Augustine Silveira, Jr. and Henry D. Bretherick, Jr. State University of New York College at Oswego Osweao. .-- = ~NY , 13126 Ei-ichi Negishi, Syracuse University Syracuse, NY 13210
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I I
NMR Determination of n-ButylliMm
Because of the need to use n-butyllithium in our organic undergraduate project-oriented laboratories (I), a more efficient method was developed to determine the concentration of commercial n-hutyllithium in bexane using proton magnetic resonance analvsis. The method used could serve also as a useful instrumental analysis, inorganic or physical chemistry experiment for the undergraduate student. Present methods generally make use of titrations which are tedious and destructive in their analyses. Other methods require special equipment and additimal purified reagents (2-9). Our method of drtmninarion "in-butvllithium isa modification of the method developed by 1rwk and Reed (9)and the concentration of n-hutyllithium was calculated by pmr analysis using an adaptation of Kasler's equation (10); Our method utilizes the fact that the terminal methylene erouu of n-hutvllithium exhibits a characteristic uvfield trip~kta t 6 -0.G ppm which is integrated readily. ~ t h z i n g benzene, which was thoroughly dried and purity checked by VPC, as an internal standard ( 8 7.38 ppm), the concentration of n-butyllithium in hexane was determined by pmr analysis to be 1.61 (average of 3 runs). The reported commercial value prnr method was 1.60 M. In other ex~erimentscomvarinp.our . to a titration method (2, 3) using a new commercial sample labeled 1.60 M, the pmr data gave an average (5 runs) of 1.63 M while the titration method (2,3) gave an average (5 runs) of 1.67 M. The titration results are slightly higher than the pmr analysis and are expected since titration gives the total lithium content comprising hutyllithium, butoxide, and hydroxide. All nmr snectra for each exneriment were intemated I'iw times and excellent repruducihility war obtained. 'I'he Dmr annlvsir m r t h d of determining the cunvmtration ofn-l,~;tSllithiumproved ro be horh rim& and fast as -.veil as non.dectrurtive in thnt the n-l~utsllithiumwas used bv students in further experiments.
Experimental To a dry nitrogen-flushed,septumed, 50 ml Erlenmeyer flask was introduced.. hv sminee. amount of drv distilled benzene as - . a weiehed " the internal standard. For optimum results, it is advisable to insert the end of the syringe needle into a rubber stopper when weighing the henzene. Benzene is a suitahle reference since it has a large single peak which does not overlap with the spectrum of butyllithium and its vapor pressure is relatively low. Preferably the amount of benzene to he added should give approximately the same size integral as the
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560 / Journal of Chemical Education
methylene triplet of the n-butyllithium; in this case approximately three times the number of moles of n-butyllithium over moles of benzene was used. Neat was added, by syringe, a known volume of n-butyllithium in hexane solution. The flask was swirled, and an aliquot of the sample was removed by syringe and transferred to a dry nitrogen-flushed,septum-capped pmr tube. The pmr spectrum of the solution was obtained and the benzene singlet and n-butyllithium triplet were integrated. The integrals were taken five times, measured and averaged. The molar concentration of n-butyllithium in hexane was calculated using the following adaptation of Kasler's equation (10).
Molarity of n-hutyllithium = W S N A M,NuHsVu where W, = weight of standard N, = number of protons in pmr signal of standard N, = number of protons in prnr signal of unknown H, = integral height of pmr signal of standard H. = integral height of prnr signal of unknown h l . = mol,~rtrlnrweight ofrtandard Y, = wlwnt
