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Teaching and tetrafluoroethane

Dear Reader,

I have noticed that a row has erupted regarding the differences between the Swedish and Chinese educational systems, before I get onto the subject of modern freon I would like to give my penny’s worth. The Swedish system as sold to me is largely about people understanding and being able to use skills (sounds very good) while the far eastern system is about students having a good knowledge of things (sounds good also). But there is a problem, students need both.

I have encountered chemical engineering graduates who have never heard of the Seveso accident in Italy, in some ways this is a chem eng accident which is in some ways as bad as Chernobyl, to my mind to be ignorant of an accident which spews toxic muck which kills wildlife and endangers the local people is not a good way for a chem eng student to be. While the student might be able to do a series of calculations of things like heat flows in a set of reactors similar to the one in which the thermal runway occurred in Italy, it is a bad idea for a student to be totally unaware that a major foul up had occurred.

In a similar way I have encountered a PhD student in chemistry who was unaware that chlorine bleach and acid should not be stored in the same wooden cupboard. Now to chemists of my generation it is “schoolboy” level knowledge that chlorine bleach and cyanide are not compatible with acid, contact of potassium cyanide or chlorine bleach with acid forms toxic gas (hydrogen cyanide and chlorine respectively). I had to say that this was not a one off case, I once found a leaking bottle of acid in a crate of chemicals next to a glass bottle of potassium cyanide. The readers will be glad to know that as soon as I found this combination of chemicals, I reached in and pulled the bottle of potassium cyanide out of the crate before putting it in a cupboard which was free of bottles of acids.

To my mind having some knowledge is part of the tool kit of abilities which a chemist needs, a chemist needs to be able to recognise a series of dangerous situations and know instantly how to react. Rather than being required to work out everything from first principles a chemist needs to know things like that “water should not be used on a sodium fire” and “to protect gas cylinders from electric arcs, corrosive substances and mechanical abuse”.

On the other hand I have encountered plenty of chemistry students who fall into a series of traps caused by failure to do anything but recall facts. This takes a range of different forms, one example would be to ignore events in other parts of the world or sectors of society thinking that because an adverse event has not occurred it is impossible while ignoring a related event which occurred either somewhere else or in another sector.

But the one which annoys me most is when people refuse to even try to put two items of information together, for example if I show a chemical reaction such as the aldol reaction and then people want a new mechanism for a closely related reaction such as the Henry reaction, or the final step in the formation of DiNOSar from nitromethane, formaldehyde and cobalt(III) tris ethylene diamine.

To be good at chemistry you need both to be able to remember things and to be able to apply chemical logic and reasoning to new problems.

But back to freon, many years ago it was found that treatment of alkyl chlorides with hydrogen fluoride and a Lewis acid formed alkyl fluorides. In the patent literature it explains how the treatment of trichloroethylene with HF and a Lewis acid forms 1,1,1-trifluoro-2-chloroethane (HCFC-133a) with ease, treatment of the HCFC-133a with more HF under forcing conditions forms the product (HFC-134a).

We should consider the mechanism by which this reaction occurs. The most likely reaction is that the Lewis acid causes the HF to add to the alkene in the trichloroethylene to form 1,1,2-trichloro-1-fluoroethane. The next stage will be that the Lewis acid will pull off a chlorine atom to form a carbocation. If the chlorine at the 2 carbon is removed then the carbocation will be a primary carbocation with no hope of hyperconjugation with a C-H bond.

On the otherhand if one of the chlorines on the 1 carbon is removed then the carbocation will be stabalised by both resonance with the remaining chlorine and hyperconjugation with the C-H bonds on the 2 carbon. This carbocation will then react with a fluoride to form 1,2-dichloro-1,1-difluoroethane.

The removal of the last chlorine on the 1 carbon will form a carbocation which is only stabilised by the hyperconjugation with a C-H bond, this carbocation will form the freon-133a. In the old days freon-133a was used as a feedstock for the production of halothane by means of a free radiacal reaction with bromine. Now days halothane is not used much as it can cause liver damage, it can form trifluoroacetic acid in a human body.

Only under forcing conditions will the last chlorine atom come off to form the carbocation which will then form freon-134a. To do this you need to form a very unstable carbocation, so I imagine that forcing conditions will be needed to do this. While it might seem to some people that I have been working some sort of special mechanistic miracle, I have not I have just applied the ideas of how alkenes react with electrophiles via carbocationic intermediates to our freon problem. Any decent organic chemist could have done the same.

For those of you who do not understand freon codes the work like this,

First number is the number of carbons in the molecule minus one, if this number is zero then you leave it out

Second number is the number of hydrogen atoms in the molecule plus one.

Third number is the number of fluorine atoms in the molecule.


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