attributed to the higher energy of the carbonyl bond (173 kcal./mole) compared with that of carbon-oxygen bond (79 kcal./mole). The carbonyl carbon atom in acetone may not be so readily available for forming Cz molecules, thus reducing the effective carbon number of the molecule and reducing Cz emission. Because of the poor molar emission response of methanol observed previously, its FE/FI ratio value appears high in comparison with that of n-propanol. Nevertheless, this may be due to the smaller ionization response rather than to a higher emission response. The ionization response of methanol has been reported t o be lower than that of the other alcohols (6). The introduction of a bromo or cyano group into the three-carbon chain markedly increases the emission response a t the 589-mp region: consequently the ratio is larger than for the other compounds. This observation is similar to the increase in FE/FI ratio observed when halogens were added to the methane nucleus, but it is opposite to the effect observed when halogens are added to the benzene nucleus. The combustion processes are therefore distinctly different for each type of compound, emphasizing the need for comparison of ratios with those of standards. The increased CJCH ratio for isobutyronitrile is probably partially due to the extra carbon. The cyanogen nitrogen does not give the very great increase observed for amine-type compounds.
This is attributed to the high energy of the CN bond (209 kcal./mole), which thus reduces the effective carbon number of this compound to three and does not make the nitrogen atom available for NHz nor NH band emission. DISCUSSION
The FE/FI dual detector that is reported here for use in gas chromatography has also been found in unpublished work to be useful in following the course of vapor-phase reactions. In applications of this type the capability of sensitivity detecting FI-insensitive molecules not containing CH bandssuch as CO, CO,, SO-becomes of considerable advantage. The present sensitivity of the dual detector is limited by the sensitivity of the F E mode. The detector sensitivity can probably be increased by improving the design of the burner assembly and possibly by selecting other fuel-oxidizer combinations for the flame plasma. For high sensitivity, however, hydrogenoxygen and hydrogen-air flames are likely best. Other fuels will likely produce either a high F I background or a high F E background. The use of an integrating sphere with burner will increase the efficiency of light collection and may permit development of a direct relationship between emitting species concentration and readout signal. The dual FE/FI detector is somewhat similar to a dual electron capture-FI detector, since both electron capture
and FE detectors are more sensitive to heteroatoms. Nevertheless, because of the variety of wavelengths available and the emission a t different wavelengths by different molecules, the FE/FI detector is more selective. The FE detection mode could also be combined with other detection modes, for example, thermal conductivity. The thermal conductivity mode is a better comparison mode than FI because it is less dependent upon the elemental makeup and structure of molecules than the FI mode. LITERATURE CITE0
( I ) Braman, R. S., Gordon, E. S., ZEEE Trans. Instrumentation and Measurement, IM-14, 11-19 (1965). (2) Gaydon, A. G., “The Spectroscopy of Flames,” Riley, New York, 1957. (3) Grant, D. W., “Gas Chromatography 1958,” D. H. Desty, ed., p. 158, Aca-
demic Press, New York, 1958. (4) Juvet, R. S., Durbin, R. P., Gas Chromatog. 1 (12), 14, 1963. (5) Juvet, R. S., Durbin, R. P., ANAL. CHEK 38, 565 (1966). (6) Perkins, G., Jr., Rouayheb, G. M., Lively, L. D., Hamilton, W. C., “Gas Chromatography,” N. Brenner, J. E. Callen, hl. D. Weiss, eds., pp. 269-285, Academic Press, New York, 1962. (7) Pitzer, K. S., J. A m . Chem. SOC.70, 2140 (1948). (8) McCormack, A. J., Tong, S. C., Cooke, W. D., ANAL.CHEM.37, 1470 (1965). (9) Sternberg, J. C., Poulson, R. E., J. Chromatog. 3,406 (1960). (10) Winefordner, J. D., Vickers, T. J., ANAL.CHEM.36, 1939 (1964). for review December 20, 1965. RECEIVED Accepted March 7, 1966.
An Assay Method for Vinyl Grignard Reagents Using Gas Chromatography ANATOLE WOWK and SALVATORE DiGIOVANNI’ M&T Chemicals Inc., Rahway, N. .I. A method for the determination of vinyl Grignard reagents has been developed which assays the reactive vinyl magnesium moiety in the product. It distinguishes the vinyl magnesium from other titratable compounds resulting from the hydrolysis, oxidation, or decomposition of the vinyl Grignard. The method involves the reaction of the vinyl Grignard reagent with an excess of tributyltin chloride followed by analysis of the reaction product mixture by gas-liquid chromatography, The content of tributylvinyltin in the mixture indicates the concentration of the reactive vinyl group in the Grignard. The method has been used for the analysis of Grignard reagent made from vinyl chloride and magnesium in tetrahydrofuran.
