Zinc chloride is the name of
inorganic
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s with the
formula ZnCl
2 and its
hydrates
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
.
Zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
chlorides, of which nine
crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
forms are known, are
color
Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are assoc ...
less or white, and are highly
soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
in
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
.
This salt is
hygroscopic and even
deliquescent
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...
. Zinc
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
finds wide application in
textile
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
processing,
metallurgical fluxes, and chemical synthesis. No mineral with this chemical composition is known aside from the very rare mineral
simonkolleite, Zn
5(OH)
8Cl
2·H
2O.
Structure and properties
Four crystalline forms (
polymorphs) of ZnCl
2 are known: α, β, γ, and δ. Each case features tetrahedral Zn
2+ centers.
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.
The orthorhombic form (δ) rapidly changes to one of the other forms on exposure to the atmosphere. A possible explanation is that the OH
− ions originating from the absorbed water facilitate the rearrangement.
[ Rapid cooling of molten ZnCl2 gives a ]glass
Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...
.
Molten ZnCl2 has a high viscosity at its melting point and a comparatively low electrical conductivity, which increases markedly with temperature. As indicated by a Raman scattering study, the viscosity is explained by the presence of polymers,. Neutron scattering study indicated the presence of tetrahedral centers, which requires aggregation of ZnCl2 monomers as well..
In the gas phase, ZnCl2 molecules are linear with a bond length of 205 pm.
Hydrates
Five hydrates
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
of zinc chloride are known: ZnCl2(H2O)''n'' with ''n'' = 1, 1.5, 2.5, 3 and 4. The tetrahydrate ZnCl2(H2O)4 crystallizes from aqueous solutions of zinc chloride.[
]
Preparation and purification
Anhydrous ZnCl2 can be prepared from zinc and hydrogen chloride:
:Zn + 2 HCl → ZnCl2 + H2
Hydrated forms and aqueous solutions may be readily prepared similarly by treating Zn metal, zinc carbonate, zinc oxide, and zinc sulfide with hydrochloric acid:
:ZnS + 2 HCl + 4 H2O → ZnCl2(H2O)4 + H2S
Unlike many other elements, zinc essentially exists in only one oxidation state, 2+, which simplifies the purification of the chloride.
Commercial samples of zinc chloride typically contain water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and products from hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
as impurities. Such 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
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
gas. Finally, the simplest method relies on treating the zinc chloride with thionyl chloride
Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
.
Reactions
Molten anhydrous ZnCl2 at 500–700 °C dissolves zinc metal, and, on rapid cooling of the melt, a yellow diamagnetic glass is formed, which Raman studies indicate contains the ion.
A number of salts containing the tetrachlorozincate anion, Zn, are known. "Caulton's reagent", V2Cl3(thf)6Zn2Cl6 is an example of a salt containing Zn2.
The compound Cs3ZnCl5 contains tetrahedral Zn and Cl− anions.[ No compounds containing the Zn ion have been characterized.][
Zinc chloride dissolves readily in water to give ZnCl''x''H2O(4−''x'') species and some free chloride. Aqueous solutions of ZnCl2 are acidic: a 6 M aqueous solution has a pH of 1.][ The acidity of aqueous ZnCl2 solutions relative to solutions of other Zn2+ salts (say the sulfate) is due to the formation of the tetrahedral chloro aqua complexes where the reduction in coordination number from 6 to 4 further reduces the strength of the O–H bonds in the solvated water molecules.
In alkali solution, zinc chloride converts to various zinc hydroxychlorides. These include Zn(OH)3Cl2−, Zn(OH)2, ZnOH, and the insoluble Zn5(OH)8Cl2·H2O. The latter is the mineral simonkolleite. When zinc chloride hydrates are heated, HCl gas evolves and hydroxychlorides result.
When solutions of zinc chloride are treated with ammonia, various complexes of "ammines" are produced. These include Zn(NH3)4Cl2·H2O and on concentration ZnCl2(NH3)2. The former contains the Zn(NH3)62+ ion,][ and the latter is molecular with a distorted tetrahedral geometry. The species in aqueous solution have been investigated and show that Zn(NH3)42+ is the main species present with Zn(NH3)3Cl+ also present at lower NH3:Zn ratio.
