resulted in an average relative decrease of 10% in intensity ratio values. Similar effects with volume changes have been noted by Eardley and Clarke (15) in rotrode techniques. Thus, in order to obtain satisfactory precision it was found necessary to use fresh electrodes and fixed volume aliquots for each exposure. Inasmuch as there are n o standard samples by which ac(15) R. P. Eardley and H. S . Clarke, Appl. Spectry., 19,69 (1965).
curacy can be checked, the best method is comparison with chemically determined values on the same samples. It is known that the samples which have been analyzed by both the chemical method (2) and the spectrographic method are not homogeneous; however, good agreement has been obtained as shown in Table V. RECEIVED for review August 30,1967. Accepted December 5, 1967. Work performed under the auspices of the U. S. Atomic Energy Commission.
A Procedurefor DeterminingGroup I,Group II,and Some Group 111 Orgalnometal Compounds C. A. Uraneck, J. E. Burleigh, and J. W. Cleary Phillips Petroleum Co., Chemical Laboratories, Bartiesuille, Okla. The cleavage of dialkyl or diary1 disulfides by organometal compounds and subsequent titration of the mercaptide formed with silver ion was developed as a procedure for analyzing Group I, Group II, and some Group Ill organometal compounds. This method has been successfully used for several years for analyses of organolithium compounds in this laboratory. Precision and accuracy are comparable to any procedure proposed to date. With the exception of lithium metal, the presence of impurities commonly found in organolithium preparations do not complicate the cleavage reaction. Stoichiometry and lack of side reactions indicate that the destruction of organolithium compounds by oxygen, water, alcohols, and ethers can be readily followed. Any active organometal compound capable of rapidlycleaving disulfidecan be determined. The procedure appears promising for the analysis of isopropylphenylpotassium, triisobutylaluminum, diethylmagnesium, and calcium acenaphthalene. Diethylzinc and dodecynyllithium did not cleave the disulfide at a rate high enough to be of practical use for this procedure.
THEPROBLEMS of analyzing active organometal compounds in solution are reflected in the number of analytical methods proposed in recent years. Some of the different analytical procedures offered for organolithium compounds are iodometric ( I ) , oxidimetric based on vanadium pentoxide ( 2 ) , thermometric titration with butanol (3), radiochemical using tritiated alcohols (4), high frequency titration with acetone (5), derivatization with dimethylphenylchlorosilane (6), direct titration with alcohols using colored indicators (7), and modification of the double titration procedure of Gilman and Cartledge (8). The advantages and limitations of the various procedures are discussed to some degree in the papers cited and will not be repeated here. (1) A. F. Clifford and R. R. Olsen, ANAL.CHEM., 32, 544 (1960). (2) P. F. Collins, C. W Kamienski, D. L. Esrnay, and R. B. Ellestad, Zbid., 33, 468 (1961). (3) W. L. Everson, Zbid., 36, 854 (1964). (4) D. R . Campbell and W. C. Warner, Zbid., 37,276 (1965). ( 5 ) S. C. Watson and J . F. Eastharn, Zbid., 39, 171 (1967). (6) H. 0. House and W. L. Respess, J . Organornerd. Chem., 4, 95 (1965). (7) S. C. Watson and J. F. Eastharn, Zbid., 9,165 (1967). ( 8 ) H. Gilman and F. K. Cartledge, Zbid., 2,447 (1964).