742
ANALYTICAL CHEMISTRY
S
the preparation of vinyl Grignard in high yields from vinyl chloride and magnesium in tetrahydrofuran (THF) was described in our laboratory in 1957, (5, 6) no specific method for assaying this reagent has been described in the literature. A general method of assaying Grignard compounds by an acid-base titration developed by H. Gilman ( I ) has been used for this purpose. This method, however, indicates the total content of C-Mg and 0-Mg bonds present in the material under test and not the content of the reactive CH2C=CHMg moiety which is desired. Thus, we have found in many instances that vinyl Grignard reagents which have deteriorated for one reason or another give, by titration, much higher assay figures than their INCE
true vinyl magnesium content as evidenced by reduced yields in coupling reactions. To utilize the vinyl Grignard agent as a synthetic tool, it became necessary to develop an accurate assay method. In our search for a more appropriate method of assay, the gas-volumetric method was investigated. The gases obtained on hydrolysis of a Grignard reagent with dilute acid were subjected to chromatographic and mass spectrometric analyses. In the case of a vinyl Grignard reagent stored for a period of time these analyses showed, besides the theoretically expected ethylene, the presence of considerable amounts of 1 Present address, Hewlett-Packard Co., Englemood, N. J.
ethane, hydrogen, and several C4and Cs alkanes and alkenes. This mixture of hydrocarbons makes the gas-volumetric method unsuitable for assay purposes. The assay method which was developed is essentially a use test in which a vinyl Grignard is used to vinylate an organometallic halide. An aliquot of the sample being tested is reacted with a large excess of tributyltin chloride. The inorganic magnesium salts formed (or their T H F complexes) are then precipitated with an alkane and filtered. The solvents are evaporated and the residue containing only butyltin compounds is analyzed by gas-liquid chromatography (GLC). Although a number of papers concerning the GLC of organotin compounds appear in the literature, most of the data are merely qualitative. In the few quantitative studies reported, Proesch and Zoepfi (4) have reported the GLC analysis of ethylmethyltins and more recently Steinmeyer et al. (‘7) have reported the analysis of butylmethyltins. Matsuda and Matsuda ( 3 ) encountered difficulties in the GLC analysis of organotin halide mixtures due to redistribution effects. This effect was not observed with the compounds encountered during this investigation. The organotin mixtures which we subjected to GLC analysis contained as their main components tributylvinyltin and the excess of tributyltin chloride. If it is assumed that the vinylmagnesium chloride couples quantitatively with the large excess of tributyltin chloride, the amount of tributylvinyltin in the coupling product is a measure of the reactive C H F C H M g moiety in the Grignard reagent being assayed. It is convenient to express the calculated “molarity by coupling” (M,) as a per cent of “molarity by titration” (XJ which is determined at the same time. The resulting value is designated as the “vinyl activity” ( V A ) of a given Grignard preparation.
I n our experience fresh preparations of vinyl Grignard reagent show a V A between 85 and 90%. It is obvious that other classes of compounds such as aldehydes and ketones may also be successfully employed in this method of assaying vinyl Grignard. Tributyltin chloride was chosen for the following reasons: In the case of carbonyl compounds the post-reaction mixture must be hydrolyzed in order to break the 0-Mg bonds formed. With organometallic halides no such bonds are formed and the hydrolysis can be eliminated. Coupling product components-large
amounts of tributylvinyltin, tributyltin chloride and small amounts of tetrabutyltin and dibutyldivinyltin (the last two resulting from the presence of tetrabutyltin and dibutyltin dichloride in the technical grade tributyltin chloride)are easily and clearly separable on the GLC column. All these components are high boiling liquids and are easily separated from T H F by stripping under an aspirator vacuum. After our work on this method was completed, a similar method for determination of methyl Grignard reagents was described in the literature (2). It consists of coupling the reagent with an excess of dimethylphenylchlorosilane in ether and determining the amount of trimethylphenylsilane formed by GLC using cumene as an internal standard. EXPERIMENTAL
