Zinc chloride#As a metallurgical flux

Zinc chloride has long been known but currently practiced industrial applications all evolved in the latter half of 20th century.

An amorphous cement formed from aqueous zinc chloride and zinc oxide was first investigated in 1855 by Stanislas Sorel. Sorel later went on to investigate the related magnesium oxychloride cement, which bears his name.{{cite book |author1=Wilson, A. D. |author2=Nicholson, J. W. | year = 1993 | title = Acid-Base Cements: Their Biomedical and Industrial Applications | publisher = Cambridge University Press | isbn = 978-0-521-37222-0 }}

Dilute aqueous zinc chloride was used as a disinfectant under the name "Burnett's Disinfecting Fluid".{{cite book | author = Watts, H. | year = 1869 | title = A Dictionary of Chemistry and the Allied Branches of Other Sciences | publisher = Longmans, Green | url = https://archive.org/details/adictionarychem11wattgoog }} From 1839 Sir William Burnett promoted its use as a disinfectant as well as a wood preservative. The Royal Navy conducted trials into its use as a disinfectant in the late 1840s, including during the cholera epidemic of 1849; and at the same time experiments were conducted into its preservative properties as applicable to the shipbuilding and railway industries. Burnett had some commercial success with his eponymous fluid. Following his death however, its use was largely superseded by that of carbolic acid and other proprietary products.{{cite journal |last1=McLean |first1=David |title=Protecting wood and killing germs: 'Burnett's Liquid' and the origins of the preservative and disinfectant industries in early Victorian Britain |journal=Business History |date=April 2010 |volume=52 |issue=2 |pages=285–305|doi=10.1080/00076791003610691 |s2cid=154790730 }}

Unlike other metal dichlorides, zinc dichloride adopts several crystalline forms (polymorphs). Four polymorph are known: α, β, γ, and δ. Each features {{chem2|Zn(2+)}} centers surrounded in a tetrahedral manner by four chloride ligands.{{cite book | author = Wells, A. F. | year = 1984 | title = Structural Inorganic Chemistry | location = Oxford | publisher = Clarendon Press | isbn = 978-0-19-855370-0 }}

Here a, b, and c are lattice constants, Z is the number of structure units per unit cell, and ρ is the density calculated from the structure parameters.{{cite journal | last1 = Oswald | first1 = H. R. | last2 = Jaggi | first2 = H. | title = Zur Struktur der wasserfreien Zinkhalogenide I. Die wasserfreien Zinkchloride | journal = Helvetica Chimica Acta | year = 1960 | volume = 43 | issue = 1 | pages = 72–77 | doi = 10.1002/hlca.19600430109 }}{{cite journal | last1 = Brynestad | first1 = J. | last2 = Yakel | first2 = H. L. | title = Preparation and Structure of Anhydrous Zinc Chloride | journal = Inorganic Chemistry | year = 1978 | volume = 17 | issue = 5 | pages = 1376–1377 | doi = 10.1021/ic50183a059 }}{{cite journal | last1 = Brehler | first1 = B. | title = Kristallstrukturuntersuchungen an ZnCl₂ | journal = Zeitschrift für Kristallographie | year = 1961 | volume = 115 | issue = 5–6 | pages = 373–402 | doi = 10.1524/zkri.1961.115.5-6.373 | bibcode = 1961ZK....115..373B }}

The orthorhombic form (δ) rapidly changes to another polymorph upon exposure to the atmosphere. A possible explanation is that the {{chem2|OH−}} ions originating from the absorbed water facilitate the rearrangement. Rapid cooling of molten {{chem2|ZnCl2}} gives a glass.{{cite journal |author1=Mackenzie, J. D. |author2=Murphy, W. K. | title = Structure of Glass-Forming Halides. II. Liquid Zinc Chloride | journal = The Journal of Chemical Physics | year = 1960 | volume = 33 | issue = 2 | pages = 366–369 | doi = 10.1063/1.1731151 |bibcode=1960JChPh..33..366M }}

