[22868-13-9] · Na2S2 · Sodium Disulfide · (MW 110.12)
(nucleophilic agent for synthesis of organic disulfides and selective reducing reagent for the nitro group1-3)
Alternate Name: disodium disulfide.
Physical Data: mp about 495 °C; d 2.01 g cm-3.
Solubility: readily sol water; sol ethanol.
Form Supplied in: light yellow, microcrystalline, very hygroscopic powder; gradually darkens on heating. N2S2.6H2O, cream colored needles. Alkali metal disulfides are not commercially available.
Preparative Methods: Na2S2 preparations are cumbersome and sometimes afford mixtures containing the polysulfides Na2S4 and Na2S5. They involve reduction of Sulfur with Sodium in liquid ammonia,4 heating Na with S in a vacuum at 500 °C,4 or heating Na2S4 with Na in absolute ethanol under inert gas.4,5 Na2S2 is formed in situ by heating the aqueous or alcoholic Sodium Sulfide solution with elemental sulfur.6-9
Handling, Storage, and Precautions: can be stored for a limited period under anhydrous and oxygen-free conditions, but generally it is used directly as prepared as the aqueous or ethanolic solution.
Alkylation of Na2S2 with alkyl halides or sulfonates is a good synthesis of symmetrical disulfides.1,2,9-11 For example, when 1-bromopentane is treated with Na2S2 (generated in situ from Na2S and S), bis(n-pentyl) disulfide is formed in satisfactory yield (eq 1).12
Activated aryl halides treated with Na2S2 produce disulfides in high yields and numerous examples, among them the preparation of bis(2-nitrophenyl) disulfide (eq 2), have been reported.1,2,13
The Sandmeyer-type displacement of the diazonium group by S2- anion gives aryl disulfides. Reaction of this type is used for synthesis of thiosalicyclic acid (eq 3).6 Although many arenediazonium salts arylate Na2S2, one should beware of possible explosions, probably caused by an accumulation of diazosulfides.1,2
Na2S2 is a useful reducing reagent for the nitro group in the presence of other functional groups. It is used for selective reduction of 3-nitrobenzonitrile to 3-aminobenzonitrile, and of one nitro group in 4,6-diamino-1,3-dinitrobenzene (eq 4).7 The reaction of 4-nitrotoluene with Na2S2 and polysulfides, generated in situ from Na2S and S, involves an interesting disproportionation. The methyl group is oxidized to formyl, and the nitro group is reduced to NH2. Finally, 4-aminobenzaldehyde is obtained as a product (eq 5).8
Jacek Mlochowski & Ludwik Syper
Technical University of Wroclaw, Poland