Figure 1 . Chromatogram of butylvinyltin compounds
Coupling Reaction. Exactly 31.4 grams of tributyltin chloride were weighed out directly in a 100-ml. 3-necked flask. The flask was then equipped with a Y adaptor, a precision bore Teflon paddle stirrer, an addition funnel (with pressure equalizing arm), thermometer, and a reflux condenser having a nitrogen inlet. After the whole system (including the addition funnel) was filled with nitrogen, the organotin was diluted with 15 ml. of THF. A 20-ml. sample of the vinyl Grignard solution was measured with a nitrogen filled pipet, dropped into the addition funnel, and added dropwise a t a rather fast rate to the rapidly stirred organotin solution. The addition funnel was rinsed with two 5-ml. portions of THF. The reaction mixture was heated with an electric mantle to 70’-75’ C., reacted a t that temperature for one hour, and cooled to room temperature. A 20-ml. sample of the post-reaction mixture (a suspension of partially precipitated magnesium salts) was pipetted out and dropped into 100 ml. of rapidly stirred hexane. The resulting heavy precipitate of magnesium salts was stirred for about 30 seconds and cooled in a refrigerator for about 2 hours. The material was filtered twice using for the second “polish filtration” a fritted disc funnel (F porosity, 60 mm. diameter) and minimum suction. The clear filtrate was stripped free of solvent in a rotary evaporator down to 2C-28 torr a t 90”-95’ C. in the water bath. The stripped product (5 to 8 ml.) was analyzed by GLC. GLC Apparatus and Operating Conditions. An F & M Model 720 dual column, linear - temperature programmed gas chromatograph equipped with a four-filament hot-wire, thermal conductivity detector with a Minneapolis-Honeywell 1-mv. recorder was used for this investigation. Peak areas were measured with a Daystrom AttenU-Matic integrator. Samples were introduced into the instrument with a Hamilton 10 pl. syringe.
Peaks are: ( I ) dodecane, (2) dibutyldivinyltin, (3) tributylvinyltin, (4) tetrabutyltin, ( 5 ) tributyltin chloride
A four foot X l/, inch coiled glass column packed with 20% DC-550 silicone oil on 60/80 mesh Gas Chrom Z operated from 120’ to 210’ C. a t a programming rate of 15’ C./min. allowed the complete separation of the butyltin compounds. Other instrument conditions are : Injection port temperature, 200’ C.; detector temperature, 300’ C.; carrier gas, helium dried over 5-A molecular sieves; tank pressure, 40 p.s.i.; recorder chart speed, 0.5 inch/min.; bridge current, 150 ma.; sample size, 4 11. Reagents and Standards. T H F (Du Pont), hexane (Industrial or %yo grade) and tributyltin chloride ( M & T Chemicals Inc., maximum 2y0 dibutyltin dichloride) were used without any purification for the coupling reaction. Tetrabutyltin, tributyltin chloride, tributylvinyltin, and dibutyldivinyltin (M&T Chemicals Inc.) were distilled a t reduced pressures to obtain a minimum purity of 99% for GLC standardization. Internal Standard. Dodecane (Matheson, Coleman & Bell) 99% (olefin free) was checked chromatographically and shown to have no components with the same retention time as the butyltin compounds. To facilitate the determination of soluble, nonvolatile materials which may be contained in the coupling product, the internal standard was added after salts and solvents were removed from the sample of the postcoupling mixture. Determination of Area Correction Factors. The Area Correction Factor (ACF) for the butyltin compounds were determined relative to dodecane by the internal standard technique. The detector response for dodecane was arbitrarily set a t unity. Three standard mixtures containing dodecane and a standard butyltin VOl. 38, NO. 6, MAY 1966
743
Table 1.
Replicate GLC Determinations of Some Coupling Products
GLC run days after Vinyltin content by GLC Bu,SnVi Bu2SnVi2 coup1ing 1
33 1 13 6 13 3 21
24.6 24.1 53.5 52.5 30.1 30.4 28.8 28.6
Grignard by titration ( M t )
0.2 0.3 0.7 0.7 0.4 0.4 0.4 0.4
3.130 2.580 1.920
=
ViMgCl
+
Moles 0.0600 Used Amounts 20 ml. of 3M soln.