Molten {{chem2|ZnCl2}} has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature.{{cite book | author = Prince, R. H. | year = 1994 | title = Encyclopedia of Inorganic Chemistry | editor = King, R. B. | publisher = John Wiley & Sons | isbn = 978-0-471-93620-6 }}{{cite book | author = Ray, H. S. | year = 2006| title = Introduction to Melts: Molten Salts, Slags and Glasses | publisher = Allied Publishers | isbn = 978-81-7764-875-1 }} As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,.{{cite book | author = Danek, V. | year = 2006 | title = Physico-Chemical Analysis of Molten Electrolytes | publisher = Elsevier | isbn = 978-0-444-52116-3 }} Neutron scattering study indicated the presence of tetrahedral {{chem2|ZnCl4}} centers, which requires aggregation of {{chem2|ZnCl2}} monomers as well.{{cite journal | last1 = Price | first1 = D. L. | last2 = Saboungi | first2 = M.-L. | last3 = Susman | first3 = S. | last4 = Volin | first4 = K. J. | last5 = Wright | first5 = A. C. | title = Neutron Scattering Function of Vitreous and Molten Zinc Chloride | journal = Journal of Physics: Condensed Matter | year = 1991 | volume = 3 | issue = 49 | pages = 9835–9842 | doi = 10.1088/0953-8984/3/49/001 | bibcode = 1991JPCM....3.9835P | s2cid = 250902741 }}

A variety of hydrated zinc chloride are known: {{chem2|ZnCl2(H2O)_{n}|}} with n = 1, 1.33, 2.5, 3, and 4.5.{{cite book |author1=Holleman, A. F. |author2=Wiberg, E. | title = Inorganic Chemistry | publisher = Academic Press | location = San Diego | year = 2001 | isbn = 978-0-12-352651-9 }} The 1.33-hydrate, previously thought to be the hemitrihydrate, consists of trans-Zn(H₂O)₄Cl₂ centers with the chlorine atoms connected to repeating ZnCl₄ chains. The hemipentahydrate, structurally formulated [Zn(H₂O)₅][ZnCl₄], consists of Zn(H₂O)₅Cl octahedrons where the chlorine atom is part of a [ZnCl₄]^2- tetrahedera. The trihydrate consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions; formulated [Zn(H₂O)₆][ZnCl₄]. Finally, the heminonahydrate, structurally formulated [Zn(H₂O)₆][ZnCl₄]·3H₂O also consists of distinct hexaaquozinc(II) cations and tetrachlorozincate anions like the trihydrate but has three extra water molecules. These hydrates can be produced by evaporation of aqueous solutions of zinc chloride at different temperatures.{{cite journal |author1=H. Follner |author2=B. Brehler |title=Die Kristallstruktur des ZnCl2.4/3H2O |journal=Acta Crystallographica B |date=1970 |volume=26 |issue=11 |pages=1679–1682 |doi=10.1107/S0567740870004715 |bibcode=1970AcCrB..26.1679F |trans-title=The crystal structure of ZnCl2.4/3H2O |language=de}}{{cite journal |author1=E. Hennings |author2=H. Schmidt |author3=W. Voigt |title=Crystal structures of ZnCl2·2.5H2O, ZnCl2·3H2O and ZnCl2·4.5H2O |journal=Acta Crystallographica E |date=2014 |volume=70 |issue=12 |pages=515–518 |doi=10.1107/S1600536814024738 |pmid=25552980 |language=en|pmc=4257420 }}

Historically, zinc chlorides are prepared from the reaction of hydrochloric acid with zinc metal or zinc oxide. Aqueous acids cannot be used to produce anhydrous zinc chloride. According to an early procedure, a suspension of powdered zinc in diethyl ether is treated with hydrogen chloride, followed by drying{{cite journal |doi=10.1039/JR9320002282 |title=Notes: The Preparation of Pure Zinc Chloride |date=1932 |last1=Hamilton |first1=R. T. |last2=Butler |first2=J. A. V. |journal=Journal of the Chemical Society (Resumed) |pages=2283–4 }} The overall method remains useful in industry, but without the solvent:

: {{chem2|Zn + 2 HCl → ZnCl2 + H2}}

Aqueous solutions may be readily prepared similarly by treating Zn metal, zinc carbonate, zinc oxide, and zinc sulfide with hydrochloric acid:{{cite book |doi=10.1002/0471238961.2609140307151504.a02.pub3 |chapter=Zinc Compounds |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2017 |first1=Frank E. |last1=Goodwin |pages=9–10 |isbn=978-0-471-23896-6 }}

: {{chem2|ZnS + 2 HCl + 4 H2O → ZnCl2(H2O)4 + H2S}}

Hydrates can be produced by evaporation of an aqueous solution of zinc chloride. The temperature of the evaporation determines the hydrates For example, evaporation at room temperature produces the 1.33-hydrate.{{cite journal |author1=F. Mylius |author2=R. Dietz |title=Über das Chlorzink. (Studien über die Löslichkeit der Salze XIV.) |journal=Zeitschrift für anorganische Chemie |date=1905 |volume=44 |issue=1 |pages=209–220 |doi=10.1002/zaac.19050440115 |language=en}} Lower evaporation temperatures produce higher hydrates.

