Crypto
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Some questions about the Al/Hg reduction system.
Hi. I know there is a lot information about this system here on the forum as well as on other sites. I read much about it but there are a couple of
things that are not clear to me. I'm experimenting with the reduction of nitromethane lately with pretty good results. I want to give it a go with a
nitrostyrene (I got it from condensing nitromethane and benzaldehyde) in the near future.
First of all. What is the needed amount of HgCl2? Should I calculate it based on the weight of Al or it's surface?
Second. What is the needed amount of Al? I went through many reports about reducing nitromethane, imines and nitrostyrenes. In all cases a big amount
of aluminum is used. Especially with nitrostyrenes. Is this necessary? A lot of it was unreacted in my trial runs.
Lastly. Can I control the rate of the reaction by gradually adding the substrate to the activated Al in solvent? When I dump it all I get a really
vigorous reaction for the first 3 minutes. Resembles a volcano actually. After that it's nice and steady. How can I control that? An ice bath is just
not good enough...
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pepsimax
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You need around the same quantity of Al as substrate, ie 10g foil per 10g nitrostyrene. Maybe a slight excess.200mg should be fine for 25g Al. Wash
the excess hg salt out.
Yes you can add the substrate slowly, this will slow it down. there's many variables you can play with, heat, Al size, addition rate...
Though I urge you to experiment with another less toxic system, it really is very nasty stuff. Elemental hg is safer, drop some in the flask and add
conc nitric acid. Then add water and Al.
Mercury chloride is volatile in case you didn't know.
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Crypto
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The thing with Al is that lots of it does not react. That lead me to think that actually not that much of it is necessary.
I was browsing some write ups about it and there were some that gave smaller numbers. Osmium stated that about 60 grams per mole of substrate is
enough, if thick aluminum is used (he was talking about the reduction of imines though, I don't know if nitrostyrenes make a difference). There was
also a French patent which gave equimolar amounts. Again it was dealing with the reduction of imines so I'm not sure about nitrostyrenes.
The other thing that bothers me is the volume of the solvent. There is a German reference giving the ratio 1:1:1:1 for nitrostyrene:acetic
acid:ethanol:water. I couldn't find any reports of attempts to reproduce it. The reference gave decent yields.
Lastly what alcohol is preferred? EtOH or IPA? Or maybe they could be omitted because acetic acid itself is a better proton donor?
I know I have to be careful with it and would love to substitute it with something safer. Unfortunately I hadn't had luck with the Al/Cu system.
I'll post the references I mentioned tomorrow. My connection is a little woobly today.
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Crypto
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Sorry for such a delay but I was cut off the internet.
Here are the references:
Osmium's variation on the Al/Hg reductive amination
Al/Hg Reductive amination under pressure
The German reference is at the very bottom here:
Reduction of Nitroalkenes with Aluminum Amalgam
Funny that most of the information on Al/Hg reductions comes from The Hive. Google throws them out as the first results.
Anyway I tried to reduce the volume of the solvent and the results were bad. Maybe because stirring is much harder when lots of sludge builds up
quickly in such a low volume. Ineffective stirring probably lowers the yield.
Two things concern me the most.
Most of the Al I use does not get consumed during the reaction. No matter how long I'll reflux. Maybe I'm using too litle HgCl2? How much mg per gram
of Al should be used?
The second thing that makes me wonder is the necessity of refluxing after the initial vigorous reaction slows pace. This is supposed to drive it to
the end but doesn't it also cause some of the nitrostyrene to polimerize? Wouldn't it be better to keep the temperature around 65 C ?
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