ELECTRIC MOMENTS OF
Oct., 1950
ORGANOLSILICON
FLUORIDES .4ND CHLORIDES
447 1
lytically from M = 0 to M = 0.001, assuming that A j is proportional to M. For the more concentrated solutions, the evaluation is carried out graphically. Table I11 contains the provisional activity coefficients for the four salts a t rounded concentrations.
the mean deviation of three or four determinations was consistently in the range 0.03-0.05%. Values of the activity coefficients for magnesium perchloratee in the literature were obtained by the isopiestic method. The differences between these data and those of the present investigation are within the range of the experimental error. TABLE I11 No attempt has been made to correlate the dif1-ALUESOF THE ACTIVITYCOEFFICIENT,T', FOR THE ferences of activity coefficients with the size and ALKALINEEARTHAND MACWESUN PERCHLORATES shape of ions. MolLim. MgAcknowledgment.--This work was made possible, ality law Ba(C1Odn Sr(ClO4h Ca(C104h (C10r)z initially, by a grant from the Defense Research 0.001 0.910 0.895 0.897 0.897 0.898 Laboratory, The University of Texas, under the .002 .841 ,858 .861 .861 .861 sponsorship of the Bureau of. Ordnance, Navy .803 ,803 .005 .760 ,798 .802 Department, Contract NO rd-9195 and, during .Ol .678 ,743 .750 .751 ,752 the year 1949-1950, by the award of a fellowship 102 .577 .682 .695 ,697 .697 from the Humble Oil and Refining Company, .05 .420 .599 .625 .629 .628 Houston, Texas. .1 ,293 .540 .579 .587 .588 .2
:< .4
5 .6
-
. I
.8 .9 1.0
,176 ,119 ,086 ,064
,489 .463 .450 .443 .441 .440 .442 .446 ,451
.546 .533 .545 .556 .572 .592 .615 ,642 .e73
.566 .572 .589 .615 .646 .683 .725 .774 .E34
.578 .591 ,619 ,655 .698 .749 .807 .872 .947
Discussion of Results It is believed that most of the freezing point data obtained in this investigation are accurate to within 0.05410% of the freezing point depression. In the analysis of the equilibrium solutions,
Summary
1. The freezing point depression method was used to determine the activity coefficients of the alkaline earth and magnesium perchlorates in aqueous solutions over a concentration range 0.003 to 1 molal. 2. The activity coefficients were calculated from j-values listed for rounded concentrations. 3. The activity coefficients are based upon freezing point data accurate to within 0.05 to 0.10% of the observed depression and upon concentration determinations consistent to within 0.03-0.05%. (6) Stokes and Levien, Tnrs JOURNAL, 68, 333 (1946).
AUSTIN,TEXAS
RECEIVED MARCH25, 1950
[CONTRIBUTION FROM THE CHEMICAL LABORATORIES OF THE UNIVERSITY OF NOTRE DAME]
Electric Moments of Organosilicon Compounds. I. Fluorides and Chlorides' BY COLUMBA CURRAN, ROBERTIf.WITUCKIAND PATRICK A. MCCUSKER
The availability of d orbitals for bond formation in elements of the sodium series suggests a potential double bond character in silicon compounds which is not possible in carbon compounds. Contributions of structures containing double bonds are generally more common in elements of the 'sodium series than in elements of the potassium series. Brockway and Coop2 have studied the bond character in the chlorosilanes by means of electron diffraction and electric moment measurements. The low moments for these compounds, SiHaCl 1.28, SiHzClz 1.17 and SiHC13 0.85, compared to the values for the chloromethanes CH3Cl 1.87, CHzC12 1.56 and CHCI, 1.00, were interpreted by these workers as suggesting contributions of structures of the type H3-Si=C1+ to the chlorosilane molecules. This double bond (1) Presented at the New York Meeting of the American Chemical Society, September, 1947. (2) Brockmy end Coop. Tranr. Faraday Sac.. U,1419 (1988).
character of the silicon-to-chlorine bond is further indicated by the relatively large moments of the corresponding germanium compounds. Sauer and Mead4 have reported a moment of 0.74 D for hexamethyldisiloxane. In calculating the oxygen valence angle in this compound, these authors neglected potential double-bond character and assumed a Si-C bond moment of 1.0, with the carbon the negative end of the dipole, and a Si-0 bond moment of 2.8. More recently Wright and Hunter6 have reported the infrared spectra of the methylpolysiloxanes. The intensity of the absorption peak attributed to the S i 4 stretching vibration was observed by these workers to be about five times as great as that for the C-0 bond in ethers. They interpreted this high intensity as due to a large degree of ionic (3) Smyth, THISJOURNAL, (IS, 57 (1941). (4) S a u u and Mead, ibid., 68, 1794 (1946). (6) Wright and Hunter, ibid., 60, 808 (1947).
character in the Si-0 bond. The strong absorytion attributed to the Si-C stretching vibration was interpreted as an indication of a large +Si-Cdipole and the low intensity of the C-H absorption as an indication of the low polarity of the C-H bonds resulting from a charge migration from silicon to carbon. These interpretations appeared to us to be inconsistent with the moments reported for the halosilanes. In order to obtain more pertinent data on the charge distribution in organosilicon compounds we have determined the electric moments of some alkyl,tnd arylhalosilanes.