Commercial samples of zinc chloride typically contain water and products from hydrolysis as impurities. Laboratory 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 sublimate to 400 °C in a stream of dry nitrogen gas.{{cite journal |doi=10.1002/047084289X.rz007.pub3|title=Zinc chloride|author=Glenn J. McGarvey Jean-François Poisson Sylvain Taillemaud|year=2016|journal=Encyclopedia of Reagents for Organic Synthesis|pages=1–20|isbn=978-0-470-84289-8}} A simple method relies on treating the zinc chloride with thionyl chloride.{{cite book | author = Pray, A. P. | title=Anhydrous Metal Chlorides|series = Inorganic Syntheses | year = 1990 | volume = 28 | pages = 321–322}}

A number of salts containing the tetrachlorozincate anion, {{chem2|[ZnCl4](2−)}}, are known. "Caulton's reagent", {{chem2|V2Cl3(thf)6] [Zn2Cl6]}}, which is used in organic chemistry, is an example of a salt containing {{chem2|[Zn2Cl6](2−)}}.{{cite book | volume = 3 |editor1=Mulzer, J. |editor2=Waldmann, H. | title = Organic Synthesis Highlights | year = 1998 | publisher = Wiley-VCH | isbn = 978-3-527-29500-5 }}{{cite journal | last1 = Bouma | first1 = R. J. | last2 = Teuben | first2 =J. H. | last3 = Beukema | first3 = W. R. | last4 = Bansemer | first4 = R. L. | last5 = Huffman | first5 = J. C. | last6 = Caulton | first6 = K. G. | title = Identification of the Zinc Reduction Product of VCl₃ · 3THF as [V₂Cl₃(THF)₆]₂[Zn₂Cl₆] | journal = Inorganic Chemistry | year = 1984 | volume = 23 | issue = 17 | pages = 2715–2718 | doi = 10.1021/ic00185a033 }} The compound {{chem2|Cs3ZnCl5}} contains tetrahedral {{chem2|[ZnCl4](2−)}} and Chloride anions, so, the compound is not caesium pentachlorozincate, but caesium tetrachlorozincate chloride. No compounds containing the {{chem2|[ZnCl6](4−)}} ion (hexachlorozincate ion) have been characterized. The compound {{chem2|ZnCl2*0.5HCl*H2O}} crystallizes from a solution of {{chem2|ZnCl2}} in hydrochloric acid. It contains a polymeric anion {{chem2|(Zn2Cl5−)_{n}|}} with balancing monohydrated hydronium ions, {{chem2|H5O2+}} ions.

