Carbon disulphide reduction and dithiols
I was interested in the reaction of potassium dithio-oxalate with nickel, the metal is reported to give an intense magenta coloured reaction and the
paper describeds the preparation of the dithio-oxalate salt (H. O. Jones & H. S. Tasker, Jour. Chem. Soc. 1909 pp1904-1909; The action of
mercaptans on acid chlorides; Oxalyl chloride). The problem with Jones & Tasker’s synthesis is that it require some pretty obnoxious and
difficult to acquire chemicals such as oxalyl chloride and thioethanol (ethyl mercaptan) and so I did nothing until I read in one of my old chemistry
books that dithionite salts were made industrially at one time by reacting sulphur dioxide with an aqueous suspension of zinc. This gave me an idea;
could carbon disulphide be similarly reduced? The idea is not without merit since stronger reducing agents such as metallic sodium reduce carbon
disulphide to dithiole-2-thione-4,5-dithiol (see Org Syn Coll Vol 9 p203); most vicinal dithiols form interesting complexes with thiophile elements.
By analogy with the production of a S-S bond in the form of dithionite by the reduction of aqueous sulphur dioxide with zinc powder it was decided to
try the reduction of carbon disulphide in a similar manor in the hope of producing a C-C bond between two molecules.
Dithionite: 2SO2 + Zn = Zn2+O2S-SO22- One time industrial process
Tetrathiooxolate 2CS2 + Mg = Mg2+S2C-CS22- A nice idea
Dithiooxalate 2CS2 + 2OH- + Mg = Mg2+OSC-CSO2- + 2SH-
Carbon disulphide dissolves in alkalis to produce partial thiocarbonates which could then be reduced by a metal reductant to a thio-oxalate
derivative, the thiocarbonate is probably an equilibrium mixture of mono-, di- and trithiocarbonates. Since some metals have a high affinity for
sulphur they may preferentially remove this element from the reduction product, to minimise this affect magnesium was chosen as the reducing agent
since it has minimal affinity for sulphur in an aqueous solution. The anticipated reactions are:
2KOH + CS2 = K2CS2O + H2O
2KOH + CS2 + Mg = K2C2S4 + Mg(OH)2
or
2KOH + CS2 + Mg = MgC2S4 + 2KOH
Or
2KOH + CS2 + Mg = K2C2S2O2+ Mg(SH)2
etc
Details
1.25gm of magnesium powder was added to 7.6gm of carbon disulphide in a 50 ml conical flask immersed in a cold water bath. 10ml of 70% potassium
hydroxide solution was added slowly with vigorous stirring. A vigorous reaction set in and the rate of addition of alkali had to be reduced to prevent
loss of carbon disulphide by evaporation though the elevation of the temperature of the reaction mixture. The rate was adjusted to maintain a
temperature below 25°C (about 30mins). After this time a greenish homogenous solution was obtained with only a little black residue which was removed
by filtration.
Twice the solution’s volume of methanol was added causing the precipitation of a dark green material. The mixture was left to stand for 12 hours
with occasional stirring. The precipitate became paler and when filtered and dried almost white. Tests suggest the solid is mostly magnesium hydroxide
with a little basic carbonate and traces of reduced sulphur compounds. The filtrate was clear yellow and was slowly evaporated down in two stages,
first to recover the methanol and then on a water bath to remove the aqueous phase. Only when the volume was reduced to about 0.5 ml did crystals
form. The colourless crystals were removed and tested, they effervesced with dilute HCl and dilute HNO3 but no sulphur fumes either sulphide or
sulphurous oxide were evolved suggesting that the crystals were potassium carbonate. The tiny volume of yellow liquid would not crystallize but when
tested on spots of various metals gave several brightly coloured reactions, notably a deep purple with cobalt.
Spot tests with the yellow liquid residue:
All of the solutions were 0.1M with respect of the metal under test
Element or radical Spot colour Comment
Cu2+ Deep turquoise
Co2+ Deep purple
Ni2+ Very pale green Hardly visible at 0.1M
Fe2+ Dark green Turns brown after a few hours
Fe3+ Intense orange brown Ferric oxide ppt?
Mn2+ n.r. Brown spot appears after 2-4 days through oxidation
Pd2+ Dark brown Only a little darker than the palladium salt solution
Hg2+ Brown Slow, appears over several minutes
Ag+ Dark brown almost black Immediate spot
Pb2+ n.r.
Pt2+ n.r.
Bi3+ n.r.
Au3+ n.r.
Ce4+ n.r.
SeO32- n.r.
TeO32- n.r.
VO43- n.r.
MoO42- Indigo blue Sensitive reaction
WO42- n.r.
ReO4- n.r.
Os8+ Pale pink Weak but continues to darken over several days
UO22+ n.r.
CrO42- Yellow? Not much more intense than the colour of chromate
Alkali, alkaline earth and most M3+ aluminium like ions do not react or precipitate only the hydroxide. “n.r.” indicates no observable reaction
within a period of 10-15 minutes
Conclusion
The identity of the compound produced is not clear but one reaction is very telling, the lack of an intense red colouration with Ni2+ suggests that
the reactive compound is not a dithio-oxalate or similar as its reaction is reported to be an intense magenta red. Further work will be required to
improve the yield of the sulphur bearing species to allow its characterization. However, the blue molybdenum compound is remarkably similar to the
blue molybdenum complex with toluene 3,4 dithiol. The black silver reaction is probably the result of the formation of silver sulphide rather than a
silver complex with the ligand.
[Edited on 3-2-2013 by Boffis]
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