Thiocyanic_acid

Thiocyanic acid

Thiocyanic acid

Chemical compound (H–S–C≡N)

Thiocyanic acid is a chemical compound with the formula HSCN and structure H−S−C≡N, which exists as a tautomer with isothiocyanic acid (H−N=C=S).[8] The isothiocyanic acid tautomer tends to dominate with the compound being about 95% isothiocyanic acid in the vapor phase.[9]

Tautomerism between thiocyanic acid (left) and isothiocyanic acid (right)
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It is a moderately strong acid,[10] with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.[11]

One of the thiocyanic acid tautomers, HSCN, is predicted to have a triple bond between carbon and nitrogen. Thiocyanic acid has been observed spectroscopically.[12]

The salts and esters of thiocyanic acid are known as thiocyanates. The salts are composed of the thiocyanate ion ([SCN]−) and a suitable cation (e.g., potassium thiocyanate, KSCN). The esters of thiocyanic acid have the general structure R−S−C≡N, where R stands for an organyl group.

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.[13] HNCS acceptor properties are discussed in the ECW model. The salts are composed of the thiocyanate ion ([SCN]−) and a suitable cation (e.g., ammonium thiocyanate, [NH4]+[SCN]−). Isothiocyanic acid forms isothiocyanates R−N=C=S, where R stands for an organyl group.

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References

  1. [1]
    Merck Index, 11th Edition, 9257.
  2. [2]
    "Thiocyanic acid" entry in PubChem (database).
  3. [3]
    "Thiocyanic acid". The Merck Index. Royal Society of Chemistry.
  4. [4]
    von Richter, Victor (1922). Organic Chemistry or Chemistry of the Carbon Compounds. Vol. 1. Translated by Spielmann, Percy E. Philadelphia: P. Blakiston's Son & Co. p. 466.
  5. [5]
    ILO and WHO staff. "Thiocyanic acid" safety card. European Commission
  6. [6]
    Birckenbach, Lothar (1942). Forschungen und Fortschritte. 18: 232–3 {{cite journal}}: Missing or empty |title= (help). As cited in CAS Common Chemistry.
  7. [7]
    Brown, Jay A. (ed.; 2024), "Thiocyanic Acid" in Haz-Map (database). Engineered IT.
  8. [8]
    Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  9. [9]
    Beard, C. I.; Dailey, B. P. (1950). "The Structure and Dipole Moment of Isothiocyanic Acid" (PDF). The Journal of Chemical Physics. 18 (11): 1437. Bibcode:1950JChPh..18.1437B. doi:10.1063/1.1747507. hdl:1721.1/4934.
  10. [10]
    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.
  11. [11]
    Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology.
  12. [12]
    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. Bibcode:2001CPL...349..227W. doi:10.1016/S0009-2614(01)01180-0.
  13. [13]
    . 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
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