Iron(III) chloride

Iron(III) chloride, generically called ferric chloride, is an iron-based salt of chemical formula FeCl₃. It is very hygroscopic and it fumes in moist air with hydrolysis and when dissolved in water, it evolves a great deal of heat and produces a brown, foul-smelling, acidic solution. This corrosive liquid is used in treating sewage and drinking water, and to etch copper-based metals (such as those found in electrical 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 FeCl₃ which in fact has the structure [FeCl₂(H₂O)₄]Cl.2H₂O (compare chromium(III) chloride).

When heated, FeCl₃ melts then boils at around 315 °C. The vapour contains dimers of Fe₂Cl₆ (compare aluminium chloride) which slowly decompose to give FeCl₂ and Cl₂.

Chemical properties

Iron(III) chloride is a moderately strong Lewis acid which with Lewis bases such as triphenylphosphine oxide forms stable adducts such as FeCl₃(OPPh₃)₂ where Ph = phenyl. With chloride ion several anionic complexes are known, but the most stable contain 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 the oxychloride FeOCl is formed. 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 [Fe(C₂O₄)₃]^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 cause reduction of 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 in 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)

Hydrated FeCl₃ can be dehydrated to the anhydrous salt by heating with thionyl chloride.

Uses

Iron(III) chloride is probably the most widely used etching material^[1]. It is commonly 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. This is used for the industrial production of vinyl chloride, the monomer for making PVC. A related reaction is also used for making 1,2-dichloroethane, an important commodity chemical.

Another industrial application 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 most commonly used 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^[7] 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.

Precautions

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

Suppliers/Manufacturers

• Sigma-Aldrich
• Strem
• Fisher
• VWR
• Alfa

References

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

See also

• iron(III) sulfate
• iron(III) chloride sulfate
• iron(II) chloride
• iron(II) sulfate
• aluminium chloride