laf rared Functional Group Analysis of Arylsilanes MARVIN MARGOSHES'
and
VELMER A. FASSEL
Institute for Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa
phenyldisilane was not studied because of its low solubility in the solvent used.) Table I lists the peak molar absorptivities of the bands a t 12.5 microns for the p-tolyl group and a t 13.5 and 14.3 microns for the phenyl group. All three bands have been assigned to out-of-plane carbon-hydrogen bending vibrations of the benzene ring by anal3gy to the similar bands in the methyl-substituted benzenes, which have been assigned to particular vibrations by Pitzer and Scott ( 8 ) . These bands were chosen for the analysis because of their high intensity and because they are, in these compounds, free from interference. It can be seen that there are considerable variations in the peak molar absorptivities of these bands, even if the monosilanes and disilanes are considered separately. This is particularly true for the 13.5- and 14.3micron bands. If the peak molar absorptivity of the 13.5-micron band of
llelting points and chemical analyses of some of the aryl silanes are so similar that other methods of characterizing their structures are required. .4n infrared spectrometric method is described for the determination of the concentration ratio of phenyl and p-tolyl groups in tetraarj-lsilanes and hexaaryldisilanes. Accurate results are possible, even though the molar absorptivities of the functional groups are not constant. Determination of group concentration ratios, rather than individual concentrations, permits use of unweighed samples.
I
SFRARED functional group analyses desrribed to date
(f-3, 5-7, 10) have depended upon the constancy of either the peak or integrated molar absorptivity of an absorption band corresponding to a vibration of the functional group being determined The criterion of constant molar absorptivity is not met by some absorption bands that are others-ise desirable for analytical use because their intensity falls within a desirable range and because they are free from interference from other absorption bands. Such absorption bands can, however, give valuable information about compounds of unknown structure if the molar a1)sorptivity is found to vary in a regular manner with some rhange in the structure of the compounds uxder study. If, for example, the molar absorptivity of a group is dependent upon the position of the group along a carbon chain, determination of the molar absorptivity will provide information about the position of the group. Alternatively, if the changes in molar absorptivity cannot be related to the structure of the conipounds under study, it may be possible to find an absorption band from another group common to all of the compounds that varies in intensity in a similar manner, so that the ratio of the two molar absorptivities IS constant. If such a band can be found, the ratio of the two group concentrations can be related to the ratio of band intensities. The advantages of this type of analysis, which is similar to the internal standard technique used so successfully in emission spectroscopy, are discussed hriefly later in this paper.
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I NUMBER
Figure 1.
EXPERIMENTAL
OF PHENYL
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3
4
GROUPS ON SILICON ATOM
3lolar absorptivity of phenyl groups in arylsilanes 13.5-micron band
The spectra meze rerorded on a Baird -4ssociates Model B infrared spectrophotometer and the densitometer attachment of a I'erkin-Elmer Model 13 infrared spectrophotometer was used for the determination of peak molar absorptivities. Sodium chloride prisms were used in both instruments. Carbon disulfide was used as the solvent and the cell thicknesses &-ere0.4 mm. on the Baird instrument and 0.5 mm. on the Perkin-Elmer instrument. The concentration of the sample in the solution varied between 5 and 10 mg. per ml. I n all cases readings were made a t the absorption maxima rather than a t definite wave lengths. The base-line technique was used for the estimation of To values on the Baird instrument, I\-hile the cell-in, cell-out technique was used on the Perkin-Elmer instrument.
Table I.
nlolar .4bsorptivities of Arylsilanes"
Compd. NO.
1 2 3 4 5
6 7 8
RESULTS
9
The compounds studied were a series of silanes and disilanes ( / t ) with only phenyl and p-tolyl groups as substituents on the silicon atoms. Infrared spectroscopic tests indicated that the compounds were 99% pure. The compounds are listed in Table I, and they can be represented by the general formulas (C6H5)nSi(C7H7)d - , and (C&),Sil( C&)G - ", with n having the values 0 to 4 for the monosilanes and 0 to 5 for the disilanes. (Hexa-
10 11 12 13 14
Name of Compd. Tetraphenylsilane Triphenyl-p-tolylsilane Diphenyldi-p-tolylsilane P henyltri- p- t olylsilane Tetra-p-tolylsilane Pentaphenyl- p-tolyldisilane l11,2.2-Tetraphenyl-l,2-di-ptolyldisilane l11,1,2-Tetraphenyl-2,2-di-p-tolyldisilane 1,1,l-Triphenyl-2,2,2-tri-p-tolyldisilane l,l,Z-Triphenyl-l ,2.2-tri-p-tolyldisilane 1,2-Diphenyl-l,1,2,2-tetra-p-tolyldisilane l,l-Diphenyl-l,2,2,2-tetra-p-tolyldisilane Phenylpenta-p-tolyldisilane Hexa-p-tolyldisilane
Molar Absorptivity, Liters/Mole-Cm. 1 2 . 5 ~ 13.5fi 14.3~ ... 50.1 167 145 59.9 193 75.9 140 226 98.0 144 300 146 ... ... 171 124 201 169
130
158
126
199
165
120
200
178
154
204
157
144
193
162 161 155
137 155
213 207
...
