Stibnut
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Alcohol for Birch reduction
I got ahold of a couple of fairly inexpensive pressure tubes capable of 150 psi, and I've distilled out about 20 mL of (hopefully) anhydrous ammonia
from a 30% solution using a simple distilling apparatus packed with CaO as a desiccant, with the receiving flask in a dry ice/denatured alcohol bath.
Storing it in the pressure tube should be safe: NH3 doesn't reach 150 psi until it reaches 26 C and I have it in a freezer at -15 C. All reactions
will be done in a dry ice/alcohol bath so the NH3 doesn't boil as I have the tube open adding things.
I've decided I want to do a Birch reduction on benzene to make 1,4-cyclohexadiene. Insert meth joke here, but rest assured I'm not making that: I
wouldn't last a second in prison. Anyway, I have 1 g of Li from an element sample and I got a little 30 mL bottle of benzene from ebay.
My question is: what alcohol should I use as my proton source? It appears most labs on Birch reduction use ethanol, but I already have 99.9% methanol
and absolute anhydrous ethanol is hard to find online. I'd use the Klean-Strip denatured alcohol I'm using if it were anhydrous, but I suspect it's
probably 95% ethanol/5% water mixed with fairly pure methanol. Is there any difference between methanol and ethanol in terms of their behavior in the
reaction? Also, I see some people use t-butyl alcohol instead - why would they use that?
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Melgar
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You can always get ethanol the same way you got ammonia, just from high-proof vodka, and without the need for dry ice. Or do the classic diethyl
ether synthesis with sulfuric acid and some form of ethanol (not denatured if you can help it, although if you have a decent fractional distillation
setup denatured alcohol might work okay). You'll get your ether, but on redistillation, there's always some ethanol that gets through too, and that
stuff's always very nearly water-free. Denatured alcohol is usually mostly ethanol, but they always throw a bunch of other crap in it too, and don't
say what it is on the label. Usually, there's acetone, ethyl acetate, methanol, and at least two other random volatile chemicals in it, sometimes
tert-butanol and/or MEK. I can recognize most solvents by smell, and the hardware store stuff always has a few surprise chemicals in it. Personally,
I'd avoid denatured alcohol for any synthetic purpose. Even a liter of vodka fractionally distilled to like 90% then distilled over a desiccant to
98-99% or so would be better, because at least you know that if your synthesis fails, it wasn't because of some unknown denaturant. Of course, I can
buy Everclear (95% ethanol vodka) at the liquor store a block away, (thank you, New York State budget crisis of 2009! I used to have to have my friend
pick it up for me in Maryland!) but I'm assuming that's not an option for you.
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Stibnut
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Right, I completely forgot that I could just dry out most of the remaining 5% water with a desiccant. D'oh! I live in Illinois, and I believe the
190-proof Everclear is legal here. I haven't ever looked for it. Off to the liquor store tomorrow!
The dry ice/denatured alcohol bath is for cooling the reaction down to below the boiling point of anhydrous ammonia so that it doesn't boil as I open
the pressure tube to add the benzene/alcohol and the lithium. I didn't consider it a good idea to try using that in the Birch reduction because I'm
not sure what it's made of besides ethanol and methanol. I have dry methanol already anyway, so if that works I'll probably try it.
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Melgar
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Quote: Originally posted by Stibnut  | Right, I completely forgot that I could just dry out most of the remaining 5% water with a desiccant. D'oh! I live in Illinois, and I believe the
190-proof Everclear is legal here. I haven't ever looked for it. Off to the liquor store tomorrow!
The dry ice/denatured alcohol bath is for cooling the reaction down to below the boiling point of anhydrous ammonia so that it doesn't boil as I open
the pressure tube to add the benzene/alcohol and the lithium. I didn't consider it a good idea to try using that in the Birch reduction because I'm
not sure what it's made of besides ethanol and methanol. I have dry methanol already anyway, so if that works I'll probably try it.
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It's still *usually* a good idea to try to replicate existing experiments before trying variants on them. That way there's fewer possibilities for
what could have gone wrong if your experiment doesn't work. While it's true that ethanol and methanol behave very similarly chemically, methanol
behaves more like water than ethanol does, and can dissolve boric acid and chloride salts, unlike other alcohols. Granted, ethanol is used more often
in lab experiments if it doesn't matter which alcohol is used, just because it's slightly cheaper and its toxicity is very low for a solvent, so odds
are that's the main reason it's usually used.
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CaptainPike
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anhydrous ethanol
you mentioned you have CaO. This will be the perfect desiccant to use to move from the azeotropic (95%?) concentration possibly attained through
fractionation, to the anhydrous holy Grail of neatness :-)
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Stibnut
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Yep, I realized that too. I got myself some 190 proof Everclear and am drying about 150 ml of it with CaO right now.
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