Melgar
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Grignard vs. organolithium
Most of the chemistry texts seem to imply there's not much difference at all, and yet organolithium reactions seem to always seem to be run at -78˚C.
Grignard, not so much.
Still, there seem to be some practical considerations here, especially since organolithium reagents often take a few extra steps to form
without leading to Wurtz coupling, if I remember right.
Still, the takeaway seems to be that organolithium reagents are more reactive, and that could often be a good thing. For instance, if we want to
react n-butyllithium with a nitrile, I imagine that would proceed better than if its Grignard counterpart were used, since nitriles often don't react
as well to Grignards. But in the case of a less-reactive substrate, does that mean temperature must be at -78˚C here too? And does the solvent have
to be some sort of dangerous, hard-to-source, expensive ether?
And finally, how difficult would it be for an amateur to synthesize n-butyllithium? I've already produced some n-bromobutane from n-butanol; is the
next step just reacting it with lithium and a bit of sodium in an appropriate solvent? And how critical is dry ice + acetone to all these reactions?
It doesn't seem like n-butyllithium needs cryogenic temperatures in storage.
Should organolithium chemistry be something I ought to be experimenting with at some point?
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clearly_not_atara
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Organolithium reagents are more air-sensitive. Keeping the whole thing on dry ice prevents deflagration if your confinement has a leak. One upside to
organolithiums is that more solvents work whereas organomagnesium requires certain ethereal solvents to work right.
Wurtz coupling is usually only an issue with benzyl/allyl lithiums unless there's copper contamination.
Not sure what to do about nitriles honestly.
[Edited on 04-20-1969 by clearly_not_atara]
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Sidmadra
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I'm interested in the synthesis of these as well. I don't think Sodium is actually necessary, but just speeds up the reaction - I could be wrong
though. If i recall, I think I have encountered synthesis of alkyl lithiums that do not require -78c. I think the important thing here isn't
necessarily the low temperature, but rather controlling the exothermic reaction and preventing run away.
I want to try to make longer-chained alkyl lithiums. Hexyllithium (85% concentration or less), and Octylithium (95=99%) are not pyrophoric and can be
handled relatively easily in air. They'll degrade to an extent, but if you work fast I imagine it would be fine. There are also longer-chain alkyl
lithiums that are solids at room temperature and can be handled in such a manner. These all have slightly less reactivity than short chained variants,
but can participate in all the same reactions.
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aga
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ISTR that Grignard's teacher, name began with B i think, had a way to do the C-C bond thing with zinc, which was less water-sensitive.
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Melgar
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@aga Yep, the Barbier reaction. It was in situ and didn't work on as many substrates. Even aluminum can work for Grignard reactions under
the right conditions.
My point was whether for less-reactive substrates like nitriles, there would any advantage to using the organolithium versions.
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Dr.Bob
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Organo lithiums do best when cooled for the deprotonation step, to avoid further reactions, like homocouplig and elimination reactions. Once
deprotonated, many can be warmed to milder temps, in some cases it is needed to allow the complete deprotonation of the substrate. Often the are
recooled before the next step to ensure no thermal runaways upon addition of the electrophile or next reagent.
I would not try to make a alkyl-lithium above the suggested temperature, as those reactions are well established, and tested, so why take chances.
Dry ice is easy to get an many stores, so just use it.
But hexylithium is a good idea, much safer than butyl. The FMC guide that was posted here recently is a great starting point to read about lithium
alkyls.
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DIBALL
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I did both reactions and for the organolithium reaction I just used elemental Lithium, which was stored under toluene. The first time I didn´t
realize how fast Lithium oxidizes and after I put the pieces, that I just cut, inside, the reaction took really long to start and then I put some
bromomethane inside, which solved the problem.
The second time I cut the lithium in the toluene and had no oxidation problems! The reaction can be controlled quite easily if you put larger lithium
chunks inside, since the overall surface areas are smaller.
I like organolithium reactions more, since they can be easily controlled, don´t need much preparation like with the magnesium and also it is cooler
to see the shiny lithium chunks bubbling and disappearing. If this stops, the reagent is ready to use. Downside is, that lithium is a lot more
expensive than magnesium...
N-butyllithium isn´t necessary, but faster.
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