Sulfide

In chemistry, a sulfide (sulphide in British and Canadian English) is a chemical compound or combination of sulfur with an oxidation number of -2, with another chemical element or a radical thereof. Some covalent sulfur compounds, such as carbon disulfide (CS2) and hydrogen sulfide (H2S), are also considered to be sulfides. Thioethers are organic compounds of the form R-S-R' (where R and R' are organic radicals), which are also referred to as sulfides or (if R,R' are alkyl) dialkyl sulfides.

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The sulfide ion is S-2, an anion with a -2 charge on it. In aqueous solutions, sulfide ions are only present in large concentrations at alkaline pH (high pH), because at lower pH, H+ will combine with sulfide ions to form HS- or H2S. HS- is the hydrogen sulfide ion, hydrosulfide ion, or sulfhydryl ion. H2S is hydrogen sulfide, a water-soluble gas which is a weak diprotic acid. Ionically bonded sulfides can be thought of as salts of the acid hydrogen sulfide. Many inorganic sulfide salts are not very water-soluble and many have very low solubility in water. If an -SH functional group is covalently bonded to another atom or group such as an organic radical R in a thiol, then it is typically called a sulfhydryl group. Such sulfhydryl groups can also be weakly acidic, and can give up an H+ to form a substituted sulfide ion. For example, ethyl hydrosulfide, C2H5SH, can give up an H+ to form an ethyl sulfide ion,C2H5S-, although often other names are used for such compounds; see Thiol.

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The sulfur in sulfides (or in the sulfide functional groups) is in its lowest oxidation state. In sulfides, this sulfur can often be oxidized to a higher oxidation state. For example, the thioether dimethyl sulfide (CH3-S-CH3) could be oxidized to dimethyl sulfoxide (CH3-SO-CH3), which could in turn be oxidized to dimethyl sulfone (CH3-SO2-CH3). Disulfides are similar compounds having two sulfur atoms covalently bonded together and covalently or ionically bonded to the rest of the molecule or compound.

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Hydrogen sulfide gas has the odor of rotten eggs, and is also highly toxic. It is formed biologically in the sediments of swamps and in the treatment of sewage sludge by anaerobic digestion of sulfur containing proteins, or bacterial reduction of sulfates. It also occurs in some natural gas and in the emissions of some volcanoes, and as a byproduct of some industrial processes.

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