tatapouette
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More than BuLi ?
Hi all !
I posted a question in another post, which maybe was not the best one for.
Here it is :
Industrially, n-BuLi seems to be made following this reaction :
n-BuX + Li -> n-BuLi + LiX
with X = Cl or Br
As LiX precipitates, a simple filtration should be enough to recover the base.
If it is this easy, how come other compounds such as n-BuNa or n-BuK or even t-BuK (which must be really badass  ) etc, aren't commercially produced ?
(even if the potassium species can be made in situ by Schlosser's method)
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Hexavalent
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An extract from; http://forums.xkcd.com/viewtopic.php?t=39927&p=1598395
"Cotton and Wilkinson has a brief section on organo-sodium and organo-potassium compounds. As would be expected from the decreasing electronegativity
down the group they are more ionic than organo-lithium. In contrast to organo-lithium compounds, organo-sodium and organo-potassium compounds are
highly dissociated into ions, and therefore insoluble in non-polar solvents. To quote C&W, they are also 'exceedingly reactive' and are very
difficult isolate - presumably many of the polar solvents that will solvate them will also react. Perhaps this is the reason they aren't used? But
this isn't really my area of chemistry, so I might be barking up the wrong tree."
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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tatapouette
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Thanks !
This gives some interesting starting points.
So :
- for these compounds, the stronger the base, the shorter half-life before reacting with solvent. Even when used in THF or diethyl ether.
- but a polar solvent could make them easier to get.
If a polar solvent is needed, which couldn't be reactive with these species, what about good ol' HMPA ?
The phosphorous atom would probably not been attacked (too much hindrance) and I guess the dimethylamino groups couldn't be deprotonated.
Problem solved ?
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kristofvagyok
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Quote: Originally posted by tatapouette  | If a polar solvent is needed, which couldn't be reactive with these species, what about good ol' HMPA ?
The phosphorous atom would probably not been attacked (too much hindrance) and I guess the dimethylamino groups couldn't be deprotonated.
Problem solved ? |
No.
There is a P=O bond what could easily react with those stong bases. This is why they use TMEDA (tetramethyl-ethylenediammine) for dissolving the
unsoluble mass at living polymerisation of styrene initiated with BuLi.
Also, I have never ever used stronger bases than BuLi, but EtNa, and MeNa could easily explode, and I have red long ago from organoceasium reagents
like C6H6Cs6 what is a black solid, prepared from benzene and metallic Cs. It explodes on contact almost anything.
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tatapouette
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Really ?
I thought that :
- as the phosphorous atom is in a tetrahedron geometry ;
- and because of the (nearly charged) oxygen proximity,
the only possible attack would have been a SN2-type one, opposite to the oxygene.
But with the three NMe2 that hinder the phosphorous, I thought this couldn't happen.
| Quote: Originally posted by kristofvagyok |
This is why they use TMEDA (tetramethyl-ethylenediammine) for dissolving the unsoluble mass at living polymerisation of styrene initiated with BuLi.
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In this case, doesn't TMEDA act both as a polar solvent which cannot be (or at least, in these conditions, is not) attacked by the BuLi and
as a lithium complexant (which then can increase the activity of the carbanion) ?
| Quote: Originally posted by kristofvagyok |
Also, I have never ever used stronger bases than BuLi, but EtNa, and MeNa could easily explode, and I have red long ago from organoceasium reagents
like C6H6Cs6 what is a black solid, prepared from benzene and metallic Cs. It explodes on contact almost anything.
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Small molecules with big reactivity are often a problem ; in a certain way, bigger molecules can 'calm down' this extreme reactivity.
For example (and even if I would prefer to avoid this  ), I wouldn't make it a big
deal of making some phenyl azide (I actually made some substituted-phenyl azide derivatives on a few grams scale). Some caution is needed of course,
but nothing impossible.
But I would certainly not make even 100mg of alkyl azide such as methyl azide or ethyl azide !!
I agree with you that methylsodium, if it exists, would probably be very unstable. I guess bigger molecules, with more carbons, may be more stable.
Let's say t-butyl ? Or even bigger ones ?
And C6H6Cs6 : wow !! 
THAT is incredible !!
I guess it is even far more sensitive than diazomethane (which needs special glassware and stuff) !!
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ThatchemistKid
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actually, tatapouette, If I remember correctly if one uses n-chlorobutane for the reaction Lithium chloride precipitates out and can be removed but
if n-bromobutane is used then the LiBr associates with the BuLi and does not precipitate. This is actually why I obtained n-chlorobutane for my BuLi
synthing purposes . here is the wiki exert that talks about this!
If the lithium used for this reaction contains 1–3% sodium, the reaction proceeds more quickly than if pure lithium is used. Solvents used for this
preparation include benzene, cyclohexane, and diethyl ether. When BuBr is the precursor, the product is a homogeneous solution, consisting of a mixed
cluster containing both LiBr and BuLi, together with a small amount of octane. BuLi forms a weaker complex with LiCl, so that the reaction of BuCl
with Li produces a precipitate of LiCl.
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