Zinc chloride

The chloride zinc is a salt of Zinc of Chemical formula ZnCl₂. It is a white or colorless solid, extremely Hygroscopique.

Four crystalline structures different were observed, but only the form δ (hexagonal compact) can be formed when the compound is pure and anhydrous. If it undergoes a fast Trempe, ZnCl₂ can be obtained in vitreous form .

Zinc the concentrated aqueous chloride solutions have the property to dissolve the Amidon, the Soie and the Cellulose. They cannot thus be filtered through paper of standard filtration.

The zinc chloride has many applications in textile industry, the Métallurgie and the chemical synthesis.

Chemical properties

The zinc chloride is a ionic salt , which has a covalent character all the same which is highlighted by its Melting point relatively low (275°C) and its Solubilité high in solvents like the ether. It behaves like a Acide of Lewis) moderated, and the zinc chloride solutions have a pH close to 4. When it is heated in a hydrated form, it Hydrolyze to form zinc oxychloride.

In aqueous solution, the zinc chloride is a source of Ion S Zn²⁺ useful for the preparation of other zinc salts such as for example the Carbonate of zinc:

ZnCl₂ (aq) + Na₂CO₃ (aq) → ZnCO₃ (S) + 2 NaCl (aq)

Preparation and purification

The anhydrous zinc chloride is obtained starting from Zinc Métal and from Chlorure of hydrogen:

Zn + 2 HCl (G) → ZnCl₂ (S) + H₂ (G)

The hydrated forms and salt in aqueous solution can be prepared directly by using a acido-basic Réaction, for example starting from the ore of Sulfure of zinc:

ZnS (S) + 2 HCl (aq) → ZnCl₂ (aq) + H₂S (L)

The commercial zinc chloride generally contains) and zinc oxychloride produced by Hydrolyze. These products can be purified as follows: 100 G of commercial ZnCl₂ are heated with backward flow in 800 ml of anhydrous Dioxane in the presence of lic dust of Zinc Métal. The mixture is filtered hot (to withdraw the Zinc), then cooled to form a precipitated white of pure zinc chloride. The anhydrous samples can be purified by sublimation under a flow of Chlorure of hydrogen, followed by a heating to 400°C under a flow of dry Azote.

Applications

The zinc chloride is in particular used for the Soudage, because of its capacity to dissolve the metal Oxyde S at the time which it is in a molten state. This property also makes it possible to use it in cements containing Oxyde of magnesium for the dental prostheses. ZnCl₂ is also used as agent of metal etching fireproofing and as solution.

The zinc chloride is also used in the Galvanisation, like in the production of batteries.

In laboratory, the zinc chloride is used as a moderate Acide of Lewis. It can play the part of Catalyze ur for the synthesis of Fisher of indol (A) or for the acylation of Friedel-Crafts bringing into play a aromatic Composé (B):

It is in particular used for the preparation of Fluorescéine by acylation of Friedel-Crafts starting from phtalic Anhydride and of Résorcinol:

The Hydrochloric acid only badly reacts with the primary education alcohols and the secondary alcohols, but a mixture of Hydrochloric acid and zinc chloride (mixture known under the name of " reagent of Lucas"), allows the preparation of chlorides alkyls. The mechanism of this reaction is probably of type SN2 with primary education alcohols and of type SN1 with secondary alcohols.

The zinc chloride is able to activate allyl or benzyl halides what allows reactions of substitution by weak Nucléophile S like the Alcène S:

The zinc chloride is also a reagent used for the synthesis of many reagents of the organozincites type, such as for example those used for the coupling of Negishi with vinyl or aryl halides. The organozincite is then generally prepared by metal exchange with one organolithié or a Réactif of Hard grindstone, such as for example in:

The zinc énolates, prepared starting from énolates of alkaline metals and zinc chloride, make it possible to control the Stéréochimie during reactions of aldol condensation, because of Chélation of zinc. In the example below, the product thréo is favoured compared to the product érythro (factor 5/1) when is used zinc chloride:

Safety

The zinc chloride is corrosive and irritant, and must be handled with precautions. It is advised to carry glasses of safety and gloves.

R: 34 (Causes burns)
R: 50 (Very toxic for the watery organizations)
R: 53 (Can involve harmful effects in the long run for the watery environment)
S: 7 (To preserve the well closed container)
S: 8 (To preserve the container safe from moisture)
S: 28 (After contact with the skin, to wash itself immediately and abundantly with… (specified by the manufacturer))
S: 45 In the event of accident or of faintness, to consult a doctor immediately (if possible to show him the label)
S: 60 (To eliminate the product and its container like a dangerous waste)
S: 61 (To avoid the rejection in the environment. To consult the instructions spéciales/la card of data of safety)

References

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