File:CSD CIF TOCMON02.png

The adduct with thf {{chem2|ZnCl2(thf)2}} illustrates the tendency of zinc chloride to form 1:2 adducts with weak Lewis bases. Being soluble in ethers and lacking acidic protons, this complex is used in the synthesis of organozinc compounds.{{cite journal |doi=10.1002/zaac.19976230163 |title=Difluorenylzink als Alkylierungsmittel zur Darstellung von Triorganometallanen der 13. Gruppe. Synthese und Kristallstruktur von [GaFl₃(THF)] · Toluol (Fl = Fluorenyl) |date=1997 |last1=Dashti |first1=Anahita |last2=Niediek |first2=Katharina |last3=Werner |first3=Bert |last4=Neumüller |first4=Bernhard |journal=Zeitschrift für Anorganische und Allgemeine Chemie |volume=623 |issue=1–6 |pages=394–402 }} A related 1:2 complex is {{chem2|ZnCl2(NH2OH)2}} (zinc dichloride di(hydroxylamine)). Known as Crismer's salt, this complexes releases hydroxylamine upon heating.{{cite book |doi=10.1002/9780470132401.ch2|chapter=Dichlorobis(hydroxylamine)zinc(II) (Crismer's Salt)|year=1967|volume=9|last1=Walker|first1=John E.|last2=Howell|first2=David M.|title=Inorganic Syntheses|pages=2–3|isbn=978-0-470-13240-1}} The distinctive ability of aqueous solutions of {{chem2|ZnCl2}} to dissolve cellulose is attributed to the formation of zinc-cellulose complexes, illustrating the stability of its adducts.{{cite journal |author1=Xu, Q. |author2=Chen, L.-F. | title = Ultraviolet Spectra and Structure of Zinc-Cellulose Complexes in Zinc Chloride Solution | journal = Journal of Applied Polymer Science | year = 1999 | volume = 71 | issue = 9 | pages = 1441–1446 | doi = 10.1002/(SICI)1097-4628(19990228)71:9<1441::AID-APP8>3.0.CO;2-G }} Cellulose also dissolves in molten {{chem2|ZnCl2}} hydrate.{{cite journal | last1 = Fischer | first1 = S. | last2 = Leipner | first2 = H. | last3 = Thümmler | first3 = K. | last4 = Brendler | first4 = E. | last5 = Peters | first5 = J. | title = Inorganic Molten Salts as Solvents for Cellulose | journal = Cellulose | year = 2003 | volume = 10 | issue = 3 | pages = 227–236 | doi = 10.1023/A:1025128028462 | s2cid = 92194004 }} Overall, this behavior is consistent with Zn²⁺ as a hard Lewis acid.

When solutions of zinc chloride are treated with ammonia, diverse ammine complexes are produced. In addition to the tetrahedral 1:2 complex {{chem2|ZnCl2(NH3)2}}.{{cite journal | last1 = Yamaguchi | first1 = T. | last2 = Lindqvist | first2 = O. | title = The Crystal Structure of Diamminedichlorozinc(II), ZnCl₂(NH₃)₂. A New Refinement | journal = Acta Chemica Scandinavica A | year = 1981 | volume = 35 | issue = 9 | pages = 727–728 | doi = 10.3891/acta.chem.scand.35a-0727 | url = http://actachemscand.org/pdf/acta_vol_35a_p0727-0728.pdf | doi-access = free }}{{cite book | author = Vulte, H. T. | title = Laboratory Manual of Inorganic Preparations | publisher = Read Books | year = 2007 | isbn = 978-1-4086-0840-1 }}

the complex {{chem2|Zn(NH3)4Cl2*H2O}} also has been isolated. The latter contains the {{chem2|[Zn(NH3)6](2+)}} ion,. The species in aqueous solution have been investigated and show that {{chem2|[Zn(NH3)4](2+)}} is the main species present with {{chem2|[Zn(NH3)3Cl]+}} also present at lower {{chem2|NH3}}:Zn ratio.{{cite journal | last1 = Yamaguchi | first1 = T. | last2 = Ohtaki | first2 = H. | title = X-Ray Diffraction Studies on the Structures of Tetraammine- and Triamminemonochlorozinc(II) Ions in Aqueous Solution | journal = Bulletin of the Chemical Society of Japan | year = 1978 | volume = 51 | issue = 11 | pages = 3227–3231 | doi = 10.1246/bcsj.51.3227 | doi-access = free }}

