Iron(III) chloride

Iron(III) chloride
General
Systematic name	Iron(III) chloride
Other names	ferric chloride iron trichloride molysite mineral form Flores martis
Molecular formula	FeCl₃
Molar mass	162.21 g/mol hexahydrate: 270.30 g/mol
Appearance	green-black by reflected light, purple-red by transmitted light hexahydrate: yellow solid aq. solutions: brown
CAS number	[10025-77-1
Properties
Density and phase	2.80 g/cm³ 40% solution: 1.4 g/ml
Solubility in water	92 g/100 ml (20 °C)
In other solvents: acetone Methanol Ethanol Diethyl ether	63 g/100 ml (18 °C) highly soluble 83 g/100 ml highly soluble
Melting point	306 °C (579 K)
Boiling point	315 °C (588 K), partial decomposition to FeCl₂+Cl₂
Acidity (pK_a)	?
Viscosity	40% solution: 12 cP
Structure
Molecular shape	?
Coordination geometry	octahedral
Crystal structure	hexagonal
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Very corrosive
NFPA 704
R/S statement	R: 22, 34 S: 26,28
RTECS number	LJ9100000
Supplementary data page
Structure & properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Iron(III) fluoride Iron(III) bromide
Other cations	Iron(II) chloride
Related metals	Manganese(II) chloride Cobalt(II) chloride Ruthenium(III) chloride
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Iron(III) chloride, generically called ferric chloride, is FeCl₃. The color depends on the viewing angle - by reflected light crystals appear dark green but by transmitted light, the crystals are purple-red. It is deliquescent, fuming in moist air due to the evolution of HCl, which hydrates, giving a mist. When dissolved in water, it undergoes hydrolysis and evolves a great deal of heat. The resulting brown, acidic solution is corrosive; such solutions are used as a coagulant in treating sewage and drinking water and as an etchant for copper-based metals (such as those found in printed circuit boards) and stainless steel.

Anhydrous iron(III) chloride is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis. The brownish-yellow hexahydrate is a common commercial form of ferric chloride, a yellowish species described by the formula ^*Cl.2H₂O (compare chromium(III) chloride).

Structure

FeCl₃ adopts the BI₃ structure, which features octahedral Fe(III) centres and three-coordinate chloride ligands. FeCl₃ has a relatively low melting point, and boils at around 315 °C. The vapour consists of the dimer Fe₂Cl₆ (compare aluminium chloride) that increasingly dissociated into the monomer at higher temperature, in competition with the reversible decomposition to give FeCl₂ and Cl₂.

Chemical properties

Iron(III) chloride is a moderately strong Lewis acid which forms adducts with Lewis bases such as triphenylphosphine oxide, e.g. FeCl₃(OPPh₃)₂ where Ph = phenyl. FeCl₃ reacts with chloride salts to give the yellow tetrahedral FeCl₄^- ion. Solutions of FeCl₄^- in hydrochloric acid may be extracted into diethyl ether.

When heated with iron(III) oxide at 350 °C, FeCl₃ gives the oxychloride FeOCl. In the presence of base, iron(III) chloride may undergo replacement of chloride, for example to produce an alkoxide:

FeCl₃ + 3 C₂H₅OH + 3 NH₃ → Fe(OC₂H₅)₃ + 3 NH₄Cl

Carboxylate salts such as oxalate, citrate or tartrate react easily with aqueous FeCl₃ to form stable complexes such as ^*^3-.

Iron(III) chloride is also a mild oxidising agent, capable (for example) of oxidising copper(I) chloride to copper(II) chloride. Reducing agents such as hydrazine convert FeCl₃ to complexes of iron(II).

Preparation and production

Anhydrous iron(III) chloride may be prepared by union of the elements:

2 Fe(s) + 3 Cl₂(g) → 2 FeCl₃(s)

The solution of iron(III) chloride is produced on industrial scale via two methods, from iron and from ore, in a closed-loop process.

Dissolving pure iron in iron(III) chloride solution:

Fe(s) + 2 FeCl₃(aq) → 3 FeCl₂(aq)

Dissolving iron ore in hydrochloric acid:

Fe₃O₄(s) + 8 HCl(aq) → FeCl₂(aq) + 2 FeCl₃(aq) + 4 H₂O

Upgrading the iron(II) chloride with chlorine:

FeCl₂(aq) + Cl₂(g) → FeCl₃(aq)

Alternatively, ferrous chloride can be oxidized with sulfur dioxide:

4 FeCl₂ + SO₂ + 4 HCl → 4 FeCl₃ + 1/8 S₈ + 2 H₂O

Hydrated FeCl₃(H₂O)_n can be converted to the anhydrous salt by heating with thionyl chloride. The hydrate cannot be converted to FeCl₃ by heat, instead HCl is evolved and FeOCl forms

Uses

Iron(III) chloride is probably most widely used for etching copper in the production of printed circuit boards.^{^*} This occurs by the redox reaction

FeCl₃ + Cu → FeCl₂ + CuCl followed by FeCl₃ + CuCl → FeCl₂ + CuCl₂

Iron(III) chloride is also used as a catalyst for the reaction of ethylene with chlorine, forming ethylene dichloride (1,2-Dichloroethane), an important commodity chemical, which is mainly used for the industrial production of vinyl chloride, the monomer for making PVC.

In another industrial application, ferric chloride is an alternative to iron(III) sulfate in water treatment, where FeCl₃ is treated with hydroxide ion to form a floc of "iron(III) hydroxide" (more correctly formulated as FeO(OH)) that can remove suspended materials.

In the laboratory iron(III) 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(III) chloride solution that has been neutralised with sodium hydroxide until a slight precipitate of FeO(OH) is formed. The mixture is filtered before use. The organic substance is dissolved in water, methanol or ethanol, then the neutralised FeCl₃ solution is added- a transient or permanent coloration (usually purple, green or blue) indicates the presence of a phenol or enol.

FeCl₃ is sometimes used by American coin collectors to identify the dates of Buffalo nickels that are so badly worn that the date is no longer visible.

FeCl₃ is also commonly used by knife craftsmen and swordsmiths to stain blades, as to give a contrasting effect to the metal, and also to view metal layering or imperfections.

Precautions

Iron(III) chloride is toxic, highly corrosive and acidic. The anhydrous material is a powerful dehydrating agent.

References

N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
Handbook of Reagents for Organic Synthesis: Acidic and Basic Reagents, (H. J. Reich, J. H. Rigby, eds.), Wiley, New York, 1999.
B. R. Tarr, "Anhydrous Iron(III) Chloride" in Inorganic Syntheses, volume III, pp 191-194 (1950).

External links

Chlorides | Iron compounds | Metal halides | Coordination compounds | Deliquescent substances

Eisen(III)-chlorid | Cloruro férrico | Chlorure de fer (III) | 三氯化铁

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