197
...
Morar absorptivities refer in all cases to the molar concentration of the functional group rather than of the compound. Thus, 1 mole of tetraphenylsilane is equivalent to 4 moles of phenyl groupa. 0
1 Present address, Biophysics Research Laboratory, Peter Bent Brigham Hospital, Harvard Medical School, Boston, Mass.
351
352
ANALYTICAL CHEMISTRY
the phenyl group is plotted against the number of phenyl groups on the silicon atom, two smooth curves may be drawn, one for the monosilanes and one for the disilanes as shown in Figure 1. Not all of the disilanes can be included in this plot (compounds 6, 8, 10, Table I), as in some there are diflerent numbers of phenyl groups on the tWo silicon ntonis I t is possible to calculate an expected peak molar absorptivity for these compounds, weighting the peak molar absorptivities obtained from Figure 1 according to the relative numbers of phenyl groups of each type in the molecule. The values thus calculated are found to agree closely with the observed values except for lJ1,2-triphenyl1,2,2-tri-p-tolyldisilaneJwhere the calculated value is 140 liters per mole-em. and the observed value is 154 liters per mole-cm.
'+
MONOSILANES
\
7
DISILANES
Figure 3. Phenj-1-p-tolylabsorbance ratio as a function of group concentration ratios 13.5-micron phenyl band
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0 DISILANES
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NUMBER OF PHENYL GROUPS ON SILICON ATOM
Figure 2.
Molar absorptivity of phenyl groups in arylsilanes 14.3-micron band
A similar graph can be made for the 14.3-micron hand of t,he phenyl group, aa shown in Figure 2. The peak molar absorptivity of this band is relatively constant, for the disilanes but, as for the 13.5-micron band, varies by a factor of about 2 in the monosilanes. The disilanes that could not be included in Figure 2 (compounds 6, 8, and 10, Table I ) all h a w peak molar absorptivities of about 200 liters per mole-cm. The peak molar absorptivity of the 12.5-micron band of the p-tolyl group does not vary as much as that of the two phenyl group bands. However, there is again a distinct difference in the intensity of this band for the monosilanes and for the disilanes, as well as smaller variations within each group. I n 1,1,2triphenyl-1,2,2-tri-p-tolyldisilane, which had ail abnormally high value for the peak molar absorptivity of the 13.5-micron band, the peak molar absorptivity of the 12.5-micron band is also somewhat higher than would be expected from comparison with the other compounds. Since the molar absorptivity of each band attributable to the phenyl group varies regularly with changes in the structure of the compounds and since the molar absorptivity of the 12.5-micron band of the ptolyl group is relatively constant, it should be possible to determine the ratio of the two group concentrations in a sample of an unknown compound. The ratio of group concentrations is independent of the weight of sample taken, so an unweighed sample may be used, with the determination of group concentrations proceeding directly from the peak absorbances of the bands rather than from their molar absorptivities. Figures 3 and 4 show t.he two possible peak absorbance ratio8
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RATIO b h e n y 3 b - t o l y a
Figure 4. Phenyl-p-tolyl absorbance ratio as a function of group concentration ratios 14.3-micron phenyl band
Iilotted against the ratio of phenyl to p-tolyl groups in the compounds studied. TWOstraight lines are obtained in each graph, one for the monosilanes and one for the disilanes. In one case the line representing the disilanes has the greater slope, in the other case the line representing the monosilanes has the greater slope Advantage can be taken of this fact by plotting the sum of the tn o absorbance ratios against the ratio of phenyl to p-tolyl groups. This graph is shown in Figure 5. A single straight line passing through the origin is obtained for all of the compounds in the group, with all of the points falling on the line within the precision o f measurement. The fact that a single straight line is obtained in Figure 5 is due to the difference in peak molar absorptivity of the 12.5micron band in monosilanes compared to disilanes. This difference corrects for a similar difference between the sums of the peak molar absorptivities of the 13.5- and 14.3-micron bands in mono- and disilanes. If it were not for this internal correction, two lines instead of one would have been obtained. Figure 5
V O L U M E 27, N O . 3, M A R C H 1 9 5 5 can therefore be employed for group ratio determinations regardless of whether the compound is a mono- or disilane, as well as for mixtures of such compounds In the analysis of mixtures no considerable error should be introduced by variations in the ~ a v length e of thr a1)sorption maxima since such variations are small. The p-tolyl group bands all fall \I-ithin the range of 12.47 to 12.50 microns, and the phenyl group bands within the range from 13.40 to 13.G microns and 14.30 to 14.33 microns. If the sample is knnxn to he a pure compound rather than a mixture, the question of x-hether it is a mono- or a disilane can be resolved by using the data of Figures 3 and 4. nlSCUSSION