Zinc chloride dissolves readily in water to give {{chem2|ZnCl_{x}(H2O)_{4−x}|}} species and some free chloride.{{cite journal | last1 = Irish | first1 = D. E. | last2 = McCarroll | first2 = B. | last3 = Young | first3 = T. F. | title = Raman Study of Zinc Chloride Solutions | journal = The Journal of Chemical Physics | year = 1963 | volume = 39 | issue = 12 | pages = 3436–3444 | doi = 10.1063/1.1734212 | bibcode = 1963JChPh..39.3436I }}{{cite journal | last1 = Yamaguchi | first1 = T. | last2 = Hayashi | first2 = S. | last3 = Ohtaki | first3 = H. | title = X-Ray Diffraction and Raman Studies of Zinc(II) Chloride Hydrate Melts, ZnCl₂ · R H₂O (R = 1.8, 2.5, 3.0, 4.0, and 6.2) | journal = The Journal of Physical Chemistry | year = 1989 | volume = 93 | issue = 6 | pages = 2620–2625 | doi = 10.1021/j100343a074 }}{{cite journal |author1=Pye, C. C. |author2=Corbeil, C. R. |author3=Rudolph, W. W. | title = An ab initio Investigation of Zinc Chloro Complexes | journal = Physical Chemistry Chemical Physics | year = 2006 | volume = 8 | issue = 46 | pages = 5428–5436 | doi = 10.1039/b610084h | issn = 1463-9076 | pmid = 17119651 |bibcode=2006PCCP....8.5428P |s2cid=37521287 }} Aqueous solutions of {{chem2|ZnCl2}} are acidic: a 6 M aqueous solution has a pH of 1. The acidity of aqueous {{chem2|ZnCl2}} solutions relative to solutions of other Zn²⁺ salts (say the sulfate) is due to the formation of the tetrahedral chloro aqua complexes such as [ZnCl₃(H₂O)]⁻.{{cite book |author = Brown, I. D. | year = 2006 | title = The Chemical Bond in Inorganic Chemistry: The Bond Valence Model | publisher = Oxford University Press | isbn = 978-0-19-929881-5 }} Most metal dichlorides form octahedral complexes, with stronger O-H bonds. The combination of hydrochloric acid and {{chem2|ZnCl2}} gives a reagent known as "Lucas reagent". Such reagents were once used as a test for primary alcohols. Similar reactions are the basis of industrial routes from methanol and ethanol respectively to methyl chloride and ethyl chloride.{{cite journal |author1=Kjonaas, R. A. |author2=Riedford, B. A. | title = A Study of the Lucas Test | journal = Journal of Chemical Education | year = 1991 | volume = 68 | issue = 8 | pages = 704 | doi = 10.1021/ed068p704 |bibcode=1991JChEd..68..704K }}

In alkali solution, zinc chloride converts to various zinc hydroxychlorides. These include {{chem2|[Zn(OH)3Cl](2−)}}, {{chem2|[Zn(OH)2Cl2](2−)}}, {{chem2|[Zn(OH)Cl3](2−)}}, and the insoluble {{chem2|Zn5(OH)8Cl2*H2O}}. The latter is the mineral simonkolleite.{{cite book | author = Zhang, X. G. | year = 1996 | title = Corrosion and Electrochemistry of Zinc | publisher = Springer | isbn = 978-0-306-45334-2 }} {{cite web |url=http://webmineral.com/data/Simonkolleite.shtml#.VEA-9SLF-vM |title= Simonkolleite Mineral Data |author=Staff writer(s)|website= webmineral.com |access-date= October 16, 2014}} When zinc chloride hydrates are heated, hydrogen chloride evolves and hydroxychlorides result.{{cite journal |author1=Feigl, F. |author2=Caldas, A. | title = Some Applications of Fusion Reactions with Zinc Chloride in Inorganic Spot Test Analysis | journal = Microchimica Acta | year = 1956 | volume = 44 | issue = 7–8 | pages = 1310–1316 | doi = 10.1007/BF01257465 |s2cid=96823985 }}

In aqueous solution {{chem2|ZnCl2}}, as well as other halides (bromide, iodide), behave interchangeably for the preparation of other zinc compounds. These salts give

precipitates of zinc carbonate when treated with aqueous carbonate sources:

:{{chem2|ZnCl2 + Na2CO3 → ZnCO3 + 2 NaCl}}

Ninhydrin reacts with amino acids and amines to form a colored compound "Ruhemann's purple" (RP). Spraying with a zinc chloride solution, which is colorless, forms a 1:1 complex RP:{{chem2|ZnCl(H2O)2}}, which is more readily detected as it fluoresces more intensely than RP.{{cite book | author = Menzel, E. R. | year = 1999 | title = Fingerprint Detection with Lasers | publisher = CRC Press | isbn = 978-0-8247-1974-6 }}

Anhydrous zinc chloride melts and even boils without any decomposition up to 900 °C. When zinc metal is dissolved in molten {{chem2|ZnCl2}} at 500–700 °C, a yellow diamagnetic solution is formed consisting of the {{chem2|Zn2(2+)}}, which has zinc in the oxidation state +1. The nature of this dizinc dication has been confirmed by Raman spectroscopy. Although {{chem2|Zn2(2+)}} is unusual, mercury, a heavy congener of zinc, forms a wide variety of {{chem2|Hg2(2+)}} salts.

