Iron(III) chloride


Iron chloride is the inorganic compound with the formula. Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The color depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red.

Structure and properties

Anhydrous

Anhydrous iron chloride has the BiI3 structure, with octahedral Fe centres interconnected by two-coordinate chloride ligands.
Iron chloride has a relatively low melting point and boils at around 315 °C. The vapour consists of the dimer which increasingly dissociates into the monomeric at higher temperature, in competition with its reversible decomposition to give iron chloride and chlorine gas.

Hydrates

In addition to the anhydrous material, ferric chloride forms four hydrates. All forms of iron chloride feature two or more chlorides as ligands, and three hydrates feature FeCl4−.
Aqueous solutions of ferric chloride are characteristically yellow, in contrast to the pale pink solutions of 3+. According to spectroscopic measurements, the main species in aqueous solutions of ferric chloride are the octahedral complex + and the tetrahedral −.

Preparation

Anhydrous iron chloride may be prepared by union of the elements:
Solutions of iron chloride are produced industrially both from iron and from ore, in a closed-loop process.
  1. Dissolving iron ore in hydrochloric acid
  2. :Fe3O4_ 8 HCl_ -> FeCl2_ 2 FeCl3_ 4 H2O_
  3. Oxidation of iron chloride with chlorine
  4. :2 FeCl2_ Cl2_ -> 2 FeCl3_
  5. Oxidation of iron chloride with oxygen
  6. :4FeCl2_ O2 4HCl -> 4FeCl3_ 2H2O_
Small amounts can be produced by reacting iron with hydrochloric acid, then with hydrogen peroxide. The hydrogen peroxide is the oxidant in turning ferrous chloride into ferric chloride
Anhydrous iron chloride cannot be obtained from the hydrate by heating. Instead, the solid decomposes into HCl and iron oxychloride. The conversion can be accomplished by treatment with thionyl chloride. Similarly, dehydration can be effected with trimethylsilyl chloride:

Reactions

Iron chloride undergoes hydrolysis to give a strongly acidic solution.
When heated with iron oxide at 350 °C, iron chloride gives iron oxychloride, a layered solid and intercalation host.
The anhydrous salt is a moderately strong Lewis acid, forming adducts with Lewis bases such as triphenylphosphine oxide; e.g., where Ph is phenyl. It also reacts with other chloride salts to give the yellow tetrahedral ion. Salts of in hydrochloric acid can be extracted into diethyl ether.

Redox reactions

Iron chloride is a mild oxidising agent, for example, it is capable of oxidising copper chloride to copper chloride.
It also reacts with iron to form iron chloride:
A traditional synthesis of anhydrous ferrous chloride is the reduction of FeCl3 with chlorobenzene:

With carboxylate anions

s react rapidly with aqueous iron chloride to give. Other carboxylate salts form complexes; e.g., citrate and tartrate.

With alkali metal alkoxides

Alkali metal alkoxides react to give the metal alkoxide complexes of varying complexity. The compounds can be dimeric or trimeric. In the solid phase a variety of multinuclear complexes have been described for the nominal stoichiometric reaction between and sodium ethoxide:

With organometallic compounds

Iron chloride in ether solution oxidizes methyl lithium to give first light greenish yellow lithium tetrachloroferrate solution and then, with further addition of methyl lithium, lithium tetrachloroferrate :
The methyl radicals combine with themselves or react with other components to give mostly ethane and some methane.

Uses

Industrial

Iron chloride is used in sewage treatment and drinking water production as a coagulant and flocculant. In this application, in slightly basic water reacts with the hydroxide ion to form a floc of iron hydroxide, or more precisely formulated as, that can remove suspended materials.
It is also used as a leaching agent in chloride hydrometallurgy, for example in the production of Si from FeSi.
Another important application of iron chloride is etching copper in two-step redox reaction to copper chloride and then to copper chloride in the production of printed circuit boards.
Iron chloride is used as catalyst for the reaction of ethylene with chlorine, forming ethylene dichloride, an important commodity chemical, which is mainly used for the industrial production of vinyl chloride, the monomer for making PVC.

Laboratory use

In the laboratory iron chloride is commonly employed as a Lewis acid for catalysing reactions such as chlorination of aromatic compounds and Friedel–Crafts reaction of aromatics. It is less powerful than aluminium chloride, but in some cases this mildness leads to higher yields, for example in the alkylation of benzene:
The ferric chloride test is a traditional colorimetric test for phenols, which uses a 1% iron chloride solution that has been neutralised with sodium hydroxide until a slight precipitate of FeO is formed. The mixture is filtered before use. The organic substance is dissolved in water, methanol or ethanol, then the neutralised iron chloride solution is added—a transient or permanent coloration indicates the presence of a phenol or enol.
This reaction is exploited in the Trinder spot test, which is used to indicate the presence of salicylates, particularly salicylic acid, which contains a phenolic OH group.
This test can be used to detect the presence of gamma-hydroxybutyric acid and gamma-butyrolactone, which cause it to turn red-brown.

Other uses

Iron chloride is harmful, highly corrosive and acidic. The anhydrous material is a powerful dehydrating agent.
Although reports of poisoning in humans are rare, ingestion of ferric chloride can result in serious morbidity and mortality. Inappropriate labeling and storage lead to accidental swallowing or misdiagnosis. Early diagnosis is important, especially in seriously poisoned patients.