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Carbon tetrachloride

Dear Reader,

I saw a chemistry world blog article about carbon tetrachloride so now I feel compelled to write to you about an organic solvent from yesteryear, it is carbon tetrachloride or more correctly tetrachloromethane. This is the substance which sent Margo Jones to the other side when her carpet was dry cleaned with it, it also almost sent a friend of mine to the same place years ago. An old chemist who I know used to mouth pipette everything, one day he mouth pipetted solutions of things in carbon tetrachloride (I think he was measuring distribution ratios) and he told that hours later he started to vomit. He told me that he continued to puke for about three days and he suspects that he had a lucky escape (He recovered and then went onto have a productive life).

Top tip of the day : Never mouth pipette anything !

The thing which interests me most about carbon tetrachloride is the free radical chemistry of this molecule. I strongly suspect that the trichloromethyl radical is a very stable radical which contributes to some of the chemistry of carbon tet. I know many years ago both the Soviets and the Americans built PUREX plants using carbon tetrachloride as the diluent. The nice thing about carbon tet is it gives a very good quality uranium and plutonium product, also it is easy to purify the process solvent to recover pure carbon tet, it is non flammable and the phase separation is easy. But it had a horrible sting in its tail, when you irradiate carbon tetrachloride you form chloride anions. I suspect that the solvated electrons are captured by the LUMO of the carbon tetrachloride and then it splits up to form anionic chloride ions and trichloromethyl radicals. The chloride impurity in the PUREX plants quickly formed aqua regia with the nitric acid used in the PUREX process. The resulting corrosion soon caused both plants to be shut down.

I will get onto the free radical chemistry which occurs in flames later, but an interesting bit of free radical chemistry using carbon tetrachloride is used in the production of cypermethrin. This is a insect killing agent which I have bought at Ica and used to kill creepy crawlies in my house. A key step in the production of cypermethrin is the formation of the cyclopropane ring. The synthesis starts with a pericyclic reaction (Claisen rearrangement) which uses an allylic alcohol and triethyl orthoacetate to form γ,δ unsaturated ester. This ester it then reacted with carbon tetrachloride in a free radical reaction. Treatment with two bases then forms the cyclopropane ring which makes up a key part of the carboxylic acid part of the insect killing agent.

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