Mercury(II) thiocyanate

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Mercury(II) thiocyanate
Hg(SCN)2 Xray.jpg
Mercury thiocyanate powder.png
Names
Other names
Mercuric thiocyanate
Mercuric sulfocyanate
Identifiers
3D model (JSmol)
ECHA InfoCard 100.008.886 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 209-773-0
PubChem CID
UNII
  • InChI=1S/2CHNS.Hg/c2*2-1-3;/h2*3H;/q;;+2/p-2
    Key: GBZANUMDJPCQHY-UHFFFAOYSA-L
  • C(#N)[S-].C(#N)[S-].[Hg+2]
Properties
Hg(SCN)2
Molar mass 316.755 g/mol
AppearanceWhite monoclinic powder
Odor odorless
Density 3.71 g/cm3, solid
Melting point 165 °C (329 °F; 438 K) (decomposes)
0.069 g/100 mL
Solubility Soluble in dilute hydrochloric acid, KCN, ammonia
slightly soluble in alcohol, ether
−96.5·10−6 cm3/mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
highly toxic
GHS labelling: [1]
GHS-pictogram-skull.svg GHS-pictogram-silhouette.svg GHS-pictogram-pollu.svg
Danger
H300, H310, H330, H373, H410
P260, P262, P270, P271, P273, P280, P284, P301+P316, P302+P352, P304+P340, P316, P319, P320, P321, P330, P361, P364, P391, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
3
1
1
Lethal dose or concentration (LD, LC):
46 mg/kg (rat, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Mercury(II) thiocyanate (Hg(SCN)2) is an inorganic chemical compound, the coordination complex of Hg2+ and the thiocyanate anion. It is a white powder. It will produce a large, winding "snake" when ignited, an effect known as the Pharaoh's serpent. [2]

Contents

Synthesis and structure

The first synthesis of mercury thiocyanate was probably completed in 1821 by Jöns Jacob Berzelius:

HgO + 2 HSCN → Hg(SCN)2 + H2O

Evidence for the first pure sample was presented in 1866 prepared by a chemist named Otto Hermes. [2] It is prepared by treating solutions containing mercury(II) and thiocyanate ions. The low solubility product of mercury thiocyanate causes it to precipitate from the solution. [3] Most syntheses are achieved by precipitation:

Hg(NO3)2 + 2 KSCN → Hg(SCN)2 + 2 KNO3

The compound adopts a polymeric structure with Hg2+ centres linearly coordinated to two S atoms with a distance of 2.381 Å. Four weak Hg2+--N interactions are indicated with distances of 2.81 Å. [4]

Reactions

Mercury thiocyanate has a few uses in chemical synthesis.

It is the precursor to other thiocyanate complexes such as potassium tris(thiocyanato)mercurate(II) (K[Hg(SCN)3]) and caesium tris(thiocyanato)mercurate(II) (Cs[Hg(SCN)3]). The Hg(SCN)3− ion can also exist independently and is easily generated from the compounds above, amongst others. [5]

Organic halides attack Hg(SCN)2 to give a mercuric halide and a mixture of the corresponding thiocyanate and isothiocyanate. [6]

Mercuric thiocyanate catalyzes HSCN or BrSCN addition (either reagent formed in situ) to alkynes. [7]

Use in chloride analysis

Mercury thiocyanate improve detection limits in the determination of chloride ions in water by UV-visible spectroscopy. This technique was a standard method for the determination of chloride ions in laboratories worldwide. The method involves the addition of mercury thiocyanate to a solution with an unknown concentration of chloride ions and iron as a reagent. The chloride ions cause the mercury thiocyanate salt to dissociate and the thiocyanate ion to bind Fe(III), which absorbs intensely at 450 nm. This absorption allows for the measurement of the concentration of the iron complex. This value allows one to calculate the concentration of chloride. [8]

Pharaoh's serpent

Pharaoh's serpent demonstration Weze faraona.png
Pharaoh's serpent demonstration

Mercury thiocyanate was formerly used in pyrotechnics causing an effect known as the Pharaoh's serpent or Pharaoh's snake. The fireworks versions were a mixture with a small amount of potassium nitrate and gum arabic as a binder. [9] This use ceased in most countries in the early 20th century due to the toxicity of mercury, and the existence of a superior alternatives.

Chemistry of the reaction

When heated, mercury(II) thiocyanate decomposes in an exothermic reaction that can produce a large mass of coiling, serpent-like solid. An inconspicuous flame, which is usually the blue color of burning carbon disulfide but which can be yellow from impurities or incidental combustion of flammable materials on the surface it is ignited on. The resulting solid can range from dark graphite gray to light tan in colour with the inside generally much darker than the outside. This was found to be due to decomposition of the produced β-HgS (black mercury sulfide) and vaporization of the resulting mercury from the othermost and hottest layers of the solid. [10]

The decomposition of Hg(SCN)2 is exothermic on its own, and the CS2 produced ignites easily and burns off. The C3N4 product is a simplification; the actual product contains 0.5% hydrogen and is likely to consist of sheets of triazine rings linked by −N= and −NH− groups similar to g−C3N4 and was found to contain nano-particles of β-HgS (black mercury sulfide). [10]

