Some years ago, I was asked to suggest an organic solvent that could be used for the quantitative analysis of hydrocarbon mixtures based on the intensity of the C–H stretching vibrations in the infrared spectrum. This clearly called for a solvent with no C–H bonds, and moreover had no chlorine atoms, since the context was to prevent the depletion of stratospheric ozone under the Montreal Protocol. The obvious (to me) choice was carbon disulfide (CS2), but I expected to be laughed out of court, so to speak, because although it’s a very good solvent, it stinks, it’s highly flammable, it’s toxic, and it boils at the inconvenient temperature of 46°C. It took a while before I got the obvious response – thanks, but no thanks – and I think the reason for the delay was that most modern-day chemists are unaware of the existence of this simple chemical substance. As for “organic”, I wasn’t sure, but my trusty reference guide, the Chemical Rubber Company’s Handbook of chemistry and physics, listed it among both the organic and the inorganic compounds; so you pays your money and you takes your choice.
Although it’s very much a forgotten chemical, CS2 (also known as carbon bisulfide) has played important roles in Australian agriculture and mining. Cuming Smith & Co. in Melbourne produced it by reaction of sulfur vapour with red-hot charcoal, and marketed it as a rabbit poison. As you can see from the relative molecular weights – 76 for CS2 and 28 for nitrogen – carbon disulfide is much heavier than air, allowing the bunnies’ living quarters to be filled with the toxic vapour. This was pumped into warrens with equipment like that shown in an advertisement placed by an equipment manufacturer in the Australian Town and Country Journal in 1917. Cuming Smith’s agricultural chemist, F.E. England (an original member of the Australian Chemical Institute), said that there was no danger to the operator when it was used in the open.
That wasn’t the case when it was used in an enclosed space to kill mice that had invaded grain stacks. In late 1947, three men pouring 40 gallons (200 L) of the liquid over 12 500 bags of wheat in a building in central Brisbane suffered severe poisoning. One, who had been working at the bottom of the stack while the others poured from the top, died. When rescuers opened the door to retrieve his body, there was an explosion that killed one of them and collapsed the wall of the building onto a nearby cottage, killing two occupants there.
Carbon disulfide is subject to nucleophilic attack in much the same way as its notorious sibling, carbon dioxide. Upon reaction with potassium or sodium alcoholates:
RO– K+ + S=C=S → R–O–C(=S)S– K+
the products are dithiocarbonates known as xanthates. From the 1920s, xanthates were used as “collectors” in the froth flotation processes for separation of the ores of lead, copper or zinc from waste rock. The finely ground ore mixture was suspended in water containing dissolved xanthate and a frothing agent by bubbling air through the mixture, with the result that the valuable minerals were trapped in the froth, which could be skimmed off the top, leaving worthless material underneath, to be discarded.
To return to the challenge, my inability to identify a suitable solvent having no C–H and no Cl brought about the relaxation of the latter criterion and we settled on perchlorethylene (Cl2C=CCl2), which worked well provided it was freed of hydrocarbon impurities. It’s not an ozone-depleter and, although it is toxic, it is less so than CS2, so it’s manageable in a laboratory setting. “Perch”, as it is known in the trade, is being phased out of industrial uses like dry-cleaning, but you can still catch a whiff of it if you patronise a dry-cleaner that has not yet gone “chlorine free”.