The use of organometal compounds in analytical chemistry is well established. The Zerewitinoff procedure is a well known example. Recently, the use of butyllithium for the determination of disulfides was described (9). The disulfide is cleaved by excess butyllithium and the lithium mercaptide is titrated by a mercurimetric procedure. In this paper the reverse-the cleavage of an excess of disulfide by reaction with an organometal compound-is the basis for determining active organometal compounds in solution. The resultant mercaptide is titrated by the silver nitrateamperometric technique (10). The basic equations are: R'M MSR
--
+ RSSR
+ Ag(NH3)2+
+ MSR RSAg + MC + 2NH3 R'SR
(1)
(2)
Any organometal compound capable of cleaving disulfides quantitatively could be determined by this means. This method has been used for several years for the analysis of organolithium at this laboratory and has been tested as a procedure for analyzing organo-potassium, -calcium, -aluminum, magnesium, and -zinc compounds. The experimental details, results, and limitations are presented in the following sections. EXPERIMENTAL
Reagents. Common chemicals were reagent grade or equivalent. Organometal or metal hydride compounds were obtained from Foote Mineral Co., Lithium Corp. of America, Orgmet, Texas Alkyls, and by synthesis in the laboratory. r-Butyl and t-heptyl disulfides were commercial products from Phillips Petroleum Co. n-Butyl and p-tolyl disulfides were white label grade from Distillation Products Industries or from Aldrich Chemical Co., Inc., Milwaukee. p-Tolyl disulfide solution was prepared to 0.25M in dry cyclohexane. Several grams of Linde 4A Molecular Sieves, 1/16-inchpellets, were added per 150 ml of solution and left in the bottle of disulfide solution to complete drying and t o keep it dry. Crown top bottles sealed with crimp-on caps over toluene-extracted, self-sealing Neoprene liners were used to keep the solution under nitrogen and to facilitate transfer L
(9) S.Veibel and M. Wronski, ANAL.CHEM., 38,910 (1966). (10) I. M. Kolthoff and W. E. Harris, IND. ENG.CHEM., ANAL, ED., 18, 161 (1946). VOL. 40, NO. 2, FEBRUARY 1968
327
naphthalene, ethylmagnesium, ethylzinc, and isopropyl phenylpotassium were analyzed by total alkalinity.
Table I. Analytical Values for 1.71 n-Butyllithium Obtained by Means of Aliphatic Disulfides Disulfide n-Bu Li found) n-Butyl 1.69 t-Butyl 1.71 t-Heptyl 1.69
RESULTS WITH BUTYLLITHIUM
Selection of a Disulfide. A number of aliphatic and aromatic disulfides are commercially available. Preliminary testing as reagents for analysis of organolithium compounds showed that the aliphatic disulfides gave values that were lower than those obtained by other analytical procedures, However, in the presence of 10 mmoles (about 23 %) of ethyl ether in the cyclohexane solvent, the aliphatic disulfides gave values that were comparable t o the reference value based upon the manufacturer’s assay (Table I). Aromatic disulfides reacted rapidly and quantitatively with organolithium compounds in the presence or absence of ethers. A comparison of quantitative results obtained with tolyl disulfide and n-butyl disulfide in the presence and absence of ether is shown in Table 11. On the basis of the consistent results obtained in the presence or absence of ether with tolyl disulfide, this aromatic disulfide was selected as the preferred reagent for the proposed analytical procedure. Analysis of Organolithium. A comparison was made of the total alkalinity, the vanadium oxide (2), and the disulfide cleavage procedure for the analysis of three dilute solutions of n-butyllithium in cyclohexane (Table 111). The results in Table I11 show the disulfide cleavage procedure gave values only slightly lower than those obtained by total alkalinity. Total alkalinity compared almost exactly with the value based on the manufacturer’s assay which for this sample was based on the double titration method of Gilman (11). The real value should be lower than the manufacturer’s value because degradation occurs on storage and loss of organolithium compound