The Analytical Chemistry of Organometallics Quantitative Determination of Orga noalkalies A.
F. CLIFFORD
and R. R. OLSEN
Purdue University, lafayette, Ind.
b The reaction of organoalkali compounds with iodine is an analytical method faster and probably more accurate than the methods currently in use. An excess of a standardized diethyl ether solution of iodine reacts with the organoalkali solution to b e analyzed. The organoalkali solution is added slowly to the iodine solution, thus minimizing coupling reactions by always maintaining an excess of iodine. The excess iodine is then titrated with a standardized aqueous thiosulfate solution to a starch end point. The method may also be used for other types of organometallic compounds, such as those of magnesium and lead.
I
previous paper (4) the authors outlined a method for the quantitative determination of certain dilithioorganics in the presence of other organoalkalies. The present paper describes an improved method for the determination of organoalkalies in general. The usual method of assaying a n organolithium solution involves hydrolysis of an aliquot sample and titration of the lithium hydroxide formed with standard acid (1, 9, 11). This method, however, can be seriously in error because of the presence of other bases besides the organometallic. For example, base may be formed as the result of hydrolysis by adventitious water vapor (14): pi
A
+ H20 LiR + LiOH
LiR
-*
LiOH
-*
+ RH
LinO
+ RH
the colloidal Lis0 settling only s l o d y , even out of hydrocarbon solvents, or from cleavage of the solvent ( 1 0 ) . RLi
+ (CPH6)d3
---t
RC2Hj
+ C2H,0Li
or from reaction with atmospheric or adsorbed oxygen (I.$) : RLi
RLI
RLi
+ GCHzC1
-+
$CHZR
+ LiCl
Hoivever, the best results reported for this reaction in the case of butyllithium n-ere 31% bibenzyl, 21% amylbenzene, and a small amount of n-octane, based on the butyllithium content as determined by acid titration. This method is tedious and suffers from certain difficulties described in a later section. A third method, applicable to both lithium and sodium compounds, is the reaction with carbon dioxide (11). RLi
+ COn
+
RC02Li
The carboxylic acid is freed by reaction lvith a strong acid RCOaLi H + -P RCOZH Li"
+
+
and either titrated or isolated and weighed, if sufficiently nonvolatilee.g., benzoic acid. This method suffers from handling losses as 11-ell as other difficulties discussed below. I n addition, it does not distinguish between organometallic and metal carboxylate already present as the result of reaction nith adventitious carbon diouide. IODINATION REACTION
I t seemed probable that, if carried out under appropriate conditions, the reaction of iodine with the organometallic should be quick, quantitative, and free of the difficulties mentioned above, because it depends entirely on redox rather than acid-base reactions. Gilman and coworkers have reported (12) that for Grignard reagents, this reaction results in very extensive coupling. 2 R>\IgBr
+ I? +.R? + 2MgIBr
+ 02 +.ROOLi
+ ROOLi
-*
2 ROLi
I n addition, the hydrolysis method is unsuitable for organosodium compounds because of the extreme violence of the hydrolysis reaction. Attempts have been made to improve the hydrolytic assay of alkyllithium 544
compounds by combining a titration for total basicity with a titration for the base left after the alkyllithium has been removed by reaction with benzyl chloride ( 2 ) .
