Boffis
International Hazard
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2,3-dimethyl-2-butene chemistry
Some 12 to 15 years ago I was given a bottle containing what the label claims to be the above named compound (sometimes refered to as
tetramethylethylene). The 1L bottle contains about 800ml of a virtually colourless mobile liquid that is a mild lachrimator. I have finally come up
with a use for it, to prepare 2,3-dihalo-2,3-dimethyl-2-butane. These, when dehalogenated with alcoholic potassium hydroxide give
2,3-dimethylbutadienes, my target compound.
I decided to run a small experiment to see how easily the dimethylbutene can be brominated. I diluted 10ml of the liquid with 10ml of dichloromethane
and then prepared a separate solution of 6ml of bromine in 10 ml of DCM. I added the bromine solution to the butene derivative dropwise, the slight
exotherm was easily controlled and the bromine decolorized almost instantly at first, however, after I had added about 10ml of the bromine solution
the bromine no longer disappeared and the solution turn first brown and then almost black. I warmed the solution a little but this did not cause
decolorization. In the end I stopped the addition at about 13ml of bromine solution. I let it stand over night and then washed the DCM solution with
sodium metabisulphite solution several times but this did not decolourize the reacion mixture. I dried it with magnesium sulphate and then distilled
off the DCM. I expected the residue to solidify as the Mp of the dibromo-dimethylbutane is reported to be 168 C but it didn't and doesn't solidify in
the freezer either.
I decided to look at the purity of the original material, particularly given it age. I fractionally distilled the whole lot and got about 100ml of
distillate between 70 and 80 C (2,3-dimethyl-2-butene reportly boils at 72-73 C) then distillation practically ceased and the temperature began to
rise steeply. I stopped distillation for fear of peroxides.
I can find very little information about this compound and even Armarego's "Purification of laboratory reagents" only gives purification details
without explaining what is being removed. (the references given are equally useless in this respect).
Does anyone know anything about this compound or similar compounds that might give me a clue as to what the other 700ml now consist of? Is it a simple
polymer (but the liquid is thin and mobile), an oxidation product (Armarengo suggests storing and handling the compound under an inert atmosphere) are
peroxides likely?. Armarengo suggests distillation from KOH so I tried this on the residue, the KOH eventually dissolved yielding a cloudy pale yellow
liquid from which nothing distilled below 80 C.
The 100ml I recovered will be fractioned again between tighter limits and will probably be sufficient for my needs but I am curious as to what the
rest has become.
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Pumukli
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I don't know much about olefines but I can tell you that last year when I was preparing my cyclohexene I was surprised to learn that this compound
will eventually produce peroxides and it is an actual hazard of cyclohexene! Maybe your tetramethyl-etene is similar in this respect.
I think what you have is a polimer, maybe just dimer/trimer at most, so it is still thin and mobile. If it doesn't decolorize bromine then it is not
an olefin for sure. I can't recall an olefin that fails the bromine test.
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Boffis
International Hazard
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Thanks Pumukli, yep Armarego's book prescribes refluxing with sodium borohydride to remove peroxides from several similar alkenes but not for
dimethylbutene but like you say the similarity is ominus. Any way I carefully fractioned the 100ml I did recover and got about 25ml of low boil
alkane, rather inert but volatile stuff, boils around 55-60 C (dimethylbutane?) and about 45ml boiling between 71-75 C that appears to be mainly the
required butene. It reacts cleanly and instantly with bromine and the colour of the excess bromine rapidly removed by sodium sulphite solution. When
the DCM dilutant evaporates away it leaves colourless moist crystals which looks promising! This was done on 2 ml in 10ml of DCM, I didn't measure the
bromine, I just pipetted it in neat.
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