WILLIAMS. TATLOCK AND EUGENE G. ROCHOW [CONTRIBUTION FROM
THE
VOI,
72
hf ALLIXCKRODT LABORATORY O F HARVARD UNIVERSITY]
The Action of Strong Base on Hexamethyldisiloxane BY WILLIAMS. TATLOCK' AND EUGEKE G. ROCHOW* It has amply been demonstrated that cleavage of silicon-carban bonds in various methyl siloxane systems can be brought about by the action of base of varying strength, depending on the negative substituents carried by the methyl groups. Even in unsubstituted hexamethyldisiloxane, it has been shown that under sufficiently drastic conditions methane is liberated and an inorganic silicate is deposited.2 However, in view of the ready rearrangements of siloxane systems which have been shown to proceed in the presence of bases (and even iq an alcoholic ammonia medium2), i t would seem that under the proper conditions cleavage of the silicon-oxygen bond of siloxane systems by bases to form salts should compete very favorably with the stripping of methyl groups from the silicon nuclei. I t has been the purpose of this investigation to determine these conditions, with the aim of preparing alkalimetal salts of the simple silanols by such basic action. Hexamethyldisiloxane, (CH&SiOSi(CH3)3, was used as starting material in the present work because of its ready availability and because of the known acidity of trimethylsilanol toward strong bases.3 It would be expected that hexamethyldisiloxane, when treated with alkali of 12 N strength or greater, should dissolve under the attack by hydroxyl ion. I n accordance with the mechanism proposed by Swain, Esteve and Jones,4 a pefitacovalent silicon intermediate CH3
CH3
1
I
+ OH-
CH3-Si-O-Si-CH~
I
CH,
CH,
J.
[:2 , CH?
CH?
Si- -0-Si-
CH? CH3
J
I CR8--Si-OH I CH,
+
CHs
I CH3-Si-0I
1
CHa
CH,
1-
\ CHI
CH,
I HO-Si-0-Si-CH, I I CHs CHP 4CHa
c1-1
7
_____
* Harvard (1)
(2) (3) (4)
University Faculty 1948Harvard University Procter and Gamble Fellon, 1948-1950 Krieble and Elliot, THISJOURNAL, 68, 2291 (1946) Sommer, Pietrusza and Whitmore, ibid , 68, 2282 (1946) Swain F s t e \ e m d Jones zbcd , 71, 965 (1949)
would be formed, and the reaction might then proceed either by cleavage of the silicon-carbon or the silicon-oxygen bond. As was pointed out by Krieble and Elliot,2 the more electronegative group should be displaced, and this should be the siloxy group. This is represented by the scheme shown. The silanol and siloxy anion would then be expected to react with the basic solution to form the metallic salt, which would be withdrawn from the equilibrium because of its low solubility. Preliminary experiments revealed, however, that a t ordinary temperatures and even with very concentrated aqueous base, hexamethyldisiloxane seemed completely insoluble and unaffected. It was found that in a sealed tube a reaction proceeded a t temperatures above 100' to yield methane. No silanolate was precipitated. Using a stainless steel reaction bomb fitted with a pressure gage, the course of subsequent reactions could be observed by following the pressure of methane developed during the reaction. I n a homogeneous medium consisting of absolute ethanol, potassium hydroxide and hexamethyldisiloxane, it again was found that a t temperatures of 170-200' the qualitative results were the evolution of methane and the deposition of a mainly inorganic silicate, in which the potassium content varied between 20-2.74. When small amounts of water were added, the over-all course of the reaction remained unaltered, but the early evolution of methane was much faster (see Fig. 1). These results indicate that in a hydroxylated solvent the displacement of methyl groups to form methane must be easier than the competing formation of the siloxy anion. If, however, a reaction could proceed in an aprotic solvent, where the only cation available would be M+, then the most stable product would be the metallic silanolate, and the cleavage of the silicon-oxygen bond would be the expected result. In order to test this hypothesis, experiments were carried out using dry ethyl ether as solvent. It was necessary to use solid, fused potassium hydroxide, which has only a small solubility in the ether. In spite of all attempts to keep the reactants anhydrous, some methane always was evolved and a mixture of products was obtained. An explanation readily is found, however, in the formation of water by the condensation of the resulting trimethylsilanol to yield hexamethyldisiloxane. The solid products then obtained contained some carbon, and the potassium content had reached 30-35y0 (theoretical for potassium trimethylsilanolate, 30.5%). No pure potassium salt was isolated, but hydrolysis of the crude product yielded a liquid which, on the basis of the infrared spectrogram and odor, was identi-
ACTIONOF STRONG BASEON HEXAMETHYLDISILOXANE
Jan., 1950
529
