Thiocyanic acid
Thiocyanic acid is a chemical compound with the formula HSCN which exists as a tautomer with isothiocyanic acid. The iso- form tends to dominate with the material being about 95% isothiocyanic acid in the vapor phase.
It is a weak acid, bordering on strong, with a pKa of 1.1 at 20 °C and extrapolated to zero ionic strength.
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.
The salts and esters of thiocyanic acid are known as thiocyanates. The salts are composed of the thiocyanate ion and a suitable metal cation. 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. HNCS acceptor properties are discussed in the ECW model.
