Hydrogen disulfide

Hydrogen disulfide is the inorganic compound with the formula H2S2. This hydrogen chalcogenide is a pale yellow volatile liquid with a camphor-like odor. It decomposes readily to hydrogen sulfide (H2S) and elemental sulfur.

Structure
The connection of atoms in the hydrogen disulfide molecule is H\sS\sS\sH. The structure of hydrogen disulfide is similar to that of hydrogen peroxide, with C2 point group symmetry. Both molecules are distinctly nonplanar. The dihedral angle between the H^{a}\sS\sS and S\sS\sH^{b} planes is 90.6°, compared with 111.5° in H2O2. The H\sS\sS bond angle is 92°, close to 90° for unhybridized divalent sulfur.

Synthesis
Hydrogen disulfide can be synthesised by cracking polysulfanes (H2S_{n}) according to this idealized equation:
 * H2S_{n} → H2S2 + S_{n−2}

The main impurity is trisulfane (H2S3). The precursor polysulfane is produced by the reaction of hydrochloric acid with aqueous sodium polysulfide. The polysulfane precipitates as an oil.

Reactions
Upon contact with water or alcohols, hydrogen disulfide readily decomposes under ambient conditions to hydrogen sulfide and sulfur.

It is more acidic than hydrogen sulfide, but the pKa has not been reported.

In organosulfur chemistry, hydrogen disulfide adds to alkenes to give disulfides and thiols.

Quantum tunneling and its suppression in deuterium disulfide
The deuterated form of hydrogen disulfide, deuterium disulfide D\sS\sS\sD (dideuterodisulfane), has a similar geometry to H\sS\sS\sH, but its tunneling time is slower, making it a convenient test case for the quantum Zeno effect, in which frequent observation of a quantum system suppresses its normal evolution. Trost and Hornberger have calculated that while an isolated D\sS\sS\sD molecule would spontaneously oscillate between left and right chiral forms with a period of 5.6 milliseconds, the presence of a small amount of inert helium gas should stabilize the chiral states, the collisions of the helium atoms in effect "observing" the molecule's momentary chirality and so suppressing spontaneous evolution to the other chiral state.

Health effects
In high concentrations, it can cause dizziness, disorientation and ultimately unconsciousness.

Historic literature

 * Georg Brauer: Handbook of Preparative Inorganic Chemistry Volume I, page 391, Wiley, 1963.
 * von Richter, Victor: Translated by Edgar F Smith, "A Text-Book of Inorganic Chemistry", Page 111, P. Blakiston, Son & Co., 1893
 * von Richter, Victor: Translated by Edgar F Smith, "A Text-Book of Inorganic Chemistry", Page 111, P. Blakiston, Son & Co., 1893