Aqueous zinc chloride reacts with zinc oxide to form an amorphous cement that was first investigated in 1855 by Stanislas Sorel. Sorel later went on to investigate the related magnesium oxychloride cement, which bears his name.
When hydrated zinc chloride is heated, one obtains a residue of Zn(OH)Cl e.g.
: ZnCl2·2H2O → ZnCl(OH) + HCl + H2O
The compound ZnCl2·HCl·H2O may be prepared by careful precipitation from a solution of ZnCl2 acidified with HCl. It contains a polymeric anion (Zn2Cl5−) with balancing monohydrated ]hydronium
In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid i ...
ions, H5O2+ ions.[
Cellulose dissolves in aqueous solutions of ZnCl2, and zinc-cellulose complexes have been detected. Cellulose also dissolves in molten ZnCl2 hydrate and carboxylation and acetylation performed on the cellulose polymer.
Thus, although many zinc salts have different formulas and different ]crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
s, these salts behave very similarly in aqueous solution. For example, solutions prepared from any of the polymorphs of ZnCl2, as well as other halides (bromide, iodide), and the sulfate can often be used interchangeably for the preparation of other zinc compounds. Illustrative is the preparation of zinc carbonate:
:ZnCl2( aq) + Na2CO3(aq) → ZnCO3(s) + 2 NaCl
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
(aq)
Role in organic chemistry
Zinc chloride is used as a catalyst or reagent in diverse reactions conducted on an industrial scale. The partial hydrolysis of benzal chloride
Benzal chloride is an organic compound with the formula C6H5CHCl2. This colourless liquid is a lachrymator and is used as a building block in organic synthesis.
Preparation and usage
Benzal chloride is produced by the free radical chlorination o ...
in the presence of zinc chloride is the main route to benzoyl chloride
Benzoyl chloride, also known as benzenecarbonyl chloride, is an organochlorine compound with the formula . It is a colourless, fuming liquid with an irritating odour, and consists of a benzene ring () with an acyl chloride () substituent. It is ...
. It serves as a catalyst for the production of methylene-bis(dithiocarbamate).
The combination of hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
and ZnCl2, known as the "Lucas reagent
"Lucas' reagent" is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl group. A pos ...
", is effective for the preparation of alkyl chlorides from alcohols. Similar reactions are the basis of industrial routes from methanol and ethanol respectively to methyl chloride and ethyl chloride
Chloroethane, commonly known as ethyl chloride, is a chemical compound with chemical formula CH3CH2Cl, once widely used in producing tetraethyllead, a gasoline additive. It is a colorless, flammable gas or refrigerated liquid with a faintly sweet ...
.
Laboratory syntheses
Zinc chloride is a common reagent in the laboratory useful Lewis acid in organic chemistry.
Molten zinc chloride catalyses the conversion of methanol to hexamethylbenzene
Hexamethylbenzene, also known as mellitene, is a hydrocarbon with the molecular formula C12H18 and the condensed structural formula C6(CH3)6. It is an aromatic compound and a derivative of benzene, where benzene's six hydrogen atoms have each ...
:
:15 → + 3 + 15
Other examples include catalyzing (A) the Fischer indole synthesis, and also (B) Friedel-Crafts acylation reactions involving activated aromatic rings
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to saturat ...
:
Related to the latter is the classical preparation of the dye fluorescein from phthalic anhydride
Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commer ...
and resorcinol
Resorcinol (or resorcin) is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or '' meta''-isomer). Resorcinol crystallizes from benzene as colorless needles that are readily soluble i ...
, which involves a Friedel-Crafts acylation. This transformation has in fact been accomplished using even the hydrated ZnCl2 sample shown in the picture above.
:
Zinc chloride also activates benzylic and allylic halides towards substitution by weak nucleophiles such as alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s:
:
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 reagent for the synthesis of many organozinc reagents, such as those used in the palladium catalyzed Negishi coupling
The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (C-C) in the process. A palladium (0) specie ...
with aryl halide In organic chemistry, an aryl halide (also known as haloarene) is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. The haloarene are different from haloalkanes because they exhi ...
s or vinyl halide
In organic chemistry, a vinyl halide is a compound with the formula CH2=CHX (X = halide). The term vinyl is often used to describe any alkenyl group. For this reason, alkenyl halides with the formula RCH=CHX are sometimes called vinyl halides. Fr ...
s. In such cases the organozinc compound is usually prepared by transmetallation from an organolithium
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...
or a Grignard reagent, for example:
:
Zinc enolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.