fied as a trimethylsilanol-hexamethyldisiloxane mixture. Only 1400 very small amounts were isolated. When dry benzene was used as 1200 solvent, a solid reaction product was obtained which contained 30-31yo potassium and considerable carbon. 1000 However, additional impurities were introduced by the participation of 6 benzene in the reaction, and the 2 phenylated product was not identi- 2 ,S fied. 600 The most satisfactory results were obtained when an excess of hexamethyldisiloxane itself was used as 400 solvent. With fused and powdered 200 sodium hydroxide, the fission of methyl groups proceeded very slowly as indicated by the pressure curve 0 6 18 30 42 54 (see Fig. 1). The white solids obtained were extremely hygroscopic Time, hours. and averaged 35% 'Odium Fig. 1 -Curve I, reaction 2, Table 11; Curve 11, reaction 1, Table 11; Curve ical for sodium trimethylsilanolate, 111, reaction 5 , Table 11; Curve IV, reaction 8, Table 11. 20.57,). Infrared absorption spectra of the solids suspended in mineral oil proved the The same procedure was used a t reflux temperature. The presence of the desired sodium trimethylsilano- sealed-tube reactions were carried out in Carius combustion tubing using the amounts of reactants listed in Table late) and demonstrated the presence Of sili- I. The pressure of methane was not measured but was cates which retained Si-0-Si bonding, formed evident upon opening the tubes. by cleavage of methyl groups to form methane. Bomb reactions were carried out in a 110-cc. stainless Hydrolysis of the reaction products yielded a Steel reaction bomb. Heat was applied by means of a specially constructed electric furnace which supported the water-insoluble liquid which was identified as bomb at a 450 angle. The reactions are summarized in hexamethyldisiloxane by its infrared absorption Table 11. In those reactions using alcoholic potassium spectrum and by rhe refractive index. ¶- hydroxide, a sample was titrated to determine the basic tion of a pure sodium silanolate from the silicate strength. When it was necessary to use solid base, the grade pellets were fused in a silver boat to remove impurity not be however, so reagent water, Initial heating of the bomb was carried out as that a t the present time this is not a satisfactory rapidly as possible and temperature and pressure readings method for the preparation of pure silanolates. were taken a t five- to ten-minute intervals until the presNevertheless, the formation and identification sure rise was gradual. Thereafter, readings a t intervals of one or more hours proved sufficient. After the reacOf the sodium proves that in aprotic tion, the bomb was cooled to room temperature or lower cleavage of the siloxane linkage can be effected by and a reading of the residual pressure was taken. The strong bases in preponderance Over cleavage of methane was then discharged and the bomb opened. The liquid and solid portions were collected and protected from methyl groups. moisture. A sample of the liquid layer was distilled, and Experimental in cases where alcohol was used as solvent a sample also ,_ I.
_c_-
The hexamethyldisiloxane was obtained from the General Electric c0.(b. p. n ~ ~1.3774). D The reactions with aqueous alkali are summarized in Table I. At temperature the reactants were stirred for long periods of time, and the aqueous layer examined for precipitation.
TABLE I M1 Mta
Concn. NaOH, hT
Solvent
Solvent, ml.
10 8 H20 50 10 10 HzO 50 10 12 HzO 50 10 12 Hz0 50 10 12 HzO-alc. 100 10 10 Hz0 5 6 12 HzO 6 2 20 Hz0 10 5 20 10 HtO-alc. 6 Satd.' Alc. 6 OM2 = hexamethyldisiloxane ROH.
Temp, OC
Room t . Room t. Room t . Reflux Reflux 115 115 150 115 140
Tube
Time, days
3 3 3 1 1 3 3 5 hr,6 3 l6 hr' exploded.
was titrated with acid to determine the loss of basic strength. A sample of the solid reaction product was dissolved in water and standard acid and the per cent. sodium or potassium determined by back-titration with standard base. Where possible, an infrared absorption spectrum of the solid material in suspension in mineral oil was taken using a Baird recording spectrophotometer. In a qualitative way it has been shown that the strong absorption band which appears near 8 U , in methyl siloxane systems is some function of the Si-CH3 grouping.6 The presence of this strong band in the absorption spectra of the solid reaction products indicated those cases where considerable carbon had been retained. Hydrolysis of the solid was carried out to determine whether any waterinsoluble liquid was formed. (Such liquid could arise only from the hydrolysis of the silanolate to yield trimethylsilanol and subsequently hexamethyldisiloxane.) Characterization of Solid from Reaction 9, Table 11.An infrared absorption spectrum of the solid suspended in mineral oil was taken and compared with the spectrum of pure sodium trimethylsilanolate prepared by reaction (5) Wright and Hunter, THIS JOURNAL, 69, 803 (1947); Young, Servais, Currie and Hunter, ibid., 70, 3758 (1948); Richards and Thompson, J . Chern. SOC.,124 (1949).
MARTINJ. SCHICK, PAULDOTYAND BRUNOH. ZIMM
530
Vol. 72
TABLE I1 Solvent,
hll. Ma"
Solvent
10 10 20 20 16
Alcohol Alc.-H20 Ether Ether Benzene
1 2 3 4 5c 6 7 8" 9
ml.
50 50-55 30 35 50
. 50 hf2 50 M2 65 . hI2 &I? .. 50 * Mz = hexamethyldisiloxane. KOH hydrolyzed t o yield trimethylsilanol.
Base
KOH KOH KOHb
KOH KOH
KOH KOH* NaOH XaOH not fused.
G. of base
Ream. temp, O C
2.13 2.41 10
IU4,320,24A
Summary The action of sodium and potassium hydroxides (6) Burkbard, Rochow, Booth and Hartt, Chcm. Reus., 41, 127 11947).
THE
Resid. At temp. P(p.s.i.) OC.
% Na or IC
680 5 22.2 93 655 6 24.2 1U6 11 "00 160 15 33.4 4 19(J 73.5 110 12 36.5 > 190 200 24 30.8 118 5 190 61.5 290 24 35.7 440 24 .. d 5.5 185 13
