Dec., 1057
HYDROLYSIS OF CHLOROSILANES. I. COXDUCTOMETRIC TITRATION
1591
THE HYDROLYSIS OF ALKYL AND ARYL CHLOROSILANES. I. CONDUCTOMETRIC TITRATION BYL. H. SHAFFER* AND E. M. FLANIGEN Contribution f r o m the Research Labora.tory of Linde Cornpang, A Division of Union Carbide Corporation, Tonawanda, N e w York Received April 16, 1967
A conductometric method for following the hydrolysis of chlorosilanes in homogeneous solution was developed. The conductometric end-points indicate that hydrolysis intermediates such as C1(R2SiO),Rm~SiCI, C12RSiOSiRCl2and RC1,Si(OR2SiC1)3-, are formed in difunctional, trifunctional and mixed chlorosilane systems, respectively. Lowering the reaction temperature and increasing the HC1 concentration was found to suppress the hydrolysis of silicon-chlorine bonds and favor the formation of the chlorine end-blocked siloxane intermediates.
Introduction The hydrolysis of chlorosilanes, with evolution of by-product hydrochloric acid is of fundamental importance in the preparation of siloxanes. Although several previous publications' already have dealt with convenient methods for obtaining various siloxanes from chlorosilanes, little or no work has been published on the details of the hydrolysis reaction itself. The development and use of a conductometric technique to study the hydrolysis of chlorosilanes is described in this and a succeediiig paper. This paper will be concerned primarily with t,he homogeneous conductometric titrat'ioii of chlorosilane solutions and an interpretation of the end-points observed in terms of hydrolysis ' intermediates. Several of these have been prepared previously by other methods; some are new but proved stable enough to be isolated easily once their existence was suspected and a few are so unstable that their transitory existence in solution could only be inferred from the shape of the titration curve and the body of other data accumulated by this method. Experimental Apparatus .-The conductivity cell designed for the hydrolysis esperiments is shown in Fig. 1 . The cell is a diptype platinum electrode system. The platinum electrodes are connected through a platinum to Pyres glass seal to platinum wire leads. The cell was designed for durability and practicality rather than accuracy in absolute conductance measurements since only relative changes in conductivity were desired. Shiny platinum electrodes were used instead of the usually preferred platinum black since 110 advantage is espected with the platinum black electrodes in non-aqueous solvent systems. The cell constant, estimated from the geometry of the electrodes, was 1.90. A rough measurement on 0.1 N KCI solution gave a value of 1.67. The 1.67 value was used for calculating specific conductivities. Since only the change in conductivity is being measured in any one experiment, no attempt was made to determine the cell constant more precisely. Two conductivity bridges were used for malting resistance measurements: a General Radio Company megohm bridge, Type 544 P, a d.c. Wheatstone Bridge with a range of 0.1 megohms to megamegohm; and an Industrial Instruments Inc. conductivity bridge, model RC16 with an a.c. power supply and a range from 0.2 to 2.5 megohms. Runs st 0 and -78" were made in ice-water and Dry Iceacetone baths, respect'ively. Runs a t other temperatures were made in appropriate constant temperature baths. Temperature Measurement.-In the majority of esperiments 110 measurement of temperature inside the conduc-
* Central Researrh Laboratory, American Machine and Foundry Co., Staniford, Conn. (1) E. D. Hughes, Quart. R e v . ( L o n d o n ) , 5 , 245 (1951); J. F. Hyde and W. H. Daudt, J . A n . C h e m . Soc.. 74, 388 (1952); W. Patnode and D. F. Wilcock, ibid., 68, 358 (1946).
REACTANT RESERVOIR
I
J
GLASS STIRRING ROD
PLATINUM LEAD
GLASS THERMOCOUPLE WELL
INLET TUBE GLASS TUBING SUPPORT FOR PLATINUM LEADS
PYREX TO PLATINUM SEALI CM. SQUARE SHINY PLATINUM ELECTRODES
RIGID GLASS LATTICE. SUPPORT FOR PLATINUM ELECTRODE
Fig. I .-Conductivity
cell.
