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Zinc chloride

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

                                 Zinc chloride
                            Zinc chloride hydrate
                                    General
       Systematic name                                          Zinc chloride
           Other names                         Zinc(II) chloride,
                                                             zinc dichloride,
                                                               butter of zinc
     Molecular formula                                                ZnCl[2]
            Molar mass                                           136.29 g/mol
            Appearance                               White crystalline solid.
            CAS number                                            [7646-85-7]
                                  Properties
     Density and phase                                     2.907 g/cm³, solid
   Solubility in water                                   432 g/100 mL (25 °C)
            in ethanol                                 100 g/100 mL (12.5 °C)
           in acetone,
         diethyl ether                                                Soluble
         Melting point                                         275 °C (548 K)
         Boiling point                                        756 °C (1029 K)
                                   Structure
          Coordination
              geometry                             Tetrahedral, 4-coordinate,
                                                     linear in the gas phase.
     Crystal structure                     Four forms known
                                                  Hexagonal close-packed (δ)
                                                      is the only stable form
                                                              when anhydrous.
         Dipole moment                                                    ? D
                                    Hazards
                  MSDS                                          External MSDS
     EU classification                                          Irritant (I),
                                                                Corrosive(C).
              NFPA 704 Image:nfpa_h3.png Image:nfpa_f0.png Image:nfpa_r0.png
             R-phrases                                          R34, R50, R53
             S-phrases                                   S7/8, S28, S45, S60,
                                                                          S61
          RTECS number                                              ZH1400000
                            Supplementary data page
         Structure and
            properties                                          n, ε[r], etc.
    Thermodynamic data                                        Phase behaviour
                                                           Solid, liquid, gas
         Spectral data                                        UV, IR, NMR, MS
                               Related compounds
          Other anions                            Zinc fluoride,
                                                                zinc bromide,
                                                                  zinc iodide
         Other cations                                   Copper(II) chloride,
                                                             cadmium chloride
               Except where noted otherwise, data are given for
                       materials in their standard state (at 25°C, 100 kPa)
                                           Infobox disclaimer and references

   Zinc chloride is the name of chemical compound ZnCl[2] or its hydrates.
   All zinc chlorides are colorless or white and highly soluble in water.
   ZnCl[2] itself is hygroscopic and can be considered deliquescent.
   Samples should therefore be protected from sources of moisture, such as
   the atmosphere.

   Four crystalline forms of ZnCl[2] and its hydrates are known, although
   the anhydrous form appears to exist only in the hexagonal close-packed
   phase. Rapid cooling of molten ZnCl[2] forms a glassy material.

   Concentrated aqueous solutions of zinc chloride have the interesting
   property of dissolving starch, silk, and cellulose. Thus, solutions of
   zinc chlorides cannot be filtered through standard filter papers.

   Zinc chloride finds wide application in textile processing,
   metallurgical fluxes and chemical synthesis.

Chemical properties

   ZnCl[2] is an ionic solid, although some covalent character is
   indicated by its low melting point (275 °C). Further evidence for
   covalency is provided by its high solubility in solvents such as
   diethyl ether. ZnCl[2] is a mild Lewis acid. Consistent with this
   character, aqueous solutions of ZnCl[2] 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 Zn^2+ for the
   preparation of other zinc salts, for example zinc carbonate:

          ZnCl[2]( aq) + Na[2]CO[3]( aq) → ZnCO[3]( s) + 2 NaCl( aq)

Preparation and purification

   Anhydrous ZnCl[2] can be prepared from zinc and hydrogen chloride.

          Zn + 2 HCl → ZnCl[2] + H[2]

   Hydrated forms and aqueous solutions may be readily prepared using
   concentrated hydrochloric acid and pieces of Zn. Zinc oxide and zinc
   sulfide react with HCl, without forming hydrogen:

          ZnS( s) + 2 HCl( aq) → ZnCl[2]( aq) + H[2]S( 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 ZnCl[2] 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),and then the filtrate is allowed to
   cool to give pure ZnCl[2] as a white precipitate. Anhydrous samples can
   be purified by sublimation in a stream of hydrogen chloride gas,
   followed by heating to 400 °C in a stream of dry nitrogen gas.

Uses

   ZnCl[2] is used as a flux for soldering because of its ability (when
   molten) to dissolve metal oxides. 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 known as
   killed spirits. because of its corrosive nature it is not a suitable
   flux for situations where any residue cannot be cleaned totally 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. ZnCl[2] has also been used as a
   fireproofing agent, for etching metals, and is also a primary
   ingredient in fabric refresheners such as Febreze.

   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 wet 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 S[N]2 mechanism with primary alcohols
   but S[N]1 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 NaBH[3]CN 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, i.e., threo rather
   than erythro.

Precautions

   Corrosive, irritant. Wear gloves and goggles.
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