This analytical method has several advant,ages in addition to yielding quantitative data on functional group conccnt,rations even 1vhi:n the molar absorptivities of t,he ahsorption bands used for the analyses are not constant. Because the sample need not be weighed and dissolved in a knoxn amaunt of solvent,, less time is required t,o obtain the necessary data. The use of peak absorbances, rather than molar absorptivities, saves t'ime in calculating the results. In addition, it waa found that virtually the same analytical curves were obtained on the two instruments evrn though different spectral slit widths were used. The data in Fignrcs 3, 4,, ?nd 5 were obtained on a Baird A4ssociates hlodcl B spectrophotometer, operated at normal slit widths and using a hase-line technique for the estimation of To. The data of Tahle I and Figures 1 and 2 were obtained on a Perkin-Elmer Model 13 spectrophotometer, using the drnsitometer attachment. Thcse readings were made a t rather narrow slit widths: 0.440 mm. a t 12.5 microns, 0.500 mm. a t 13.5 microns, and 0.815 mm. at, 14.3 microns. In spite of these differences, very neai,ly t,he same analytical curves were obtained for the t'wo instruments. For the Baird instrument the slope of the analytical curve is 1.89 and xit,h the Perkin-Elmer instru-
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353 ment the slope is 2.10. The average deviations of the points from the respective curves were 0.11 on the absorbance ratio scale for the Baird instrument and 0.27 for the Perkin-Elmer instrument, indicating that the base-line technique was, in this case, self-correcting for some factor affecting the absorptivities. The greater part of the standard deviation value for the Raird instrument Came from one point, the absorbance ratio for pentaphenyl-p-tolyldisilane, and the comparatively large error here was probably caused by the low absorbance of the 12.5micron p-tolyl group band. If this point was not considered in the calculations, the average deviation in Figure 5 was 0.04. Since the closest values of the absorbance ratio are 0.95 for a phenyl-p-tolyl group ratio of 1 to 2 and 0.63 for a group ratio of 1 to 3, it is unlikely that this method of analysis will fail to distinguish between different possible structures for a compound. The variations of peak molar absorptivity of the various bands xith the number of phenyl groups on a silicon atom, shonn in Figures 1 and 2, probably reflect the electron-donating character of the p-tolyl group, affecting the electronic structures of thc rings. Apparently, this effect is not propagated through more than one silicon atom. It is not clear why the 14.3-micron band of the disilanes is not affected in this manner. Other groups attached to the same silicon atom can also affect the band intensities. TWO chlorosilanes, diphenyl-p-tolylchlorosilane and phenyl-di-p-tolylchlorosilane, containing both phenyl and p-tolyl groups have been studied Neither of these compounds would fit on the analytical curve of Figure 5, but an extension of a straight line connecting the t a o points on a similar plot passed through the origin. This general approach is probably also applicable to compounds in which the silicon atom is replaced hv another metal, though such compounds have not yct been made available for study. Certain substituent groups on the silicon atom can give rise to interfering absorption bands. Aliphatic groups on the Rilicon atom absorb strongly in the 12- to 15-micron region (11) and silanols have a strong absorption band, presumably caused by a deformation vibration of the hydroxyl group, a t about 11.8 microns in the solid and 12.4 microns in solution, the shift representing the effect of hydrogen bonds in the crystal. [This band has previously been observed by Richards and Thompson (9).] The silicon-hydrogen group absorbs strongly a t about 12 5 microns, presumably corresponding to a deformation vibration. ACKNOWLEDGhIENT
The authors are grateful to Henry Gilman and T. C. 'Xu of the Department of Chemistry of Iowa State College for the compounds used in this study. LITERATURE CITED
(1) .Inderson, J. d.,and Seyfried, W. D., ANAL.CHEM.,20, 998
(1948).
(2) Evans, A., Hibbard, R. R., and Powell, A. S., Ibid., 23, 1604
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I 2 3 PHENYL/PARA-TOLYL
4 5 GROUP RATIO
Figure 5. Analytical curve for functional group analysis of tetraarylsilanes and hexaaryldisilanes containing phenyl and p-tolyl groups
(1951). (3) Francis, S.A., Ibid..25, 1466 (1953). (4) Gilman, H., and Wu, T. C., ,J. Am. Chem. Soc., 7 5 , 3762 (1953). (5) Hastings S.H., Watson, A. T., Williams, R. B., and Anderson, .J. .4.,Jr., AivaL. CHEM.,24,612 (1952). (6) Hibbard, R. R., and Cleaves,A. P., Ibid., 21,486 (1949). Kier, D. S., and Dobriner, K., (7) Jones, R. N., Ramsay. D. -4., J . Am. Chem. Soc., 74, 80 (1952). (8) Pitaer, K. S., and Scott, D. W., Ibid.,65,803 (1943). (9) Richards, R. E., and Thompson, H. W., J. Chem. SOC.,1949, 124. (10) Rose, F. W., J . Research A-atl. Bur. Standards, 20, 129 (1938). (11) Wright, N.. a n d Hunter, 121. J., J. Am. Chem. Soc., 69, 803 (1947). RECEIVED for review August 18. 19.54. Accepted November 17, 1954. Contribution N o . 352 from the Institute for Atomic Research and Deparb mcnt of Chemistry, Iowa State College.lAmes, Is. Work waa performed in t h e Ames Laboratory of t h e .4tomic Energy Commission.
