Chemistry:Thiocyanic acid

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Thiocyanic acid[1]
Skeletal formula of thiocyanic acid with the explicit hydrogen added
Spacefill model of thiocyanic acid
Names
Preferred IUPAC name
Thiocyanic acid[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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|Section1=! colspan=2 style="background: #f8eaba; text-align: center;" |Identifiers

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3D model (JSmol)

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|- | 3DMet

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| ChEBI

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|- | ChemSpider

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| EC Number

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  • 207-337-4

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| 25178 |-

| KEGG

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|- | MeSH | thiocyanic+acid |-

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|- |Section2=! colspan=2 style="background: #f8eaba; text-align: center;" |Properties

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| CHNS

|- | Molar mass

| 59.09 g·mol−1

|- | Appearance | colorless, oily liquid |- | Odor | pungent |- | Density | 2.04 g/cm3 |- | Melting point | 5 °C (41 °F; 278 K)

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| Miscible |-

| Solubility | soluble in ethanol, diethyl ether |-




| log P | 0.429 |-


| Acidity (pKa) | 0.926 |- | Basicity (pKb) | 13.071 |- |Section3=! colspan=2 style="background: #f8eaba; text-align: center;" |Hazards

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| GHS pictograms | GHS07: Harmful |- | GHS Signal word |Warning

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| H302, H312, H332, H412 |-

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| P261, P264, P270, P271, P273, P280, P301+312, P302+352, P304+312, P304+340, P312, P322, P330, P363, P501 |- |Section4=! colspan=2 style="background: #f8eaba; text-align: center;" |Related compounds

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Related alkanenitriles

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|- }} Thiocyanic acid is a chemical compound with the formula HSCN which exists as a tautomer with isothiocyanic acid (HNCS).[3] The iso- form tends to dominate with the material being about 95% isothiocyanic acid in the vapor phase.[4]

Tautomerism between thiocyanic acid (left) and isothiocyanic acid (right)

It is a moderately strong acid,[5] with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.[6]

HSCN is predicted to have a triple bond between carbon and nitrogen. It has been observed spectroscopically but has not been isolated as a pure substance.[7]

The salts and esters of thiocyanic acid are known as thiocyanates. The salts are composed of the thiocyanate ion (SCN) and a suitable metal cation (e.g., potassium thiocyanate, KSCN). The esters of thiocyanic acid have the general structure R–SCN.

Isothiocyanic acid, HNCS, is a Lewis acid whose free energy, enthalpy and entropy changes for its 1:1 association with a variety of Lewis bases in carbon tetrachloride solution at 25 °C have been reported.[8] HNCS acceptor properties are discussed in the ECW model.


References

  1. Merck Index, 11th Edition, 9257.
  2. International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. pp. 784. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4. 
  3. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
  4. Beard, C. I.; Dailey, B. P. (1950). "The Structure and Dipole Moment of Isothiocyanic Acid". The Journal of Chemical Physics 18 (11): 1437. doi:10.1063/1.1747507. Bibcode1950JChPh..18.1437B. https://dspace.mit.edu/bitstream/1721.1/4934/1/RLE-TR-112-04711196.pdf. 
  5. Munegumi, Toratane (23 January 2013). "Where is the Border Line between Strong Acids and Weak Acids?". World Journal of Chemical Education 1 (1): 12–16. http://pubs.sciepub.com/wjce/1/1/4/index.html. 
  6. Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology. 
  7. Wierzejewska, M.; Mielke, Z. (2001). "Photolysis of Isothiocyanic Acid HNCS in Low-Temperature Matrices. Infrared Detection of HSCN and HSNC Isomers". Chemical Physics Letters 349 (3–4): 227–234. doi:10.1016/S0009-2614(01)01180-0. Bibcode2001CPL...349..227W. 
  8. . Barakat, T. M.; Nelson, J.; Nelson, S. M.; Pullin, A. D. E. (1969.) “Spectra and hydrogen-bonding of characteristics of thiocyanic acid. Part 4.—Association with weak proton acceptors”. Trans. Faraday Soc., 1969,65, 41-51