Thiocyanic acid – Wikipedia

Thiocyanic acid[1]
Skeletal formula of thiocyanic acid with the explicit hydrogen added
Spacefill model of thiocyanic acid
Preferred IUPAC name

Other names

ECHA InfoCard 100.006.672 Edit this at Wikidata
EC Number
MeSH thiocyanic+acid
  • InChI=1S/CHNS/c2-1-3/h3H checkY
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)
Solubility soluble in ethanol, diethyl ether
log P 0.429
Acidity (pKa) 0.926
Basicity (pKb) 13.071
GHS labelling:
GHS07: Exclamation mark
H302, H312, H332, H412
P261, P264, P270, P271, P273, P280, P301+P312, P302+P352, P304+P312, P304+P340, P312, P322, P330, P363, P501
Related compounds

Related alkanenitriles

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Chemical compound

Thiocyanic acid is a chemical compound with the formula HSCN which exists as a tautomer with isothiocyanic acid (HNCS).[4] The iso- form tends to dominate with the material being about 95% isothiocyanic acid in the vapor phase.[5]

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

It is a moderately strong acid,[6] with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.[7]

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.[8]

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.[9] HNCS acceptor properties are discussed in the ECW model.


  1. ^ Merck Index, 11th Edition, 9257.
  2. ^ Richter, Victor von; Spielmann, Percy E., trans. (1922). Organic Chemistry or Chemistry of the Carbon Compounds. Vol. 1. Philadelphia, Pennsylvania, U.S.A.: P. Blakiston’s Son & Co. p. 466.
  3. ^ International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 784. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
  4. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  5. ^ 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.
  6. ^ 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.
  7. ^ Martell, A. E.; Smith, R. M.; Motelaitis, R. J. (2001). NIST Database 46. Gaithersburg, MD: National Institute of Standards and Technology.
  8. ^ 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.
  9. ^ . 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