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Zinc chloride
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Zinc chloride is the name of chemical compound with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white and highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from sources of moisture, including the water vapor present in ambient air. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis.
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Zinc chloride is the name of chemical compound with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white and highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from sources of moisture, including the water vapor present in ambient air. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis. No mineral of above given composition was found yet. However, a very rare mineral simonkolleite, Zn5(OH)8Cl2·H2O, is known.
Structure and properties
Four crystalline forms, (polymorphs) , of ZnCl2 are known, and in each case the Zn2+ ions are tetrahedrally coordinated to four chloride ions. The pure anhydrous orthorhombic form rapidly changes to one of the other forms on exposure to the atmosphere and a possible explanation is that the presence of OH- facilitates the rearrangement. Rapid cooling of molten ZnCl2 gives a glass, that is, a rigid amorphous solid and this ability has been related to the structure in the melt.
The covalent character of the anhydrous material is indicated by its relatively low melting point of 275 °C. Further evidence for covalency is provided by the high solubility of the dichloride in ethereal solvents where it forms adducts with the formula ZnCl2L2, where L = ligand such as O(C2H5)2. In the gas phase, ZnCl2 molecules are linear with a bond length of 205 pm.
Molten ZnCl2 has a high viscosity at its melting point and a comparatively low electrical conductivity that increases markedly with temperature.. A raman spectra study of the melt indicated the presence of polymeric structures and a neutron scattering study indicated the presence of tetrahedral complexes.
Hydrates
Five hydrates of zinc chloride are known, ZnCl2(H2O)n where n = 1, 1.5, 2.5, 3 and 4. The tetrahydrate ZnCl2(H2O)4 crystallizes from aqueous solutions of zinc
chloride.
Preparation and purification
Anhydrous ZnCl2 can be prepared from zinc and hydrogen chloride.
- Zn + 2 HCl ? ZnCl2 + H2
Hydrated forms and aqueous solutions may be readily prepared similarly by treating Zn metal with hydrochloric acid. Zinc oxide and zinc sulfide react with HCl:
- ZnS(s) + 2 HCl ? ZnCl2(aq) + H2S(g)
Unlike many other elements, zinc essentially exists in only one oxidation state, 2+, which simplifies purification of the chloride.
Commercial samples of zinc chloride typically contain water and products from hydrolysis as impurities. Such samples may be purified by recrystallization from hot dioxane . Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating the submlimate to 400 °C in a stream of dry nitrogen gas. Finally, the simplest method relies on treating the zinc chloride with thionyl chloride.
Reactions
Molten anhydrous ZnCl2 at 500 - 700 °C dissolves zinc metal and on rapid cooling of the melt a yellow diamagnetic glass is formed which Raman studies indicate contain the Zn22+ ion.
A number of salts containing the tetrachlorozincate anion, ZnCl42-, are known . "Caulton's reagent," V2Cl3(thf)6Zn2Cl6 is an example of a salt containing Zn2Cl62-.
The compound Cs3ZnCl5 contains tetrahedral ZnCl42- and Cl- anions. No compounds containing the ZnCl64- ion have been characterised.
Whilst zinc chloride is very soluble in water, solutions cannot be considered to contain simply solvated Zn2+ ions and Cl- ions, ZnClxH2O(4-x) species are also present.
Aqueous solutions of ZnCl2 are acidic: a 6 M aqueous solution has a pH of 1. The acidity of aqueous ZnCl2 solutions relative to solutions of other Zn2+ salts is due to the formation of the tetrahedral chloro aqua complexes where the reduction in coordination number from 6 to 4 further reduces the strength of the O-H bonds in the solvated water molecules.
In alkali solution in the presence of OH- ion various zinc hydroxychloride anions are present in solution, e.g.ZnOH3Cl2-, ZnOH2Cl22-, ZnOHCl32-, and Zn5OH2Cl3.H2O (simonkolleite) precipitates.
When ammonia is bubbled through a solution of zinc chloride the hydroxide does not precipitate, instead compounds containing complexed ammonia (ammines) are produced, Zn(NH3)4Cl2.H2O and on concentration ZnCl2(NH3)2. The former contains the Zn(NH3)62+ ion and the latter is molecular with a distorted tetrahedral geometry. The species in aqueous solution have been investigated and show that Zn(NH3)42+ is the main species present with Zn(NH3)3Cl+ also present at lower NH3:Zn ratio.
Aqueous zinc chloride reacts with zinc oxide to form an amorphous cement which was first investigated in the 1855 by Sorel,who later he went on to investigate the related magnesium oxychloride cement which bears his name.
When hydrated zinc chloride is heated, one obtains a residue of Zn(OH)Cl e.g.