Bonding and structure
Enolate anions are electr ...
s, prepared from alkali metal enolates and ZnCl2, provide control of stereochemistry in aldol condensation
An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties (of aldehydes or ketones) react to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), and this is then followed by dehydration to ...
reactions due to chelation
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
on to the zinc. In the example shown below, the '' threo'' product was favored over the ''erythro
In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have dif ...
'' by a factor of 5:1 when ZnCl2 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''.
:
Other uses
As a metallurgical flux
The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride
Zinc ammonium chloride is the inorganic compound with the formula (NH4)2ZnCl4. It is the ammonium salt of tetrachlorozincate. It used as a flux in the process of hot-dip galvanizing.
Uses
Steel to be galvanized passes through an acidic clea ...
), involves the production of HCl and its subsequent reaction with surface oxides.
Zinc chloride reacts with metal oxides (MO) to give derivatives of the idealized formula MZnOCl2. This reaction is relevant to the utility of ZnCl2 solution as a flux for soldering — it dissolves passivating oxides, exposing the clean metal surface. Fluxes with ZnCl2 as an active ingredient are sometimes called "tinner's fluid".
Zinc chloride forms two salts with ammonium chloride: (NH4)2ZnCl4 and (NH4)3ClZnCl4, 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 H2 gas and ammonia fumes.
In textile and paper processing
Concentrated aqueous solutions of zinc chloride (more than 64% weight/weight zinc chloride in water) have dissolving starch, silk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons. The best-known silk is obtained from the ...
, and cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
.
Relevant to its affinity for these materials, ZnCl2 is used as a fireproofing agent and in fabric "refresheners" such as Febreze. Vulcanized fibre
Vulcanized fibre or red fibre is a laminated plastic composed of only cellulose. The material is a tough, resilient, hornlike material that is lighter than aluminium, tougher than leather, and stiffer than most thermoplastics. The newer wood-lamin ...
is made by soaking paper in concentrated zinc chloride.
Smoke grenades
The zinc chloride smoke mixture ("HC") used in smoke grenades
Smoke grenades used at demonstrations in Paris, 2008
upBritish L83A1 Smoke Grenade manufactured in May 2008. This grenade has already been used.
A smoke grenade is a canister-type grenade used as a signaling device, target or landing zone mark ...
contains zinc oxide
Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cement ...
, hexachloroethane
Hexachloroethane, also known as perchloroethane is the organochlorine compound with the chemical formula . It is white solid at room temperature with a camphor-like odor. It has been used by the military in smoke compositions, such as base-eject ...
and granular aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
powder, which, when ignited, react to form zinc chloride, carbon and aluminium oxide smoke, an effective smoke screen
A smoke screen is smoke released to mask the movement or location of military units such as infantry, tanks, aircraft, or ships.
Smoke screens are commonly deployed either by a canister (such as a grenade) or generated by a vehicle (such as ...
.
Fingerprint detection
Ninhydrin reacts with amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s and amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
s to form a colored compound "Ruhemann's purple" (RP). Spraying with a zinc chloride solution forms a 1:1 complex RP:ZnCl(H2O)2, which is more readily detected as it fluoresces more intensely than RP.
Disinfectant and wood preservative
Dilute aqueous zinc chloride was used as a disinfectant under the name "Burnett's Disinfecting Fluid".
From 1839 Sir William Burnett
Sir William Burnett, Knight Commander of the Order of the Bath, KCB, Fellow of the Royal Society, FRS (16 January 1779 – 16 February 1861) was a British physician who served as Physician-General of the Royal Navy.
Early life
Burnett was born ...
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
Phenol (also called carbolic acid) is an aromaticity, aromatic organic compound with the molecular chemical formula, formula . It is a white crystalline solid that is volatility (chemistry), volatile. The molecule consists of a phenyl group () ...
and other proprietary products.
Safety
Zinc chloride is a chemical irritant of the eyes, skin, and respiratory system.
Additional reading
*N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
*
*''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.
References
External links
Grades and Applications of Zinc Chloride
PubChem ZnCl2 summary
{{DEFAULTSORT:Zinc Chloride
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
Inorganic compounds
Metal halides
Deliquescent substances