ANALYTICAL CHEMISTRY
Table I. Apparent Normality of Phenyllithium by Various Methods Time, Hours Acld GO?Iz 0 1 386 1 120 1 2 1 1 6 1 311 1 163 1 247 1 011 1 121 32 48 1 077 . 1 002
which n-ould make it nonquantitative; however, these n-orkers always added the iodine to the organometallic solution, so that the organometallic mas always in excess during all but the last stages of the addition. On the other hand, Datta and Mitter (5) reported that if phenylmagnesium bromide were added slonly to an excess of iodine, the yield of iodobenzene was in excess of 90%. Consequently, this reaction was investigated as a means of quantitatively assaying organoalkali solutions. Procedure. Iodine was dissolved in ether t o form a n approximately 0.6N solution. T h e iodine solution was standardized using sodium thiosulfate solution which was standardized with copper wire after t h e method of Mellon ( I S ) . [The standardiz R t'ion of the thiosulfate was cross-checked with a copper solution which had in turn been standardized against (ethylenedinitri1o)tetraacetic acid.] Because 1 mole of phenyllithium reacts with 1 mole or 2 equivalents of iodine, at least 2 moles of iodine was taken for every mole of phenyllithium expected to be found. The standardized iodine solution was diluted with 20 ml. of anhydrous, peroxide-free ether. T o this solution a known volume of phenyllithium solution was added carefully, dropwise, with efficient stirring (magnetic stirrer) and in a n inert atmosphere (nitrogen). biter the reaction mixture had been stirred for 3 minutes, 25 ml. of water, acidified t o p H 5 with hydrochloric acid, vias added to the ether solution. Three grams of potassium iodide was added to increase the solubility of the iodine in the aqueous phase. The unreacted iodine was then titrated Fyith standardized thiosulfate solution, using a starch end point, and shaking well to ensure its sharpness. COMPARISON OF METHODS
Phenyllithium. Phenyllithium solutions were analyzed simultaneously b y acid titration, carbonation, and iodination a t intervals over 48 hours t o compare results as a function of time. The data from a typical set of analyses are shown in Table I. The benzoic acid obtained in the carbonation method was extracted, purified, and weighed. Each method s h o w a regular decrease in phenyllithium content, the decreases
300
310
/i
320 330 340
MILLIMICRONS Figure 1 . solution
Ultraviolet fluorescence of biphenyl in ether Figure 2. solution
02-free. 2537-A. excitation. 1.281 N phenyllithium. Iodine method 1 . Ether-soluble materials from hydrolysis of 2 ml. of phenyllithium in 50 ml.of diethyl ether 2. Solution of 2 ml. o f phenyllithium in diethyl ether 3. Ether-soluble moterials from analysis o f 2 ml. of phenyllithium in 50 ml. o f diethyl ether
being parallel for all three methods. The acid titration method averaged 12% higher than the iodination method, while the carbonation method averaged 8.5% lower, as expected. The change in normality by all methods was about 10% in the first 32 hours and 207, in the first 4s hours. The coupling reactions, 2$Li 2+Li
+ I,
+ ?COS
-
+ 2LiI
.-f
41,
$2
iLi2C03
+ CO
if exteneil-e. could be major sources of error in the iodination and carbonation rractions. The nearest reaction knon n to the coupling reaction suggested above as taking place during the carbonation reaction is that reported (8) t o occur betn een triphenylgerniyllithium or triphen~-lsilyllithiumand methyl triphenylgermane carboxylate.
