CHEMISTRY

A. A. GUNDYREV, N. S. NAMETKIN, and Academician A. V. TOPCHIEV

DIPOLE MOMENTS OF HEXAALKYL DERIVATIVES OF DISILOXANE

In the present communication we give the values of the dipole moments for four hexaalkyl derivatives of disiloxane: hexamethyldisiloxane, hexaethyldisiloxane, hexapropyldisiloxane, and hexabutyldisiloxane, obtained by hydrolysis of the corresponding trialkylhalide (ethoxy) silanes ((^1)).

The dipole moment of hexamethyldisiloxane has been measured by a number of authors ((^{2,3})), whereas the dipole moments of the other hexaalkyl derivatives of disiloxane have been measured by us for the first time. We calculated the dipole moments from the values of the dielectric constants of dilute solutions of these substances in the nonpolar solvent—(n)-hexane.

The dielectric constants were measured in an apparatus operating by the beat method. The stabilization of the instrument was piezoquartz, with a frequency of (7.95 \times 10^5) Hz. The measuring capacitor consisted of three concentric platinum cylinders. The outer and inner cylinders were connected to one another at several points by platinum wire, and this electrode was grounded, whereby the middle electrode was shielded.

The interelectrode capacitance of this capacitor, as measured with benzene, proved to be 12.363 pF. Using this value and the results of experimental data on the measurement of the capacitance of the capacitor filled with substance, we calculated the dielectric constants of toluene, (n)-hexane, and hexamethyldisiloxane at (25^\circ). The values obtained and the literature data are given in Table 1. Comparison of these data indicates the very high accuracy of the instrument.

Table 1

Dielectric constants of toluene, (n)-hexane, and hexamethyldisiloxane at (25^\circ)

* For (t = 20^\circ).

The measurement of the dielectric constant (\varepsilon), density (d), and refractive index (n_D) was carried out at (25 \pm 0.05^\circ). The dielectric constant was determined with an accuracy of 0.02%. The density of the solutions was determined with a pycnometer with an accuracy of 0.02%, and the refractive index on an IRF-23 refractometer with an accuracy of 0.01%.

The dipole moments of the hexaalkyl derivatives of disiloxane were calculated by the method of Hedestrand ((^7)) from the formula:

[
\mu = 0.012813 \cdot 10^{-18}\sqrt{P_{\mathrm{op}}T}
\tag{1}
]

where (\mu) is the dipole moment, (P_{\mathrm{op}}) is the orientational polarization, and (T) is the absolute temperature.

Orientational polarization is related to the total polarization of the dissolved substance (P_\infty) and to the deformation polarization (P_{D\infty}), taken as equal to the molar refraction, by the relation

[
P_{\mathrm{op}}=P_\infty-P_{D\infty}.
\tag{2}
]

The subscript (\infty) indicates that the values (P_\infty) and (P_{D\infty}) have been extrapolated to infinite dilution by the Hedestrand method.

The total polarization at infinite dilution was calculated by the following formulas, in which subscript 1 refers to the pure solvent and subscript 2 to the dissolved substance; the dielectric constant (\varepsilon), density (d), and refractive index (n_D) without subscripts are quantities characterizing the solution; (C_2) is the mole fraction of the dissolved substance, and (M_1) and (M_2) are the molecular weights of the solvent and the dissolved substance.

Total polarization of the dissolved substance:

[
P_\infty=A\left(M_2-\beta d_1\frac{M_1}{d_1}\right)+B\alpha\varepsilon_1,
\tag{3}
]

where

[
A=\frac{\varepsilon_1-1}{\varepsilon_2+2}\frac{1}{d_1}, \qquad
B=\frac{3M_1}{d_1(\varepsilon_1+2)^2}, \qquad
\beta d_1=\frac{d-d_1}{c_2}
\quad\text{and}\quad
\alpha\varepsilon_1=\frac{\varepsilon-\varepsilon_1}{c_2}.
]

Deformation polarization of the dissolved substance:

Table 2

Constants of n-hexane

[
P_{D\infty}=A'\left(M_2-\beta d_1\frac{M_1}{d_1}\right)+B'\gamma n_1^2,
\tag{4}
]

where

[
A'=\frac{n_1^2-1}{n_1^2+2}\frac{1}{d_1}; \qquad
B'=\frac{3M_1}{d_1(n_1^2+2)^2}
]

and

[
\gamma n_1^2=\frac{n^2-n_1^2}{c_2}.
]

Table 2 gives the constants of the pure solvent for calculating (P_\infty) and (P_{D\infty}).

Table 3 gives the results of measurements of the dielectric constant ((\varepsilon)), density ((d)), and refractive index ((n_D)) of solutions of hexaalkyl derivatives of disiloxane in n-hexane for different concentrations of the dissolved substance, expressed in mole fractions ((C_2)).

From the data of Tables 2 and 3, the constants necessary for calculating dipole moments by the Hedestrand method were calculated. These constants are given in Table 4.

For a number of compounds, (\alpha\varepsilon), (\beta d_1), and (\gamma n_1^2) depend nonlinearly on concentration; therefore, extrapolation to zero concentration was necessary. For this purpose the curves (\varepsilon=f_1(c_2)), (d=f_2(c_2)), and (n_2=f_3(c_2)) were constructed, and tangents were drawn at the point (c_2=0). The tangent of the angle of inclination of the tangent to the concentration axis gave the desired value.

As indicated above, the literature contains data only on the dipole moment of hexamethyldisiloxane. The dipole moment of this compound, measured in benzene, proved to be (0.80\,D) (²). According to other data (³), the dipole moment of hexamethyldisiloxane, measured in benzene at (30^\circ), is (0.79\,D). These values agree well with the value obtained by us, equal to (0.78\,D).

Table 3

Experimentally found values of (\varepsilon), (d), and (n_D) for solutions of hexaalkyl derivatives of disiloxane in (n)-hexane

Table 4

Constants for calculating the dipole moments of hexaalkyl derivatives of disiloxane by the Hedestrand method

The values of the dipole moments of the hexaalkyl derivatives of disiloxane given in Table 4 show that the Si—O bond is strongly polar. This is due to the considerable difference in the electronegativities of Si and O.

The observed decrease in the dipole moment of molecules containing ethyl, propyl, and butyl radicals in comparison with molecules containing methyl radicals can apparently be explained by an increase in the valence angle caused by repulsion of bulky groups.

Received
13 V 1958

CITED LITERATURE

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