Hydrogen disulfide

Hydrogen disulfide (H2S2) is an inorganic compound. This malodorous oil decomposes readily to hydrogen sulfide (H2S) and elemental sulfur.

Structure
The structure of hydrogen disulfide is similar to that of hydrogen peroxide, with two central sulfur atoms and two outer hydrogen atoms. However, the skew angle of the molecule is different, as the H-S-S bond in hydrogen disulfide has a near-standard 90° torsion angle of the syn conformer.

Skew angle in the H2S2 molecule is 90.6°, compared with 113.7° in H2O2. Molecular dimensions in these two molecules are: O-O, O-H, S-S and S-H bonds have lengths of 1.490, 0.970, 2.055 and 1.352 ångströms respectively.

Synthesis and reactions
Hydrogen disulfide can be synthesised by dissolving alkali or alkali earth metal polysulfides in water. When the solution is mixed with concentrated hydrochloric acid at &minus;15 °C, a yellow oil consisting a mixture of polysulfanes (H2Sn) will pool below the aqueous layer. Fractional distillation of this oil gives hydrogen disulfide separate from any other polysulfides (mostly trisulfide).

Hydrogen disulfide readily decomposes under ambient conditions to hydrogen sulfide and sulfur. In organosulfur chemistry, hydrogen disulfide adds to alkenes to give disulfides and thiols.

Health effects
Hydrogen disulfide has been described as "having a severe and irritating odour" that is similar to camphor or sulfur chloride, causing "tears and a smarting sensation in the nostrils". If it is present in high concentrations, it can cause dizziness, disorientation and ultimately unconsciousness.