$aGeLi
Ultraviolet fluorescence of biphenyl in ether
02-free. 2537-A. excitotion 1. Same as 1, Figure 1 (different slit opening) 2. 0.487 gram o f biphenyl in 50 ml. o f diethyl ether 3. 0.0682 gram o f biphenyl and 0.4342 gram o f iodabenzene ( o p proximate amounts expected from I2 analysis, 3, Figure 1 ) in 50 ml. o f diethyl ether
+ &eC02CH3
.-f
$eGez
+ CH30Li + CO
By analogy the reaction suggested above may go through the steps +Li
-+
+
$Li COP $C02Li $COzLi -,$2 Li20 Liz0 COa + LinC03
+
+
+ CC)
(On the other hand, coupling during the hydrolysis reaction does not affect the assay, as the same amount of lithium hydroxide results whether coupling takes place or not. 2+Li
+ 2H20
.-f
+Z
+ 2LiOH + H2)
Consequently. it n as very important t o establish the extent to nhich coupliig took place. To accomplish this the original phenyllithium solution and products of
reaction were examined by a rnodified Perkin-Elmer Spectracord Model 4000, using 2537 A. excitation (xenon), for the fluorescence peak of biphenyl (2900 to 3600 A.) (3). On a typical sample, the original ether solution of phenyllithium contained an amount of biphenyl equal to 3.7% of the phenyllithium content as determined by iodination assay. That the phenylithiuin exerted no quenching effect can be seen from the sharpness of the peak (curve 2) shoi5-n in Figure 1. (A quenching effect would characteristically have rounded off the peak.) Because iodobenzene is known (16) to quench the fluorescence of biphenyl, it n as necessary to correct for this in the analysis of the iodination products. Figure 2 shon s curves for the fluorescence of approximately the same amounts of biphenyl in ether: curve 2. biphenyl alone, VOL. 32, NO. 4, APRIL 1960
545
and curve 3, biphenyl in the presence of approximately the amount of iodobenzene expected from a n iodination reaction. From these data, it can be shown that the total biphenyl among the iodination products was not more than 9.4% of the phenyllithium content based on the iodination assay. Because 3.7% was present in the original phenyllithium solution, however, not more than 5.7% was produced during iodination. Because 0.5 mole of iodine is used up per mole of phenyllithiurn going to biphenyl as compared with 1 mole of iodine per mole of phenyllithium going to iodobenzene, the analytical error is actually only half as much or not more than 2.8%. Additional evidence on the accuracy of the determination comes from work on the reaction of triphenylantimony dichloride with phenyllithium in ether (16, 17). The reaction of the first 2 moles of phenyllithium gives pentaphenylantimony, which is soluble in ether. Excess phenyllithium gives lithium hexaphenyl antimonate, Li [Sb(Cd%)6], which, on the other hand, is insoluble. I t was invariably necessary to add 2.2 moles of phenyllithium, as assayed by acid titration, before the first lithium hexaphenyl antimonate appeared, strongly indicating that the acidimetric assay was 10% too high. All of the above observations indicate that the iodination assay is more accurate than either acid titration or carbonation. Biphenyl production during the carbonation reaction was similarly investigated. The biphenyl produced during carbonation was isolated and assayed gravimetrically. On a solution having a n original assay of biphenyl equivalent to 3.7% of the phenyllithium as determined by iodination, the biphenyl content after carbonation was equivalent to 7.1% of the phenyllithium, a n increase of 3.4%, which clearly demonstrates the occurrence of coupling during carbonation. At the same time, the phenyllithium content as determined by carbonation was only 90.3% of that determined by iodination. The amount of biphenyl produced during carbonation was found, however, to vary greatly from time to time, possibly as a function of the humidity of the day. The carbonation assay is low not only because of the occurrence of side reactions, but also because of handling difficulties. [It is interesting (though not significant to the analytical results) to note that it was found by fluorescence analysis that during hydrolysis a n amount of biphenyl equivalent to 31.1y0 of the phenyllithium (by iodination assay) was produced.] Butyllithium. A similar comparison of t h e acidimetric, iodometric, and
546
ANALYTICAL CHEMISTRY
Table I I . A p p a r e n t Normality of Benzyllithiurn by Various Methods
Time, Hours 0 3 16 32 48
Acid
A
B
0 694 .. 0.665 0:442 0 640 0:467 0.640 0:387 0.483 0 570 0.347 0.519
and organocadniiuni solutions. The iodination method has been used extensively for determination of tetraethyllead and found to be very accurate (6).