In the presence of oxygen, zinc chloride oxidizes to zinc oxide above 400 °C. Again, this observation indicates the nonoxidation of Zn²⁺.{{cite journal |author1=Frida Jones |author2=Honghi Tran |author3=Daniel Lindberg |author4=Liming Zhao |author5=Mikko Hupa |title=Thermal Stability of Zinc Compounds |journal=Energy & Fuels |date=2013 |volume=27 |issue=10 |pages=5663–5669 |doi=10.1021/ef400505u |language=en}}

Concentrated aqueous zinc chloride dissolves zinc oxide to form zinc hydroxychloride, which is obtained as colorless crystals:{{cite book|author1=F. Wagenknecht|author2=R. Juza|chapter=Zinc Hydroxychloride|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY,NY|volume=2pages=1071}}

:{{chem2|ZnCl2 + ZnO + H2O -> 2 ZnCl(OH)}}

The same material forms when hydrated zinc chloride is heated.{{cite book | author = House, J. E. | year = 2008 | title = Inorganic Chemistry | publisher = Academic Press | isbn = 978-0-12-356786-4 }}

The ability of zinc chloride to dissolve metal oxides (MO){{cite book|last=Wiberg|first=Nils|title=Lehrbuch der Anorganischen Chemie |trans-title=Holleman & Wiberg, Textbook of Inorganic chemistry |language=de|publisher=de Gruyter, Berlin|year=2007 |page = 1491|isbn=978-3-11-017770-1}} is relevant to the utility of {{chem2|ZnCl2}} as a flux for soldering. It dissolves passivating oxides, exposing the clean metal surface.

Zinc chloride is an occasional laboratory reagent often as a Lewis acid. A dramatic example is the conversion of methanol into hexamethylbenzene using zinc chloride as the solvent and catalyst:{{cite journal|title = Hydrocarbons from Methanol|first = Clarence D.|last = Chang|pages = 1–118|doi = 10.1080/01614948308078874|journal = Catal. Rev. - Sci. Eng.|volume = 25|issue = 1|year = 1983}}

:{{chem2|15 CH3OH → C6(CH3)6 + 3 CH4 + 15 H2O}}

This kind of reactivity has been investigated for the valorization of C1 precursors.{{cite journal|title = Onium Ylide chemistry. 1. Bifunctional acid-base-catalyzed conversion of heterosubstituted methanes into ethylene and derived hydrocarbons. The onium ylide mechanism of the C₁ → C₂ conversion|first1 = George A.|last1 = Olah|first2 = Hans|last2 = Doggweiler|first3 = Jeff D.|last3 = Felberg|first4 = Stephan|last4 = Frohlich|first5 = Mary Jo|last5 = Grdina|first6 = Richard|last6 = Karpeles|first7 = Takashi|last7 = Keumi|first8 = Shin-ichi|last8 = Inaba|first9 = Wai M.|last9 = Ip|first10 = Koop|last10 = Lammertsma|first11 = George|last11 = Salem|first12 = Derrick|last12 = Tabor|journal = J. Am. Chem. Soc.|year = 1984|volume = 106|issue = 7|pages = 2143–2149|doi = 10.1021/ja00319a039}}

Examples of zinc chloride as a Lewis acid include the Fischer indole synthesis:{{OrgSynth | last1 = Shriner | first1 = R. L. | last2 = Ashley | first2 = W. C. | last3 = Welch | first3 = E. | title = 2-Phenylindole | year = 1942 | volume = 22 | pages = 98 | collvol = 3 |collvolpages = 725 | doi = 10.15227/orgsyn.022.00981955 | prep = cv3p0725 }}

:File:ZnCl2 aromatics (cropped indole).gif

Related Lewis-acid behavior is illustrated by a traditional preparation of the dye fluorescein from phthalic anhydride and resorcinol, which involves a Friedel-Crafts acylation.{{cite book | author = Furnell, B. S. | title = Vogel's Textbook of Practical Organic Chemistry | edition = 5th | publisher = Longman/Wiley | location = New York | year = 1989 }} This transformation has in fact been accomplished using even the hydrated {{chem2|ZnCl2}} sample shown in the picture above. Many examples describe the use of zinc chloride in Friedel-Crafts acylation reactions.{{OrgSynth | last = Cooper | first = S. R. | title = Resacetophenone | collvol = 3 | collvolpages = 761 | prep = cv3p0761 | year = 1941 | volume = 21 | pages = 103 | doi = 10.15227/orgsyn.021.0103 }}{{cite journal | last1 = Dike | first1 = S. Y. | last2 = Merchant | first2 = J. R. | last3 = Sapre | first3 = N. Y. | title = A New and Efficient General Method for the Synthesis of 2-Spirobenzopyrans: First Synthesis of Cyclic Analogues of Precocene I and Related Compounds | journal = Tetrahedron | year = 1991 | volume = 47 | issue = 26 | pages = 4775–4786 | doi = 10.1016/S0040-4020(01)86481-4 }}