0.00000 .00726 ,00790 .01126
Diphenyldichlorosilane 2.276 2,347 2.353 2.388
0 . 0000u .00900 iJ171fi I E20fi
Phenyltrichlorosilane 3 376 2.351 2 418 2 -459
TABLE1 DIELECTRIC CONSTANTS AND DENSITIES O F BENZENE SOLUTIONS AT 25 O C,
0.00000 .01113 01461 .01720 a
0.oc)Ooo ,01339 .(I1786 .01908
0.oooOO .00562
.01259
.02062
c
Triethylchlorosilane 2.276 2.339 2.359 2.374 Diethyldichlorosilane 2.276 2.379 2.413
2.422
Ethyltrichlorosilane 2.276 2.307 2.345
2.3813
d
10.8737 .8739 .8740 .8741 0.8737 .8772 .8785 .8788 0.8740 .8766
.w2 .w1
,8792 ,8793 .8819 0.8735 ,8804
,8868 .8RQ-I
I riethylfluorosilane 2.276 2 320 2 340
Experimental Reparation and purification of Compounds.-Ethyltrichlorosilane and diethyldicblorosilane were obtained from now-Corning and were purified by fractional distillation in a 53-inch, glass helix packed column. The purified liquids had the properties: ethyltrichlorosilane, P. p. 100-100.1 at 746 mm., dUd 1.2342; diethyldichlorosilane, 1.0472. IJ.p. 129.0 at 744 mm., ~ Z S D1.4291, Triethylchlorosilane was prepared by the reaction of ethylmagnesium bromide with diethyldichlorosilane and purified by fractional distillation in the 53 in. column. The product obtained had the following properties: b. p. 145.8-146.0, n% 1.4299. Phenyltrichlorosilane was prepared by the reaction of silicon tetrachloride with phenylmagnesium chloride and the product obtained was purified by fractional distillation. The purified material boiled at 85.1' at 15 mm. The purity was checked by analyzing for cldoride by titration of an aqueous suspension with standard base. Diphenyldichlorosilane and triphenylchlorosilane were obtained from Anderson Laboratories, Adrian, Michigan, and were purified by fractional distillation. The samples med for measurements had the following properties : diphenyldichlorosilanc, 13. p. 126.0 a t 3 tnm., ~ E I .,5778 ~ ~ D and ds+ 1.20214; tripheiiylchlornsilatle, 11. p. 156" a t I mm., in. p. 9C-95". Triethylfluorosilane, diethyldifluorosilane, triphenylAuorosilane, diphenyldifluorosilane and phenyltriflnorosilane were all piepared by the reaction of anhydrous zinc fluoride on the corresponding chlorine compounds and were puriced by *fractional distillation. The purified fluorine compounds had thf following properties : triethyliluoro~ 0.8326; diethylddsilane, b. p. 109.2 , 2 % 1.3879, fluorosilane, b. p. 62.0 ,phenyltrifluorosilane, b. p. 101.9 ,
0.8736
0.87YG ,8724 .8721 ,8720
L' ?-UJ
IXethyldifluorosilane 3 276 2 347 1 768 2 117
0.8737 .8747 .8749 .87.55
l)ipiieiiyltiifl~roro~ilatlr 3 "(j
0.8736
,8781 .8781 .87R7 .8815
2 .j5d 3 353 3 3x0
2 402
Phenyltri€iuorosilane 2,376 3 39.5 3.407 1: 461;
0.8737 .8790 .8795 .8822
i1.00000 .00887 .01176 ,01535
Triphenylchlorosilane 2 276 2.344 2 364 2 395
0.8727 .8799 ,8822 .8849
0. ooooo ,01238 .01456 ,01738
Triphenylfluorosilane 2.276 2.347 2.359 2.375
0.8728 .8815 .8830 .a51
TABLE11 POLARIZATIONS AND ELECTRIC MOMENTS
Triethylchlorosilane Diethyldichlorosilane Ethyltrichlorosilane Triphenylchlorosilane Diphenyldichlorosilane Phenyltrichlorosilane T riethylfluorosilane Diethyldifluorosilane Triphenylfluorosilane Diphenyldifluorosilane Phenyltritluorosilane
Pi
MRn
F
134 158 121 188 207 170.4 100.5 132.4 157.2 196
43.5 38.68 33.85 89.4 69.2 48.94 38.04 27.8 83.7 57.8 31.9
2.07 2.39 2.04 2.14
190.4
2.56
2.41 1.72 2.23 1.84 2.57 2 77
Oct.. 1950
ELECTRIC MOMENTS
OF ORGANOSILICON
diphenyldifluorosilane, b. p. 93.5-94.0" a t 3 mm., n% 1.5221, triphenylfluorosilane,b. p. 162-165Oat3 mm. The triphenylfluorosilane was further purified ,by several recrystallizations from dry ether, m. p. 61.5 Measurements and Calculations.-The measuremente of dielectric constants and densities and the calculation7of electric moments have been previously described. The molar refractions of the diethyldichlorosilaneand triethylfluorosilane were obtained from the refraction and density data, and the values for the other compound were taken from or calculated from the data and bond refractions listed by Warrick.8
.
