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Italian: Clorato di zinco
English: Chloride of Zinc
Pure chloride of zinc is triturated, as directed under Class VII, but owing to the deliquescent of the salt, the lower triturations will not keep well.
Minerals; Inorganic; Zinc-Group
Proven by Dr. Franz.
Allen: Encyclop. of Mat. Med., Vol. X, 215; Clarke: A Dictionary of Pract. Mat. Med., Vol. III, 1599.
Description of the substance
Zinc chloride (ZnCl2) is a colorless or white compound of zinc and chlorine that is extremely hygroscopic.
Four crystalline structures have been reported, although in pure form (i.e. water-free) only the delta (hexagonal close-packed) phase can form. It can be quenched from the melt to form a glassy material.
Concentrated aqueous solutions of zinc chloride have the interesting property of dissolving starch, silk and cellulose, so that solutions cannot be filtered through standard filter papers.
Zinc chloride finds wide application in textile processing, metallurgical fluxes and chemical synthesis.
Zinc chloride is an ionic salt, though some covalent character is indicated by its low melting point (275 °C) and its high solubility in solvents such as diethyl ether. It behaves as a mild Lewis acid, and aqueous solutions have a pH around 4. It is hydrolyzed to an oxychloride when hydrated forms are heated.
In aqueous solution, zinc chloride is a useful source of Zn2+ for the preparation of other zinc salts, for example zinc carbonate:
ZnCl2(aq) + Na2CO3(aq) → ZnCO3(s) + 2 NaCl(aq)
Preparation & purification
Anhydrous zinc chloride can be prepared from zinc and hydrogen chloride.
Zn + 2 HCl(g) → ZnCl2(s) + H2(g)
Hydrated forms and aqueous solutions may be readily prepared using standard acid-base methods, or from one of its ores, zinc sulfide:
ZnS(s) + 2 HCl(aq) → ZnCl2(aq) + H2S(l)
Commercial samples of zinc chloride typically contain water and zinc oxychloride, the main hydrolysis product. Such samples may be purified as follows: 100 g of crude ZnCl2 are heated to reflux in 800 mL anhydrous dioxan in the presence of zinc metal dust. The mixture is filtered while hot (to remove Zn), then allowed to cool to give pure ZnCl2 as a white precipitate. Anhydrous samples can be purified by sublimation in a stream of hydrogen chloride gas, followed by heating up to 400 °C in a stream of dry nitrogen gas.
One use for zinc chloride is as a flux for soldering. This is because of its ability (when molten) to dissolve metal oxides. This property also leads to its use in the manufacture of magnesia cements for dental fillings. ZnCl2 has also been used as a fireproofing agent and for etching metals.
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 has in fact been done successfully using the wet 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") at 130 °C is effective for the preparation of alkyl chlorides. This probably reacts via an SN2 mechanism with primary alcohols but via SN1 with secondary alcohols.
Zinc chloride is also able to activate 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 point 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. This is because the chelate is more stable when the bulky phenyl group is pseudo-equatorial rather than pseudo-axial, i.e., threo rather than erythro.
Zinc Chloride Solution slowly corrodes steel. Madison Industries recommends rubber-lined steel tanks or the more economical fiber glass reinforced polyester tanks for storage. Since solutions of Zinc Chloride above 67 percent crystallize at 40oF,it should not be stored outside unless heating coils and insulation are provided. Coils should be fabricated from Monel, Inconel or Teflon. Alternately an electrical heating tape may be affixed to the outside of the tank.
Centrifugal pumps constructed of type 316 stainless steel, rubber-lined steel, FRP, glass-filled epoxy resins or other high quality engineering plastics are suggested for transfer of Zinc Chloride Solution.
Zinc Chloride Solutions are acidic, astringent and may cause skin irritation or even burns. Contact with skin or eyes should be avoided through the use of face shields, rubber aprons and rubber gloves. Protective clothing should be employed in cases where contact with the solution is probable. In case of contact, flush eyes and skin with water for 15 minutes. Remove contaminated clothing and consult a physician. A paste of baking soda can be applied to help neutralize acid remaining on the skin. Obtain prompt medical attention for eyes. Contaminated clothing should be thoroughly laundered prior to reuse. Do not take internally. If Zinc Chloride Solution is ingested, administer large quantities of water or milk. Do not induce vomiting. Avoid inhaling mist or fumes. The Threshold Limit Value (TLV) of Zinc Chloride is 1 milligram per cubic meter of air for a 8 hour work-day. Exposure to fumes can be minimized by adequate ventilation or exhaust fans. In case of spills, flush area with plenty of water to chemical sewers. Zinc Chloride Solution is non-flammable.