The number of resonance structures of heptazine and triazine, varying molecular weights of samples, and the fluorescense of the product made acquiring spectra difficult even by relatively exotic methods of NMR (with one spectrum acquisition being run for 12 days straight to get a mostly clean reading). Because of this, a heptazine-based structure similar to Liebig's melon, a compound initially prepared around the same time that the pharoah's snake reaction was discovered, was not ruled out by the authors as a partial component of the solid material. [10] The generalized reaction is as follows:

C3N4 is not a product of this decomposition. Cyanogen is generally only produced when Hg(CN)2 or similar is heated to decomposition, and early attempts to form (SCN)2 via the same route starting at this compound failed and only generated SO2, CO2 and N2. [10]

Related Research Articles

In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are Lewis bases.

<span class="mw-page-title-main">Zinc chloride</span> Chemical compound

Zinc chloride is an inorganic chemical compound with the formula ZnCl2·nH2O, with n ranging from 0 to 4.5, forming hydrates. Zinc chloride, anhydrous and its hydrates, are colorless or white crystalline solids, and are highly soluble in water. Five hydrates of zinc chloride are known, as well as four forms of anhydrous zinc chloride.

<span class="mw-page-title-main">Thiocyanate</span> Ion (S=C=N, charge –1)

Thiocyanates are salts containing the thiocyanate anion [SCN]−. [SCN]− is the conjugate base of thiocyanic acid. Common salts include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyrotechnics.

Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms Ps–Ps or Ps–X, such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.

<span class="mw-page-title-main">Thiourea</span> Organosulfur compound (S=C(NH2)2)

Thiourea is an organosulfur compound with the formula SC(NH2)2 and the structure H2N−C(=S)−NH2. It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom ; however, the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. Thioureas are a broad class of compounds with the general structure R2N−C(=S)−NR2.

<span class="mw-page-title-main">Black snake (firework)</span> Type of firework which smokes and spews out ash resembling a snake, staying on the ground

"Black snake" is a term that can refer to at least three similar types of fireworks: the Pharaoh's snake, the sugar snake, or a popular retail composition marketed under various product names but usually known as "black snake". The "Pharaoh's snake" or "Pharaoh's serpent" is the original version of the black snake experiment. It produces a more impressive snake, but its execution depends upon mercury (II) thiocyanate, which is no longer in common use due to its toxicity. For a "sugar snake", sodium bicarbonate and sugar are the commonly used chemicals.

<span class="mw-page-title-main">Mercury(II) cyanide</span> Chemical compound

Mercury(II) cyanide, also known as mercuric cyanide, is a poisonous compound of mercury and cyanide. It is an odorless, toxic white powder. It is highly soluble in polar solvents such as water, alcohol, and ammonia, slightly soluble in ether, and insoluble in benzene and other hydrophobic solvents.

Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water.

<span class="mw-page-title-main">Mercury(II) sulfate</span> Chemical compound

Mercury(II) sulfate, commonly called mercuric sulfate, is the chemical compound HgSO4. It is an odorless salt that forms white granules or crystalline powder. In water, it separates into an insoluble basic sulfate with a yellow color and sulfuric acid.

<span class="mw-page-title-main">Mercury(II) acetate</span> Chemical compound

Mercury(II) acetate, also known as mercuric acetate is a chemical compound, the mercury(II) salt of acetic acid, with the formula Hg(O2CCH3)2. Commonly abbreviated Hg(OAc)2, this compound is employed as a reagent to generate organomercury compounds from unsaturated organic precursors. It is a white, water-soluble solid, but some samples can appear yellowish with time owing to decomposition.

<span class="mw-page-title-main">Potassium thiocyanate</span> Chemical compound

Potassium thiocyanate is the chemical compound with the molecular formula KSCN. It is an important salt of the thiocyanate anion, one of the pseudohalides. The compound has a low melting point relative to most other inorganic salts.

<span class="mw-page-title-main">Organomercury chemistry</span> Group of chemical compounds containing mercury

Organomercury chemistry refers to the study of organometallic compounds that contain mercury. Typically the Hg–C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury(II) cation, CH3Hg+; ethylmercury(II) cation, C2H5Hg+; dimethylmercury, (CH3)2Hg, diethylmercury and merbromin ("Mercurochrome"). Thiomersal is used as a preservative for vaccines and intravenous drugs.

Mercury(I) sulfide or mercurous sulfide is a hypothetical chemical compound of mercury and sulfur, with elemental formula Hg
2S. Its existence has been disputed; it may be stable below 0 °C or in suitable environments, but is unstable at room temperature, decomposing into metallic mercury and mercury(II) sulfide.

<span class="mw-page-title-main">Thiocyanogen</span> Chemical compound

Thiocyanogen, (SCN)2, is a pseudohalogen derived from the pseudohalide thiocyanate, [SCN]−, with behavior intermediate between dibromine and diiodine. This hexatomic compound exhibits C2 point group symmetry and has the connectivity NCS-SCN.