tivity cell was made. The temperature ot the outside cooling-bath was measured with a calibrated Hg therrnometer a t 0 and 25", with a Hg-Tl thermonicter (HBASTM kinematic viscosity Type HB Cat. KO. 22400-C) in the range -65 to -15", and a propane thermometer (H. S. Morton Company, No. 240) a t -78'. The reaction temperature in all esperiments was assumed to be the same as the temperature of the surrounding bath since the small heat of reaction in such dilute solution is easily dissipated by the rapid stirring employed. To verify this assumption, the temperature inside the conductivity cell and in the surrounding constant temperature bath was measured with a calibrated copper-constantan thermocouple during some of the runs a t 0, -30, and -78". The measurements were made in the conductivity cell by inserting a glass thermocouple well in the cell as shown in Fig. 1 to support the thermocouple. Potentiometric measurements were made with a Ruhicon potentiometer using a water-ice reference junction. In all cases the temperature of the reaction niisture was the same as the constant temThe temperature of the Dry perature bath within +0.5'. Ire-acetone bath varied between -70 and -78" in duplicate experiments. Reagents.-Ansul Chemical ethylene glycol-dimetjhyl ether was purified by passing it through a LINDE Sodium S zeolit8e2column immediately before use. Such purification (2) D. W. Breck, W. G . Eversole and R. R9. Milton, ibid., 78, 2338 (1956). The term "LINDE" is a registered Trade-Mark of Union Carbide Corporation.
L. H. SHAFFER AND E. M. FLANIGEN
1592
Vol. 61
TABLE I THEORETICAL CHLOROSILANE HYDROLYSIS PRODUCTS Systems
Product
Difunctional
R2SiCl(OH)" R2Si(OH)z C1(R2SiO),R2Si(CI)
1 1 1 and 2 or 3 1 and 2 or 1 and 3 1 and 2 or 1 and 3
Cl(RzSiO),R2Si(OH) (HO)(R2SiO),RzSi(OH)
a
Titration end-point moles HnO/mole chlorosilane
Reaction(s)
RzSiO), Trifunctional RSiC1,- ,l(OH)2 RSiC120SiCI2R (RSiClO), (RSiOvd, [RSi(OH)O], The formation of significant quantities of such products is improbable.
1 and 2 or 3 1 1 and 2 01' 3 1 and 2 or 3 1 and 2 or 3 1 and 2 or 3
1.0 2.0 Between 0 . 5 and (equal to x / ( x 1.0 Between 1.0 and (equal to (x (x 1)) 1.0
1.0
+ 1)) 2,O
+ 2)/
+
n
0.5 1.o 1.5 2.0
obtain the titration curve. In these experiments, the changes in conductivity were so large that it was convenient to use a logarithmic scale rather than the more conventional linear scale to plot conductivity. Detailed conventional plots of the end-point region were made in several cases, and while the end-point was more sharply defined, its position relative to the water/chlorosilane axis was not altered. Typical titration curves are shown in Figs. 2 and 3. The end-point is determined readily by a sharp rise in the donductivity of the reaction mixture when hydrolysis is complete and unreacted water is present in the reaction mixture. Aqueous titration of a solution of anhydrous HCl in the same solvent system established that the sharp rise in conductivity a t the end-point of the chlorosilane titration is due to the ionization of the hydrochloric acid formed in the presence of unreacted water. The nature of the hydrolysis products can be deduced from the mole ratio of water/chlorosilane a t which the end-point occurs. A single product was sometimes present a t the end-point but frequently mixtures of a homologous series of silicone polymers were obtained. Relatively simple confirmatory tests, such as boiling point, index of refraction, molecular weight and weight-% GI, were used in a few cases to check the interpretations offered here.
Results and Discussion In the hydrolysis of chlorosilanes, the following reactions can take place =Sic1
/
I
I
I
l
I
I
I
,
I
I
/
l
0.5 1.0 MOLE RATIO: H20/ RSiC13 >> RzSiC12 > RaSiCl
And second, for any chlorosilane containing two or more chlorines on the same silicon, the first chlorine reacts very much faster than those remaining? Experimental The apparatus previously describedl was used to obtain hydrolysis rates by conductivity measurements. As is illustrated in Fig. 1, there is a linear relation between water concentration and conductivity when water is added to et,hylene glycol-dimethyl ether-HC1 and dioxane-HC1. Therefore, the hydrolysis rate can be determined by measuring the rate of change of the resistance of a chlorosilaneHC1 solution after an addition of water. The fraction of unreacted water remaining after any time interval is given by ( R L - Itm)/(Ra - R m ) . R, is the resistance a t the time the first measurement is made; R Lis the resistance a t time t (3) See also W. C. Schumb and A. J. Stevens, J . Am. Chem. 78,3178 (1950).
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