Discussion The moment obtained for triethylchlorosilane agrees with the value reported for this compound by Malatesta and P i z ~ o t t i . ~The large moments of the organohalosilanes compared to the values for the chlorosilanes reveal that the ethyl and phenyl groups are much more positive with respect to silicon than is hydrogen, on contrast to the interpretations of Wright and Hunter5 and the assumptions of Sauer and Mead.4 This polarity suggests a hyperconjugation effect with contributions of such structures as H+C!==Si--Cl and H+C=SiCl- to the ethylchlorosilanes. The increase in moment from chlorosilane to triphenylchlorosilane, 0.86, compares to an increase of 0.32 from chlorogermane to triphenylbromogermane,3 indicating, as expected, less double bond character in carbon-to-germanium bonds. The moments obtained for the silicon compounds investigated along with the values for a few corresponding carbon compounds reported by other investigators are listed in Table 111. TABLE 111 XTS IN DEBYEUNITS (CHa)aCCI 2.15 (CHa)zCClz 1.99 CHaCCls 1.6 CaHjCHzCl 1.85 (CsHj)&Ch 2.39 CaHrCC13 2.04 CzHs(CH8)zCF 1.92
...........
..
Ce,HsCH*F
1.77
CeHsCFs
2.56
...........
..
The moments of benzotrichloride and benzotrifluoride are the values reported by Freiser, Hobbs and Grosslo and the value for t-amyl fluoride is that of Rogers." Values for the other carbon compounds listed were taken from Technical Report No. 11, Laboratory for Insulation Research, Massachusetts Institute of Technology, 1947. The moments of the chloroalkanes, chloromethanes and chlorosilanes all show the characteristic decrease from the mono- to the tri(6) Moede and Curran, THISJOURNAL, T I , 852 (1949). (7) Curran and Chaput, ibid., 19, 1134 (1947). (8) Wanick, ibid., 16, 2465 (1946). (9) Malatesta and Pizzotti, Gam chkim. it&, 711, 143 (1943). (10) Freiser, Hobbs and Gross,THISJOURNAL, T i , 111. (1949). (11) Rogers, ibid., W, 457 (1947).
FLUORIDES AND
4473
CHLORIDES
chlorides, attributed to the inductive effect. Diethyldichlorosilane, on the contrary, has a larger moment than the triethylchlorosilane, and the increase is almost exactly that calculated in the absence of an inductive effect. This increase reveals that the RzSi group is much more polarizable than the H2C, H2Si or R2C groups, suggesting an increased contribution of the structure H+C= Si- in diethyldichlorosilane. The moment of triethylfluorosilane is much smaller than the value for the corresponding chloride, whereas the increase in moment from the mono- to the dihalide is much greater for the fluoro compounds. This is consistent with a larger contribution of R3-Si=F+ than Ra-Si=Cl+ as expected from considerations of the relative tendencies of chlorine and fluorine to form double bonds, and suggests that the introduction of the second fluorine atom in the molecule results in the contribution of the structure R&i/
F+ F-
.
We were
unable to obtain a reliable value for the moment of ethyltrifluorosilane (b. p. -4') in benzene, but it appears to be a t least as large as the moment of the difluoride, in contrast to the decrease observed for moment of the trichloro compound. Unfortunately the moments of the di- and trifluoroalkanes are not available for comparison. The phenylhalosilanes all have appreciably larger moments than the corresponding carbon compounds. The chloro compounds show a maximum moment for the phenyldichlorosilane, whereas phenyltrifluorosilane has the largest moment of all the compounds investigated, indicating large contributions of such structures F
as +CaH5=Si
