Quantitative Determination of Organolithiums

Organolithium compounds are highly sensitive to air and humidity. The commercial RLi usually contains varying amounts of alkoxides as impurities. The concentration of the organoUthium solution can be easily checked by several methods, which can be divided into two categories (i) direct titration methods, based usually on a color change of self-indicators, and (ii) double titration methods. Such different methods have been reviewed in detail [12, 151], The traditional double titration method provides the content of active organohthium and of lithium alkoxides/hydroxide (Fig. 26.9). 

the total base is determined by acid-base titration of LiOH formed by the hydrolysis of a sample of organolithium solution. Therefore, a known amount of the organolithium solution is quenched with excess of water and titrated with O.IN aq. H SO, or HCl using phenolphthalein as color indicator. 

In the second step, the organolithium reagent is destroyed with an organic halide (e.g., benzyl chloride or 1,2-dibromoethane). It is supposed that the organolithium reagent reacts quantitatively affording LiCl or LiBr and that the residual base (LiOH and lithium alkoxides) does not react. After hydrolysis with an excess of water, the amount of free (residual) base is determined. 

Active base RLi = Total base-Residual base 

The classical double titration method has certainly the advantage that it can also be employed to titrate organomagnesium intermediates, metal dialkylamides, and even radical anions. Moreover, self-indicators and anhydrous and oxygen-free conditions are not required. 

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Part of the NMR spectroscopy techniques mentioned in Section IV.B can be used for quantitative determination of organolithium compounds in solution. 

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