<span class="mw-page-title-main">Tetramethyltin</span> Chemical compound

Tetramethyltin is an organometallic compound with the formula (CH3)4Sn. This liquid, one of the simplest organotin compounds, is useful for transition-metal mediated conversion of acid chlorides to methyl ketones and aryl halides to aryl methyl ketones. It is volatile and toxic, so care should be taken when using it in the laboratory.

<span class="mw-page-title-main">Amalgam (chemistry)</span> Alloy of mercury with another metal

An amalgam is an alloy of mercury with another metal. It may be a liquid, a soft paste or a solid, depending upon the proportion of mercury. These alloys are formed through metallic bonding, with the electrostatic attractive force of the conduction electrons working to bind all the positively charged metal ions together into a crystal lattice structure. Almost all metals can form amalgams with mercury, the notable exceptions being iron, platinum, tungsten, and tantalum. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore. Dentistry has used alloys of mercury with metals such as silver, copper, indium, tin and zinc.

<span class="mw-page-title-main">Ammonium thiocyanate</span> Chemical compound

Ammonium thiocyanate is an inorganic compound with the formula [NH4]+[SCN]−. It is an ammonium salt of thiocyanic acid. It consists of ammonium cations [NH4]+ and thiocyanate anions [SCN]−.

<span class="mw-page-title-main">Copper(I) thiocyanate</span> Chemical compound

Copper(I) thiocyanate is a coordination polymer with formula CuSCN. It is an air-stable, white solid used as a precursor for the preparation of other thiocyanate salts.

<span class="mw-page-title-main">Sulfur dicyanide</span> Chemical compound

Sulfur dicyanide is an inorganic compound with the formula S(CN)2. A white, slightly unstable solid, the compound is mainly of theoretical and fundamental interest given its simplicity. It is the first member of the dicyanosulfanes Sx(CN)2, which includes thiocyanogen ((SCN)2) and higher polysulfanes up to S4(CN)2. According to X-ray crystallography, the molecule is planar, the SCN units are linear, with an S-C-S angle of 95.6°.

References

  1. ↑ "Mercuric thiocyanate (Compound)". pubchem.ncbi.nlm.nih.gov. Retrieved 31 May 2023.
  2. 1 2 Davis, T. L. (1940). "Pyrotechnic Snakes". Journal of Chemical Education. 17 (6): 268–270. doi:10.1021/ed017p268.
  3. ↑ Sekine, T.; Ishii, T. (1970). "Studies of the Liquid-Liquid Partition systems. VIII. The Solvent Extraction of Mercury (II) Chloride, Bromide, Iodide and Thiocyanate with Some Organic Solvents". Bulletin of the Chemical Society of Japan. 43 (8): 2422–2429. doi: 10.1246/bcsj.43.2422 .
  4. ↑ Beauchamp, A.L.; Goutier, D. "Structure cristalline et moleculaire du thiocyanate mercuric" Canadian Journal of Chemistry 1972, volume 50, p977-p981. doi : 10.1139/v72-153
  5. ↑ Bowmaker, G. A.; Churakov, A. V.; Harris, R. K.; Howard, J. A. K.; Apperley, D. C. (1998). "Solid-State 199Hg MAS NMR Studies of Mercury(II) Thiocyanate Complexes and Related Compounds. Crystal Structure of Hg(SeCN)2". Inorganic Chemistry. 37 (8): 1734–1743. doi:10.1021/ic9700112.
  6. ↑ Kitamura, T.; Kobayashi, S.; Taniguchi, H. (1990). "Photolysis of Vinyl Halides. Reaction of Photogenerated Vinyl Cations with Cyanate and Thiocyanate Ions". Journal of Organic Chemistry. 55 (6): 1801–1805. doi:10.1021/jo00293a025.
  7. ↑ Kočovský, Pavel. "Mercury(II) thiocyanate". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rm045.{{cite encyclopedia}}: CS1 maint: multiple names: authors list (link)
  8. ↑ Cirello-Egamino, J.; Brindle, I. D. (1995). "Determination of chloride ions by reaction with mercury thiocyanate in the absence of iron(III) using a UV-photometric, flow injection method". Analyst. 120 (1): 183–186. doi:10.1039/AN9952000183.
  9. ↑ Weingart, George W. (1947). "Part III. Products of Manufacture and Formulas". Pyrotechnics (2d, rev. and enl ed.). Brooklyn: Chemical Pub. Co. pp. 182–183. ISBN   9780820601120.
  10. 1 2 3 4 Miller, Thomas S.; d'Aleo, Anita; Suter, Theo; Aliev, Abil E.; Sella, Andrea; McMillan, Paul F. (17 November 2017). "Pharaoh's Serpents: New Insights into a Classic Carbon Nitride Material". Zeitschrift für anorganische und allgemeine Chemie (Journal of Inorganic and General Chemistry). 643 (21): 1572–1580. doi: 10.1002/zaac.201700268 .
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