is likely to occur during the handling steps. In such event, no change should occur in total alkalinity which should always be equal to or higher than the actual active organolithium content. The vanadium oxide method gave a significantly lower value for solutions A and B as has been our experience with this procedure in analysis of organolithium compounds; however, others have obtained satisfactory results with the procedure (2,14). Analyses were performed on stock solutions as received (Table IV). The results indicated the procedure is directly applicable to high concentrations of butyllithium compounds as well as to methyllithium which is not readily analyzed by other than the total alkalinity procedure (11). In the course of another experiment, a dilute solution of nbutyllithium in cyclohexane was standardized in duplicate repeatedly (Table V). The standard deviation reflects the
of reagent with a hypodermic syringe. Ammonium nitrate solution, the supporting electrolyte for the titration, was made by dissolving 60 grams in 200 ml of concentrated ammonium hydroxide. Standard aqueous silver nitrate solutions were 0.02N or 0.005N. Apparatus. Hamilton one-way microvalves, Whittier, Calif., were used on syringes for transferring organometal or dry reagents. The disulfide cleavage was performed in 25-ml mixing cylinders (Kimble 20040) fitted with 13- by 18-mm serum stoppers. The amperometric apparatus used was similar to that described by Kolthoff and Harris (IO). Procedure. Glassware, syringes, and needles that contacted dry reagents or organometal solutions were baked at least overnight at 11G120” C. The mixing cylinders sealed with dry serum stoppers were connected in series with dry rubber tubing and needles so that dry nitrogen could be passed through the cylinders during cooling t o room temperature (JJ). The Hamilton valves were dried over phosphorus pentoxide in a desiccator at least overnight since the Teflon plugs of these valves might become distorted if subjected to the heat of the oven. Hydrocarbon solvents were dried by passing the liquid through a three-foot column filled with Burl saddles counter current to a stream of dry nitrogen, and then the stripped liquid was run through another column filled with aluminum oxide. At least 50% excess or 4.0 ml of 0.25M p-tolyl disulfide solution was added to 0.6 mmole of active organometal compound in a serum-stoppered mixing cylinder. The sample and disulfide were mixed by swirling the cylinders in a manner to avoid contact of the contents with the stopper. After 10 minutes, the contents were diluted to the 25-m1 mark with nitrogen-sparged isopropyl alcohol from a pressured bottle, capped with a self-sealing gasket, by means of a two-pointed syringe needle. For the most accurate results, samples and diluted solutions were weighed. Aliquots of diluted cleavage solution were titrated amperometrically the same day in 100 ml of nitrogen-sparged methanol in presence of the supporting electrolyte under a blanket of nitrogen with silver nitrate solution. All reactions took place at room temperature. Different criteria were used as assay reference values. For organolithiums, the manufacturers’ values were selected as the reference values, the triisobutylaluminum was titrated by the isoquinoline procedure (12, 13), and calcium ace(11) Foote Mineral Co., Exton, Pa., “Official Method of Analysis, n-Butyllithium,” BUN. 109 (Revised), 1966. (12) E. Bonitz, Chern. Ber., 88, 742 (1955). (13) T.R. Crompton, ANAL.CHEM., 39, 268 (1967).
(14) Am. SOC. Testing Materials, ASTM Standards, Vol. 31, E-233-64T3 p. 650 (1965).
Table 11. Comparison of Tolyl and n-Butyl Disulfides as Reagents for Butyllithium Analysis in Presence and Absence of Ether n-Butyl disulfide as reagent Tolyl disulfide as reagent n-BuLi, mmoles (RS)*,mmoles EtzO, mmoles Time for cleavage, min n-BuLi found,a Dilution based on assay,
z
0
328
a
0.612 1.69
... 2 1.60 1.71
ANALYTICAL CHEMISTRY
z.
0.612 1.69
...
10 1.62
0.612 1.69
...
20 1.64
0.612 1.69
...
30 1.62
0.612 1.69
... 61 1.64
0.620 1.46 10 30 1.65
0.568 2.5
...
