Thiocarbonic acid

Thiocarbonic acid is an acid with the chemical formula H2CS3 (or S\dC(SH)2). It is an analog of carbonic acid H2CO3 (or O\dC(OH)2), in which all oxygen atoms are replaced with sulfur atoms. It is an unstable hydrophobic red oily liquid.

It is often referred to as trithiocarbonic acid so as to differentiate it from other carbonic acids containing sulfur, such as monothiocarbonic O,O-acid S\dC(OH)2, monothiocarbonic O,S-acid O\dC(OH)(SH), dithiocarbonic O,S-acid S\dC(OH)(SH) and dithiocarbonic S,S-acid O\dC(SH)2 (see thiocarbonates).

Discovery and synthesis
It was first reported in brief by Zeise in 1824 and later in more detail by Berzelius in 1826, in both cases it was produced by the action of carbon disulfide on a hydrosulfide salt (e.g. potassium hydrosulfide).
 * CS2 + 2 KSH → K2CS3 + H2S

Treatment with acids liberates the thiocarbonic acid as a red oil:
 * K2CS3 + 2 HX → H2CS3 + 2 KX

Both the acid and many of its salts are unstable and decompose via the release of carbon disulfide, particularly upon heating:
 * H2CS3 → CS2 + H2S

An improved synthesis involves addition of barium trithiocarbonate to hydrochloric acid at 0 °C. This method provided samples with which many measurement have been made.
 * BaCS3 + 2 HCl → H2CS3 + BaCl2

Despite its lability, crystals of thiocarbonic acid have been examined by X-ray crystallography, which confirms the anticipated molecular structure of a trigonal planar molecular geometry at the central carbon atom. The C-S bond lengths range from 1.69 to 1.77 Å.

Reactions and derivatives
Thiocarbonic acid is acidic, with the first pKa being around 2. The second pKa is near 7. It dissolves S8, but does not react with it.

Salts and esters of trithiocarbonic acid are called trithiocarbonates, and they are sometimes called thioxanthates.

Thiocarbonic acid reacts with bifunctional reagents to give rings. 1,2-Dichloroethane gives ethylene trithiocarbonate (S\dCS2(CH2)2). Oxalyl chloride gives oxalyl trithiocarbonate (S\dCS2(C\dO)2).

Applications
Thiocarbonic acid currently has no significant applications. Its esters find use in RAFT polymerization.