Thiocyanic acid

Thiocyanic acid is a chemical compound with the formula HSCN and structure H\sS\sC\tN, which exists as a tautomer with isothiocyanic acid (H\sN\dC\dS). The isothiocyanic acid tautomer tends to dominate with the compound being about 95% isothiocyanic acid in the vapor phase.


 * Thiocyanic Acid Tautomerism V.1.svg

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

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

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\sS\sC\tN, 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. < 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\sN\dC\dS, where R stands for an organyl group.