30 1.58
0.614 2.7 15 2 1.60
0.614 2.7 10 30 1.63
storage stability of the solution of butyllithium as well as the day-to-day precision of the analysis. Interferences. Substances that might interfere in the proposed analysis can arise during the preparation of the organometal compound, by reaction with air or water, or by reaction with solvent components. Substances considered as possible interferers in the proposed analysis were lithium hydride, lithium metal, water, and oxygen, or their products from reaction with RLi, ethers, thioethers, and acetylenes. Lithium Hydride and Lithium Metal Dispersion. Commercial lithium hydride suspended in ether did not cleave tolyl disulfide in cyclohexane, the ether-to-cyclohexane ratio being about 1 :1 in the final reaction medium. Lithium metal dispersion cleaved the disulfide slowly in the presence of ether as well as in an all-cyclohexane solution. In 15 minutes, 3 6 x of the disulfide was cleaved in the presence of ether and 5 % in an all-cyclohexane medium, Any suspended lithium metal remaining after preparation of organolithium compounds can be considered an interfering substance in the proposed analysis. Water. The effect of water was tested by adding an excess of 0.03M n-butyllithium solution to the solvent containing a known amount of water. The unreacted butyllithium was determined at various times by the disulfide cleavage procedure; final titrations for mercaptide were made with 0.005N silver nitrate solution (Table VI). The data in Table VI indicate that slightly more than a stoichiometric amount of butyllithium reacted on a 1 :1 molar basis. Exposure of a butyllithium solution to excess water prior to addition of the disulfide solution resulted in no cleavage of the disulfide and neither did solid lithium hydroxide. These experiments indicate lithium hydroxide caused no interference in the analysis. Alcohol and Oxygen. Addition of excess methyl, isopropyl, n- or t-butyl alcohols to n-butyllithium or exposure of the butyllithium to excess air or water resulted in little cleavage of disulfide, indicating negligible interference of lithium alkoxides, oxides, and hydroxides. Although lithium alkoxides did not react with disulfide under the mild conditions of the analysis, potassium t-butylalkoxide slowly cleaved the disulfide. Air in a 7-02 crown-top bottle was dried over Linde 4A Molecular Sieves. Five milliliters of dry air was added to an excess of 0.03Mn-butyllithium solution with a dry syringe and after 10 minutes an excess of tolyl disulfide was injected. After 4 minutes 2-propanol was added and aliquots were titrated for mercaptide. Assuming 2 0 x oxygen in air, we found the oxygen showed a 2 :1 butyllithium to oxygen reaction (Table VII). Ethers. The use of ethyl ether as a solvent required the testing of the stability of butyllithium in the presence of ether solvents. n-Butyllithium in cyclohexane or ethyl ether as solvent was analyzed over a period of 90 hours (Table VIII). The butyllithium in a crown-top bottle sealed with a Neoprene gasket was relatively stable in cyclohexane for over 90 hours. In ethyl ether, 98% disappeared in the same period. In tetrahydrofuran as solvent, a 1.52% solution of nbutyllithium was 95% destroyed within 1 hour. In view of the reactivity of butyllithium with ethers, the use of these compounds as solvents for the disulfide cleavage procedure should be avoided if possible. Thioethers. n-Butyl thioether reacts with butyllithium almost as rapidly as does the disulfide. Fortunately the reaction with thioethers produces a mercaptide almost quantitatively and no apparent interference occurs.
Table 111. Comparison of the Disulfide Cleavage Procedure with the Total Alkalinity and the Vanadium Oxide Procedures for Analysis of n-Butyllithium n-Butvllithium found Solution A Solution B Solution C Z M Z M Z M Based on Mfgr.'s value Total alkalinity Disulfidecleavage Vanadium oxide
1.71 1.71 1.69 1.47
0.202 0.202 0.200 0.174
1.56 0.184 1.75 0.206 ,.. 1.77 0.208 1.55 O:i83 1.71 0.202 1.38 0.163 . . . ...
Table IV. Direct Analysis of Various Stock Solutions of Alkyllithium Compounds Manufacturer's assay, Z 15.06 11.65 5.08
Alkyllithium n-Butyl s-Butyl Methyl
Found by disulfide cleavage, Z 14.32 11.49 4.64s
Reaction time between methyllithium and disulfide was 30 minutes, as methyllithium apparently reacts at a lower rate
Table V.