File:Preparation_of_Fluorescein.svg

Zinc chloride also activates benzylic and allylic halides towards substitution by weak nucleophiles such as alkenes:{{cite journal |author1=Bauml, E. |author2=Tschemschlok, K. |author3=Pock, R. |author4=Mayr, H. | title = Synthesis of γ-Lactones from Alkenes Employing p-Methoxybenzyl Chloride as ⁺CH₂-CO₂⁻ Equivalent | journal = Tetrahedron Letters | year = 1988 | volume = 29 | issue = 52 | pages = 6925–6926 | doi = 10.1016/S0040-4039(00)88476-2 | url = http://epub.ub.uni-muenchen.de/3799/1/086.pdf }}

File:ZnCl2 benzylation (cropped).gif

In similar fashion, {{chem2|ZnCl2}} promotes selective sodium cyanoborohydride reduction of tertiary, allylic or benzylic halides to the corresponding hydrocarbons.

Zinc enolates, prepared from alkali metal enolates and {{chem2|ZnCl2}}, provide control of stereochemistry in aldol condensation reactions. This control is attributed to chelation at the zinc. In the example shown below, the threo product was favored over the erythro by a factor of 5:1 when {{chem2|ZnCl2}}.{{cite journal |author1=House, H. O. |author2=Crumrine, D. S. |author3=Teranishi, A. Y. |author4=Olmstead, H. D. | title = Chemistry of Carbanions. XXIII. Use of Metal Complexes to Control the Aldol Condensation | journal = Journal of the American Chemical Society | year = 1973 | volume = 95 | issue = 10 | pages = 3310–3324 | doi = 10.1021/ja00791a039 }}

File:ZnCl2 aldol (cropped).gif

Being inexpensive and anhydrous, ZnCl₂ is a widely used for the synthesis of many organozinc reagents, such as those used in the palladium catalyzed Negishi coupling with aryl halides or vinyl halides. The prominence of this reaction was highlighted by the award of the 2010 Nobel Prize in Chemistry to Ei-ichi Negishi.{{cite journal |doi=10.1002/anie.201101380 |title=Magical Power of Transition Metals: Past, Present, and Future (Nobel Lecture) |date=2011 |last1=Negishi |first1=Ei-Ichi |journal=Angewandte Chemie International Edition |volume=50 |issue=30 |pages=6738–6764 |pmid=21717531 }}

File:ZnCl2 Negishi.gif

Rieke zinc, a highly reactive form of zinc metal, is generated by reduction of zinc dichloride with lithium. Rieke Zn is useful for the preparation of polythiophenes{{cite journal|author1=Chen, T.-A. |author2=Wu, X. |author3=Rieke, R. D. |title=Regiocontrolled Synthesis of Poly(3-alkylthiophenes) Mediated by Rieke Zinc: Their Characterization and Solid-State Properties|journal=Journal of the American Chemical Society |year=1995|volume=117|pages=233–244|doi=10.1021/ja00106a027}} and for the Reformatsky reaction.{{cite journal|author1=Rieke, R. D. |author2=Hanson, M. V. |title= New Organometallic Reagents Using Highly Reactive Metals| journal=Tetrahedron|year= 1997|volume= 53|pages=1925–1956| doi=10.1016/S0040-4020(96)01097-6|issue=6}}

{{See also|1=Zinc–carbon battery#Zinc-chloride "heavy duty" cell|l1=Zinc-chloride "heavy duty" cell}}

Zinc chloride is used as a catalyst or reagent in diverse reactions conducted on an industrial scale. Benzaldehyde, 20,000 tons of which is produced annually in Western countries, is produced from inexpensive toluene by exploiting the catalytic properties of zinc dichloride. This process begins with the chlorination of toluene to give benzal chloride. In the presence of a small amount of anhydrous zinc chloride, a mixture of benzal chloride are treated continuously with water according to the following stoichiometry:{{cite book |doi=10.1002/14356007.a03_463.pub2 |chapter=Benzaldehyde |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |last1=Brühne |first1=Friedrich |last2=Wright |first2=Elaine |isbn=978-3-527-30385-4 }}

