow}}$

The silver halide precipitate is then examined more closely with ammonia water:

${\ displaystyle \ mathrm {Ag ^ {+} + 2 \ NH_ {3} \ longrightarrow [Ag (NH_ {3}) _ {2}] ^ {+}}}$
• 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.

All silver halides decompose when exposed to light and dissolve in concentrated sodium thiosulphate solution (fixing salt).

### Evidence as elemental bromine and iodine

Halide detection with chlorine water and hexane

Another way of differentiating bromine and iodine is to add chlorine water or chloramine T with hydrochloric acid, whereby bromide and iodide are oxidized to halogen by chlorine .

${\ displaystyle \ mathrm {2 \ I ^ {-} + Cl_ {2} \ longrightarrow I_ {2} +2 \ Cl ^ {-}}}$
Iodide and chlorine react to form purple iodine and chloride
${\ displaystyle \ mathrm {2 \ Br ^ {-} + Cl_ {2} \ longrightarrow Br_ {2} +2 \ Cl ^ {-}}}$
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.

${\ displaystyle \ mathrm {Br_ {2} + Cl_ {2} \ longrightarrow 2 \ BrCl}}$
Bromine and chlorine react to form wine-yellow bromine chloride

### Titration method

Three titration methods are used for the quantitative detection of halide ions , which are also based on the poor solubility of the silver halides:

## Individual evidence

1. Dirk Häfner: Workbook qualitative inorganic analysis , 2nd revised edition, Govi-Verlag, Eschborn 2003, p. 124, ISBN 3-7741-0997-4 .