Variation in Concentration of a Dilute Solution of n-Butyllithium in Cyclohexane with Time
Days after preparation 2
Days after preparation 16 17 20 22 23 24
3 6 10 15
n-BuLi found, M 0.0292,O.0290 0.0284,O. 0305 0.0278,0.0290 0.0286,0.0287 0.0286,O. 0287
n-BuLi found, M 0.0282,O. 0279 0.0275,O. 0278 0.0282,0.0285 0.0280,0.0283 0.0284,0.0282 0.0284,0.0282
Standard Deviation: 0.0005M
Table VI. Destruction of n-Butyllithium by Water in Cyclohexane
HzOand n-BLi, minutes
BuLi destroyed, mmole HzOadded, mmolea
2-3 15 30 60
1.13 1.02 1.12 1.08
Reaction time of
a Prepared by adding 27.4 ppm water to dried cyclohexane. A microsyringe was used to add 0.0107 gram of water to the dry cyclohexane in a 26-02dried capped bottle. The water in the solvent and the amount injected made up the quantity of 27.4 ppm.
Table VII. Destruction of Butyllithium by Air
Time air was dried, hours 2.5 6
O2added,
mmole 0.0409 0.0409
BuLi destroyed, mmole 0.0826 0.0826
BuLi destroyed , mmole Ozadded, mmole 2.05 2.05
VOL. 40, NO. 2, FEBRUARY 1968
329
Table VIII. Stability of n-Butyllithium in Cyclohexane or in Ethyl Ether n-BuLi found, Cyclohexane Ethyl ether Hours 0.0 0.8 0.9 19.2 24.8 90.8
1.71
f /
1.69 1.68
...
...
1.54 1.74
1.12 1.01 0.03
*..
1.72
I .. //
2 e
20 30 5
0.203 0.198
0.204
Time for disulfide cleavage reaction.
Mono- or Dilithiododecyne. Work with 1-dodecynyllithium showed that this compound did not cleave ptolyl disulfide. A dilithiododecyne, prepared by a procedure given in the literature (13,reacted with only 1 mole of disulfide. These experiments indicated the acetylide lithium did not cleave the disulfide.
w
- 0
TOLUENE
1
0.2
I//
Table IX. Analysis of Diethylmagnesium in Presence of Ethyl Ether Time, minutes“ EtzMg, M 10
CYCLOHEXANE
0-
0
0
ETHER L__
0
10
20 sn 40 50 60 WITHDISULFIDE, MIN REACT IONI~IME
I
70
Figure 1. Analysis of 0.44N triisobutylaluminum (TBA) in various solvents Diethylzinc. Diethylzinc did not react with tolyl disulfide rapidly enough at room temperature to justify the use of this procedure for analyzing organozinc compounds. Lithium Aluminum Hydride. An excess of lithium aluminum hydride in ether cleaved tolyl disulfide to two thiols per disulfide. This result indicated aluminum hydride would interfere in the analysis of alkylaluminums. The uses of lithium aluminum hydride for reducing disulfides (16) and for analyzing polysulfides (17, 18)are known.
RESULTS WITH OTHER ORGANOMETAL COMPOUNDS The cleavage procedure has been tested with little modification for analyzing organo-potassium, -calcium, -aluminum, -magnesium, and -zinc compounds. Isopropylphenylpotassium. The suspension of this organopotassium compound in heptane which analyzed 0.742M by total alkalinity gave a value of 0.631M a t the end of 15 minutes and 0.651M at the end of 45 minutes by disulfide cleavage. The slow disappearance of the color of the organopotassium suspension indicated a longer time would be needed for analyzing this compound in comparison to the alkyllithium compounds in solution. Calcium Acenaphthalene. A preparation of calcium acenaphthalene intended to be 0.033M in dimethoxyethane analyzed 0.0286M by disulfide cleavage compared to 0.033M by total alkalinity. The speed of the reaction and the stability of the cleavage product indicate the proposed procedure should work satisfactorily for this type of compound. Diethylmagnesium. A suspension of diethylmagnesium in a mixture of cyclohexane and toluene reacted very slowly with tolyl disulfide, and the medium was considered unsatisfactory for the procedure. In the presence of ether, the reaction occurred rapidly; and the procedure seems promising for this type of compound (Table IX). Analysis by acidbase titration of the solution of the diethylmagnesium in the ether-toluene solvent gave 0.202M diethylmagnesium. Triisobutylaluminum. Triisobutylaluminum reacted with the disulfide more rapidly in hydrocarbon solvents than in polar solvents. Differences in rates of reaction in various solvents are shown by the curves in Figure 1. They are attributed to the complexes formed between alkylaluminum and Lewis bases, These complexes are apparently less reactive than the dimeric form of the alkylaluminum. (15) H. L. Hsieh and J. A. Favre (to Phillips Petroleum Co.), U. S.Patent 3,303,225 (Feb. 7, 1967).
