Everything about Zinc Chloride totally explained
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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. ZnCl
2 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.
Structure and basic properties
Four crystalline forms, so-called
polymorphs, of ZnCl are known, and in each case the Zn
2+ ions are tetrahedrally coordinated to four chloride ligands. Rapid cooling of molten ZnCl
2 gives a glass, that is, a rigid amorphous solid. Additionally ZnCl
2 forms
hydrates and at least one mixed hydroxide, ZnClOH.
Four hydrates of zinc chloride are known. ZnCl
2(H
2O)
4 crystallizes from aqueous solutions of zinc chloride. Also characterized are ZnCl
2(H
2O)
n where n = 1, 1.5, 2.5, and 3.
Applications
As a metallurgical flux
Zinc chloride has the ability to attack metal oxides (MO) to give derivatives of the formula MZnOCl
2. This reaction is relevant to the utility of ZnCl
2 as a
flux for
soldering - it dissolves oxide coatings exposing the clean metal surface., 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 ZnCl
2 sample shown in the picture above.
Hydrochloric acid alone reacts poorly with
primary alcohols and
secondary alcohols, but a combination of HCl with ZnCl
2 (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, ZnCl
2 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 ZnCl
2, 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 ZnCl
2 in
DME/
ether was used. The chelate is more stable when the bulky phenyl group is pseudo-
equatorial rather than pseudo-
axial, for example,
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 can't be filtered through standard filter papers. Relevant to its affinity for these materials, ZnCl
2 is used as a fireproofing agent and in fabric "refresheners" such as Febreze
Safety considerations
Zinc salts are relatively non-toxic. Precautions that apply to anhydrous ZnCl
2 are those applicable to other anhydrous metal halides, for example 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.
Further Information
Get more info on 'Zinc Chloride'.
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