From: A De (ocad$##$mahendra.iacs.res.in)
Date: Sat Mar 25 2000 - 05:57:48 EST
We regularly prepare both n-BuLi and sec-BuLi in the lab. The most
important factors needed for good quality alkyl lithium are :
(a) The Li metal must be very finely dispersed. This done by heating Li
shots in liquid paraffin in a 3-neck flask fitted with an outlet at the
bottom and then vigorously stirring with a mechanical stirrer.
(b) VERY slow addition of alkyl halide to the suspension of the dispersed
metal (cyclohexane in the case of sec-BuLi and Petroleum ether in the case
of n-BuLi). 4 drops in 15 seconds.
(c)Addition of few pieces of Na for initiating the reaction.
Obviously all the operations are to be carried out in Argon atmosphere and
all the solvents should be thoroghly dried. Our experience tells that
commercially available Li dispersions are unsuitable. The dispersed metal
is washed free of Liquid paraffin with pet.ether. For sec-BuLi overnight
stirring is needed after the addition of sec-butylchloride. We do heat the
mixture in the case of n-BuLi but Chris' suggestion of sonication is good.
The preparation must be done under good care. Otherwise accidents are
likely.Pressure must be released from time to time while the metal is
dispersed.
We titrate our alkyl lithiums with diphenyl acetic acid as Chris does. It
is simple and reliable. The reference is :
W.G.Kofron and L.M.Baclawski, J.Org.Chem., 1976,Vol 41, 1879.
Cheers
Asish
On Fri, 24 Mar 2000, [ISO-8859-15] Olga Tšubrik wrote:
> Dear Colleagues,
>
> I hope somebody can give me a good advice for my problem.
> I synthesized butyllithium by reacting BuCl with excess Li (seven hours
> boiling in dry hexane). Then I let the solid phase to settle and
> then I siphoned the solution with the argon pressure to the
> dry flask. The solution is clear.
> All the procedure was done in oven-dried apparatus with dry inert gas
> flow (Ar). But now I have a problem with the determination of the BuLi
> concentration. I did it twicely.
> 1. Firstly I hydrolyzed the aliquot of BuLi
> by some water and then I titrated it with HCl. I got 1.76M for the
> concentration.
> 2. Secondly I did the so named "double-titration". I react BuLi with
> benzylchloride in ether (five minutes for reaction). BuLi must react
> with PhCH2Cl and the impurities will remain (BuOLi, LiOH).
> Then I decomposed the mixture with water and titrated it with HCl. Now I
> got 0.60M for the impurities concentration and so only 1.16M for the BuLi
> concentration. Is it possible I have so many impurities in my reagent?
> (It means that air humidity and oxygen reacted with the product!)
> What do you think about such "double-titration method"?????
> Can I trust this method? Or can somebody advice me something better?
> All the reagents were pure and dry.
>
> Can someone give me some ideas and suggestions? Help!
>
> Thank in advance.
>
> Olga Tshubrik
>
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>
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