330
ANALYTICAL CHEMISTRY
DISCUSSION The disulfide cleavage procedure is proposed for the analysis of organometal compounds because of its apparent versatility. Use of one procedure for analyses of a variety of organometal compounds has obvious advantages. The method seems adaptable to any organometal compound capable of rapidly cleaving an alkyl or aromatic disulfide. Interferences. In the analysis of organolithium compounds, substances such as lithium metal, lithium hydride, lithium hydroxide, and lithium alkoxides should be considered as possible interfering ingredients. The first arises from the preparation, the second from thermal stability, and the last two are products of the organolithium compound reacting with water and air. In the proposed procedure only lithium metal cleaved tolyl disulfide under the conditions of the analysis. Because lithium metal comes from unreacted metal during the preparation which is not removed by filtration, the metal is present as a microscopic dispersion or probably as atomic clusters, In the latter state, lithium metal will interfere with most of the procedures proposed to date. Whenever necessary, unreacted lithium metal can be readily determined (19). Stability of organolithium compounds in the presence of ethers has been studied previously (20, 21). Fortunately, (16) E. E. Reid, “Organic Chemistry of Bivalent Sulfur,” Vol. I, Chemical Publishing Co., New York, 1958, p 36. (17) M. Porter, B Saville, and A. A. Watson, J . Chem. Soc., 346 (1963). (18) S. Chakravarty, P. K. Chatterjee, and A. K. Sircar, J . Applied Polymer Sci., 9, 1395 (1965). (19) I. Kniel, “The Determination of Active Metal Content in Alkali Metal Dispersions,” Office of Rubber Reserve, CD-2817 June 11, 1952. (20) H. Gilman and G. L. Schwebke, J . Organornetal. Chem., 4, 483 (1965). (21) R . Waack, M. A. Doran, and P. E. Stevenson, J. Am. Chem. SOC.,88, 2109 (1966).
ethers are not required for the disulfide cleavage reaction of organolithium compounds and are undesirable in the analysis of alkylaluminum compounds. However, ethyl ether does not react rapidly with organolithium compounds; and the analysis can be completed before the ether decomposition of organolithium compounds is serious. Furthermore in presence of stoichiometric amounts of ether, stable complexes can form which do not decompose the organolithium compounds (22) appreciably. Aliphatic thioethers, RSR, react with butyllithium to give stoichiometric amounts of mercaptide. An excess of thioether could be used in place of the disulfide as an analytical reagent. However, the thioether would be unsatisfactory for analysis of less reactive compounds such as lithium aluminum hydride and organomagnesium compounds. The problem of reaction of thioethers for the analysis of disulfide was handled by Veibel and Wronski (9) by controlling the reaction conditions. This approach could also be used in our analysis, but so far it has been unnecessary. Oxygen, water, and alcohols and the products of their reactions with organolithium compounds do not interfere with the disulfide cleavage procedure except for the destruction of a stoichiometric amount of organometal compound. Actually, the reactions with water (22) and with alcohol (3, 7) are the bases for procedures reported for analyzing organolithium compounds. The reaction product with oxygen has been used in a procedure for analyzing dilithioaromatic compounds (23). The stoichiometry of the water-RLi reaction (24) is being used in these laboratories as a basis for studying the effect of water on various organometal systems, and the results will be reported in the future. Organolithium Compounds. The disulfide cleavage procedure for the analysis of organolithium compounds has been used in these laboratories