- ZnCl2.2H2O ? ZnCl(OH) +2HCl +H2O
The compound ZnCl2.½HCl.H2O may be prepared by careful precipitation from a solution of ZnCl2 acidified with HCl and it contains a polymeric anion (Zn2Cl5 -)n with balancing monohydrated hydronium ions, H5O2+ ions.
The formation of highly reactive anhydrous HCl gas formed when zinc chloride hydrates are heated is the basis of qualitative inorganic spot tests.
The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride, involves the production of HCl and its subsequent reaction with surface oxides. Zinc chloride forms two salts with ammonium chloride, (NH4)ZnCl4 and (NH4)3ClZnCl4, which decompose on heating liberating HCl just as zinc chloride hydrate does. The action of zinc chloride/ammonium chloride fluxes, for example, in the hot dip galvanizing process produces H2 gas and ammonia fumes.
Thus, although many zinc salts have different formulas and different crystal structures, these salts behave very similarly in aqueous solution. For example, solutions prepared from any of the polymorphs of ZnCl2 as well as other halides (bromide, iodide) and the sulfate can often be used interchangeably for the preparation of other zinc compounds. Illustrative is the preparation of zinc carbonate:
- ZnCl2(aq) + Na2CO3(aq) ? ZnCO3(s) + 2 NaCl(aq)
Applications
As a metallurgical flux
Zinc chloride has the ability to attack metal oxides (MO) to give derivatives of the formula MZnOCl2. This reaction is relevant to the utility of ZnCl2 as a flux for soldering - it dissolves oxide coatings exposing the clean metal surface. Fluxes with ZnCl2 as an active ingredient are sometimes called "Tinner's Fluid." Typically this flux was prepared by dissolving zinc foil in dilute hydrochloric acid until the liquid ceased to evolve hydrogen; for this reason, such flux was once known as killed spirits. Because of its corrosive nature, this flux is not suitable for situations where any residue cannot be cleaned away, such as electronic work. This property also leads to its use in the manufacture of magnesia cements for dental fillings and certain mouthwashes as an active ingredient.
In organic synthesis
In the laboratory, zinc chloride finds wide use, principally as a moderate-strength Lewis acid. It can catalyse (A) the Fischer indole synthesis, and also (B) Friedel-Crafts acylation reactions involving activated aromatic rings
Related to the latter is the classical preparation of the dye fluorescein from phthalic anhydride and resorcinol, which involves a Friedel-Crafts acylation. This transformation has in fact been accomplished using even the hydrated ZnCl2 sample shown in the picture above.
Hydrochloric acid alone reacts poorly with primary alcohols and secondary alcohols, but a combination of HCl with ZnCl2 (known together as the "Lucas reagent") is effective for the preparation of alkyl chlorides. Typical reactions are conducted at 130 °C. This reaction probably proceeds via an SN2 mechanism with primary alcohols but SN1 pathway with secondary alcohols.
Zinc chloride also activates benzylic and allylic halides towards substitution by weak nucleophiles such as alkenes:
In similar fashion, ZnCl2 promotes selective NaBH3CN reduction of tertiary, allylic or benzylic halides to the corresponding hydrocarbons.
Zinc chloride is also a useful starting reagent for the synthesis of many organozinc reagents, such as those used in the palladium catalysed Negishi coupling with aryl halides or vinyl halides. In such cases the organozinc compound is usually prepared by transmetallation from an organolithium or a Grignard reagent, for example:
Zinc enolates, prepared from alkali metal enolates and ZnCl2, provide control of stereochemistry in aldol condensation reactions due to chelation on to the zinc. In the example shown below, the threo product was favored over the erythro by a factor of 5:1 when ZnCl2 in DME/ether was used. The chelate is more stable when the bulky phenyl group is pseudo-equatorial rather than pseudo-axial, i.e., threo rather than erythro.
In textile processing
Concentrated aqueous solutions of zinc chloride (more than 64% weight/weight zinc chloride in water) have the interesting property of dissolving starch, silk, and cellulose. Thus, such solutions cannot be filtered through standard filter papers. Relevant to its affinity for these materials, ZnCl2 is used as a fireproofing agent and in fabric "refresheners" such as Febreze
Smoke bombs
Smoke bombs ("HC") contain zinc oxide and hexachloroethane which when ignited react to form zinc chloride smoke.
Safety considerations
Zinc chloride is a skin and respiratory irritant according to its MSDS. Precautions that apply to anhydrous ZnCl2 are those applicable to other anhydrous metal halides, i.e. hydrolysis can be exothermic and contact should be avoided. Concentrated solutions are acidic and corrosive and specifically attack cellulose and silk as Lewis acids. See MSDS in table.
Bibliography
- 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.
- G. J. McGarvey, in Handbook of Reagents for Organic Synthesis, Volume 1: Reagents, Auxiliaries and Catalysts for C-C Bond Formation, (R. M. Coates, S. E. Denmark, eds.), pp. 220-3, Wiley, New York, 1999.
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