:{{chem2|C6H5CHCl2 + H2O -> C6H5CHO + 2 HCl}}

Similarly zinc chloride is employed in hydrolysis of benzotrichloride, the main route to benzoyl chloride. It serves as a catalyst for the production of methylene-bis(dithiocarbamate).{{Ullmann | author1=Dieter M. M. Rohe | author2=Hans Uwe Wolf | title=Zinc Compounds | year=2007 | pages=1–6 | doi=10.1002/14356007.a28_537|.pub2}}

The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride), involves the production of HCl and its subsequent reaction with surface oxides.

Zinc chloride forms two salts with ammonium chloride: {{chem2|[NH4]2[ZnCl4]}} and {{chem2|[NH4]3[ZnCl4]Cl}}, 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 {{chem2|H2}} gas and ammonia fumes.{{cite book | title = ASM handbook | year = 1990 | author = American Society for Metals | publisher = ASM International | isbn = 978-0-87170-021-6 }}

Relevant to its affinity for these paper and textiles, {{chem2|ZnCl2}} is used as a fireproofing agent and in the process of making Vulcanized fibre, which is made by soaking paper in concentrated zinc chloride.{{cite journal |author1=Yiqun Fang |author2=Aojing Xue |author3=Fengqiang Wang |author4=Zhijun Zhang |author5=Yongming Song |author6=Weihong Wang |author7=Qingwen Wang |title=The influence of zinc compounds on thermal stability and flame retardancy of wood flour polyvinyl chloride composites |journal=Construction and Building Materials |date=2022 |volume=320 |page=126203 |doi=10.1016/j.conbuildmat.2021.126203 |language=en}}{{cite journal |author1=Junji Nemoto |author2=Keiichi Nakamata |title=All-cellulose material prepared using aqueous zinc chloride solution |journal=Cellulose |date=2022 |volume=29 |issue=5 |pages=2795–2803 |doi=10.1007/s10570-021-04344-1 |language=en}} Zinc chloride is also used as a deodorizing agent and to make zinc soaps.

Zinc and chloride are essential for life. Zn²⁺ is a component of several enzymes, e.g., carboxypeptidase and carbonic anhydrase. Thus, aqueous solutions of zinc chlorides are rarely problematic as an acute poison. Anhydrous zinc chloride is however an aggressive Lewis acid as it can burn skin and other tissues. Ingestion of zinc chloride, often from soldering flux, requires endoscopic monitoring.{{cite journal |doi=10.1056/nejmra1810769 |title=Ingestion of Caustic Substances |date=2020 |last1=Hoffman |first1=Robert S. |last2=Burns |first2=Michele M. |last3=Gosselin |first3=Sophie |journal=New England Journal of Medicine |volume=382 |issue=18 |pages=1739–1748 |pmid=32348645 }} Another source of zinc chloride is zinc chloride smoke mixture ("HC") used in smoke grenades. Containing zinc oxide, hexachloroethane and aluminium powder release zinc chloride, carbon and aluminium oxide smoke, an effective smoke screen.{{cite book | author = Sample, B. E. | year = 1997 | title = Methods for Field Studies of Effects of Military Smokes, Obscurants, and Riot-control Agents on Threatened and Endangered Species | publisher = DIANE Publishing | isbn = 978-1-4289-1233-5 }} Such smoke screens can lead to fatalities.{{cite book |doi=10.1016/C2011-0-07884-5 |title=Handbook on the Toxicology of Metals |date=2015 |isbn=978-0-444-59453-2|publisher = Academic Press|editor=Gunnar F. Nordberg, Bruce A. Fowler, Monica Nordberg}}

{{Reflist|colwidth=30em}}

• N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
• {{RubberBible86th}}
• The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
• D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
• 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.

• [http://muby.itgo.com/zincchloride.html Grades and Applications of Zinc Chloride]
• [https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=5727 PubChem ZnCl₂ summary].

{{Zinc compounds}}

{{Chlorides}}

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zinc

chloride

Category:Inorganic compounds

Category:Metal halides

Category:Deliquescent materials