for several years and has been more convenient than the benzyl chloride coupling reaction and more accurate than the acid-base titration procedure and the vanadium oxide procedure in our hands. Precision is good as is shown by the analysis of a standard solution over a period of time (Table V). The possibility of analyzing methyllithium by this procedure is also worth noting (Table IV). The differences between the manufacturer’s assay of the stock solutions and that found by the disulfide procedure for stored solutions are similar to those reported by others (3,25), Table IV. The procedure can be used for analyzing dilute and concentrated solutions of organolithium compounds. Although the lower limit of sensitivity has not been established, application in different research problems indicates the sensitivity is comparable to that estimated for the most sensitive methods previously reported (5). Other Organometal Compounds. Any compounds capable of cleaving disulfides without side reactions could be analyzed (22) R. Adams, Ed., Org. Reactions, 6 , 353 (1951). (23) A. F. Clifford and R. R Olsen, ANAL. CHEM ,31,1860 (1959). (24) D. K. Polyakov, Polymer Sci. USSR,7 (4), 668 (1965). (25) R. L. Eppley and J. A. Dixon, J. Organometal. Chem., 8, 176 (1967).
by this procedure. Organo-potassium, -calcium, and -aluminum compounds seem to have reacted quantitatively with the disulfide in hydrocarbon solvents at a practical rate. Diethylmagnesium and diethylzinc reacted too slowly in hydrocarbon solvents for analytical purposes, but diethylmagnesium reacted quantitatively in ether at a satisfactory rate. The disulfide procedure seemed unsatisfactory for the analysis of organozinc compounds. Triisobutylaluminum reacted in 30 minutes with 3 moles of disulfide in cyclohexane or in heptane as solvent (Figure 1). The presence of toluene decreased the rate of reaction somewhat, but in the presence of 3377, ether or isoquinoline the organoaluminum reacted too slowly for practical purposes. This slow reaction in the presence of ether is attributed to the formation of complexes with polar solvents (23, 26). Either the complexes react slowly with the disulfide or an unfavorable equilibrium exists and only the free organoaluminum compound cleaves the disulfide. The sensitivity of the disulfide cleavage to the reaction medium indicates this procedure might be a simple means of studying the relative stability and kinetics of alkylaluminum-polar compound complexes. The slow drift of thiol determined in analyses of triisobutylaluminum to a value higher than a stoichiometric amount expected for a quantitative reaction suggests the presence of a side reaction. Hydrides which are known components of commercial alkylaluminum compounds (22, 27) were suspected. Excess lithium aluminum hydride cleaved tolyl disulfide quantitatively without any appreciable side reaction. Thus the presence of hydride could account for the higher than stoichiometric amounts of alkylaluminum if not corrected for. RECEIVED for review September 27, 1967. Accepted November 13,1967. (26) E. B. Baker and H. H. Sisler, J . Am. Chem. SOC.,75, 5193 (1953). (27) Ethyl Corp., Baton Rouge, La., “Technical Information on Aluminum Alkyls and Alkyl Aluminum Halides,” p. 28, 1959.
Correction Colorimetric Determination of Iron in Plutonium Metal Using a Nitrobenzene Extraction Technique In this article by C. E. Plock and C. E. Caldwell [ANAL.
CHEM. 39, 1472 (1967)J there is an error in column 2 of page 1472. The first sentence of the Reagents section should read as follows. “The hydroxylamine hydrochloride solution (2.2 %) was prepared by dissolving 108 grams of NH20H.HCl in water, adding 60 ml of glacial acetic acid, and diluting to 5000 ml with water.”
VOL 40, NO. 2, FEBRUARY 1968
331