The theoretical content of tributylvinyltin in the total product is therefore 61.6%. When the actually determined content of tributylvinyltin is a% then the actual Grignard molarity by cou-
744
ANALYTICAL CHEMISTRY
100
a (M,)is 3.00 or M, 61.6
=
0.0487 a. To correct t'his formula for the presence of dibutyldivinyltin in the coupling product (b%) the vinyl content of this compound is converted to tributylvinyltin. One mole of dibutyldivinyltin (FW = 287.0) corresponds to two moles of tributylvinyltin (FW = 317.1); therefore, 1.00% of dibutyldivinyltin to 2.21y0 of tributylvinyltin.
x
100
The formula, adjusted for the presence of dibutyldivinyltin, becomes:
M,
= 0.0487 (a
+ b X 2.21)
RESULTS AND DISCUSSION
The assaying procedure described was applied to the analysis of several preparations of vinyl Grignard. One particular preparation was allowed to deteriorate over a period of several months and was analyzed periodically. Deterioration was not detected by the titration method but was clearly evidenced by the new coupling-GLC technique. I n addition to causing a decrease in molarity, the decomposition
-
THF
BuaSnC1 (excess) 325.5 0.0965 (60% excess) 3 1 . 4 g.
x
62.7 62.1 85.6 84.1 58.4 59.0 75.2 74.7
Wt. of int. std. X peak area of component X ACF peak area of int. std. X wt. of coupling mixture
For most samples of freshly prepared vinyl Grignard reagent, the total product is volatile and determinable by GLC to within &2% of the sample weight, However, in the case of an aged vinyl Grignard the volatiles may total only 70% of the sample weight indicating the presence of 30% nonvolatile material in the coupling product. Where the sole objective is to determine the concentration of the vinyl Grignard reagent, only the amounts of tributylvinyltin and dibutyldivinyltin need be determined. Calculation of Assay and Activity. Assuming that the vinyl Grignard under test is exactly 3.00 molar, the stoichiometry of the coupling reaction is as follows:
FW
pling
Vinvl actiGtY
(Mc) 1.219 1.206 2.680 2.632 1.508 1.524 1.445 1.435
1.944
compound were prepared. The amount of butyltin compound varied between 20 and 80%. Analyses in triplicate were made of each calibration mixture and correction factors were calculated. GLC Analysis of the Coupling Product. Approximately 1.0 gram of the solvent-free coupling product and 0.50 gram of dodecane, accurately weighed, were analyzed by GLC. A representative chromatogram is shown as Figure 1. The weight per cent composition of the mixtures was determined by the following formula:
yo component
Molarity by coupling-GLC
Bu3SnVi 317.1 0.0600 19.03 g.
+
Bu3SnC1 325.5 0.0365
+ MgClz
11.87 g.
30.9 g. total product
in a formation Of unknown products, Some Of which were nonvolatile and not detected by GLC. The value of the new assay method for differentiating reactive from non-
reactive vinyl Grignard was illustrated in two experiments where dry air was bubbled through fresh vinyl Grignard preparations to cause deactivation by air oxidation. The ensuing exothermic reaction caused only a slight change in the appearance of the Grignard. The titration value of the product also remained essentially unchanged; however, assay by the coupling-GLC method showed clearly the destruction of the C H F C H M g moiety. Reproducibility of Results. Reproducibility of the GLC analysis is dependent upon the reproducibility of the peak area correction factors. However, under the conditions of analysis, these values were essentially constant. Reproducibility of the GLC part of the assaying method was also checked as follows : Coupling product samples were analyzed by GLC in duplicate. Results calculated as vinyl activity varied by not more than 1% absolute. Several coupling products were redetermined by GLC after a period of 7 to 33 days. Vinyl activity figures varied by not more than 1.5% absolute showing the good reproducibility of the GLC analyses as well as the chemical stability of the coupling products. The corresponding analytical data on four representative samples are given in Table I. The coupling part of the procedure appears to be quite reproducible. In some Grignard preparations the identity of the hydrocarbon used to precipitate the magnesium salts appears to have a small effect on the composition of the coupling product. Pentane, heptane, and cyclohexane were also investigated in addition to hexane during the development of the method. Variations of up to 2% of the assay were obtained with different precipitating solvents. LITERATURE CITED
(1) Qilman, H., J . Am. Chem. SOC.51, 1576 (1929). (2) House, H. O., Respess, W. L., J. Organomet. Chem. 4, 95 (1965). (3) Matauda, H., Matsuda, S., Kogyo Kagaku Zasshi 63, 1960. (4) Proesch, V., Zoepfi, H., 2. Chem. 5, 97 (1963). (5) H. E.. et al.. J. Oro. Chem. . ,22,Ramsden. 1602-5'(1957).' (6) U. S. Patent 2,959,597, H. E. Ramsden, A. Halint to Metal & Thermit Corp., 11/8/60. (7) Steinmeyer, R. D. et al., ANAL.CHEM. 37, 520 (1965).
RECEIVED for review January 13, 1966. Accepted March 11, 1966.