12
0.581 0.557
SUMMARY
0 509 0.487
The method outlined gives results which have invariably lain between those of the acid titration and carbonation methods. The indications are that it is certainly more accurate than the former and probably more accurate than the latter. There is, however, no absolute method against which analyses can be checked. The method is not subject to the obvious errors of the other methods. The iodination method is as convenient as any method available.
benzyl chloride methods for butyllithium assay mas made. T h e butyllithium was made by the reaction of n-butyl chloride \vith metallic lithium after the manner of Gilman ( 7 ) in pentane purified by successive washings with concentrated sulfuric acid, permanganate, and water, followed by drying and distillation a t 36.5” C. The two sets of data for the benzyl chloride analysis (Table 11) correspond (A) to a constant amount of benzyl chloride added, estimated to represent a sixfold excess based on the acidimetric analysis a t zero time, and (B) to a constant amount of benzyl chloride added, estimated to represent a 4.5-fold excess based on the acidimetric analysis at zero time. The methods (except for benzyl chloride, B) give parallel results, the iodination assay again falling between the other two. Inasmuch as it is difficult to conceive of any reason why the iodination results should be high and inasmuch as the acidimetric assay was high because of the presence of suspended colloidal lithium oxide (the solution showed a decided Tyndall effect), it is concluded that the benzyl chloride A assay is too low and the iodination method gives much more accurate results. The discrepancy increases with time, the benzyl chloride assay being low by 21, 24, and 29% a t 3, 32, and 48 hours, respectively. The benzyl chloride method appears to be very sensitive to conditions and experience generally has been that it is very erratic. Amylsodium. The success of t h e iodometric method n ith lithium compounds suggested t h a t it could be applied also to the assay of organosodium compounds. A single analysis of a n amylsodium solution in pentane yielded 0.123 mole by carbonation and 0.152 mole by iodination. Again it is diffcult to conceive of any means by which the iodination results could be high, Consequently, it would seem that the iodination method must be much more accurate (by about 20%). Other Organometallics. I n view of the results of Datta and Mitter it seems likely that the iodination method could also be used for the assay of Grignard reagents and probably also organozinc
ACKNOWLEDGMENT
Thanks are due to J. P. Paris of this laboratory for determining the fluorescence spectra and to T. V. Liston, who prepared the amylsodium solution and ran the carbonation assay on it. LITERATURE CITED
(1) Adams, R., ed., Org. Reactions 6, 353 (1951). --
,-l
7
r
(3) Bowen, E . J . , Williams, A. H., Trans. Faraday Soc. 35, 765 (1939). ( 4 ) Clifford, A. F., Olsen, R. R., ANAL. CHEY.31, 1860 (1959). 15) Datta. R. L.. Rlitter. H. I(. J. Am. ’. Chem. ioc. 41, ’287 (1919). (6) Frey, F. W,,P.O. Box 341, Baton \
I
Rouge, La., private communication.
( 7 ) Gilman, H., Beel, J. A., Brannen, C. G., Bullock, M. W., Dunn, G. E., Miller, L. S., J . d m . Chem. SOC. 71, 1499 (1949). (8) Gilman, H., Gerowi, C. W., Ibid., 77, 4670 (1955). (9) Gilman, H., Haubein, A. H., Ibid., 66, 1515 (1944). (10) Gilman, H., Haubein, A. H., Hartzfeld, H., J . Org. Chern. 19, 1034 (1954). (11) Gilman, H., Langham, W.,Moore, F. W.,J . Am. Chern. SOC.62, 2327 i1940). \ - - - - ,
(12) Gilman,
H., Wilkinson, P. D., Fishel, W. P.. l l e y r s , C. H., Ibid.,
45, 150 (1923). (13) Mellon, 31.G , “Quantitative Analysis.” D. 447. Thomas Y. Crowell Co.. Sew Pork, 1955 (14) Muller, E.. Topel, T., Ber. 72B, 273 (1939). (15) Olsen, R. R , >I. S. thesis, Purdue University, August 1957. (16) West, R.,.‘Chemical Applications of Spectroscopy,” -4.Teissberger, ed., Chap. VI, pp. 728, 731. Interscience, S e w York, 1956. (17) n’ittig, G , Clauss, I< d n n . 277, 26 (1952).
RECEIVED for review October 14, 1959. .4ccepted December 24, 1959. Division of Inorganic Chemistry, Symposium on Organometallic Compounds, 133th meeting, ACS, Boston, Mass., April 1959. Work supported by the Shern-in-Williams co.
