[CONTRIBUTION FROM
THE
DHIPARTMENT OF CHEMISTRY OF
THE
UNIVERSITY OF
BUFFALO]
STUDIES IN SILICO-ORGANIC COMPOUNDS. (11). THE REACTIONS OF SILICOORTHOESTERS WITH CERTAIN ACID ANHYDRIDES HOWARD W. POST
AND
CHARLES H. HOFRICHTER, JR.
Received April 4, I940
The purpose of this investigation was to study the acylation of silicoorthoesters, including the alkyl orthosilicates. As far back as 1866, Friedel and Crafts reported that ethyl orthosilicate reacted with acetic anhydride (l), to give triethoxysilicomethyl acetate and the simple ester, thus: I. Si(OCzH6)4 (CH&0)20 G CH&OOS~(OCZH~)~ CH~COOCZHS The same authors attempted the acetylation of ethyl orthosilicate with acetyl chloride. The products obtained from this reaction were ethyl acetate and triethoxysilicomethyl chloride. Later, Friedel and Ladenburg attempted a similar reaction with acetyl chloride and ethyl ethane orthosiliconate (2). The product was not obtained as such, but was hydrolyzed to an impure ethane siliconic acid. Dearing and Reid also attempted the acylation of ethyl orthosilicate by the action of phthalic anhydride (3). The products of the reaction were written as diethyl phthalate and diethyl silicate. The latter product was not reported as identified. Evidence for the dissociation of the carbon orthoesters into ions in the presence of a polar medium such as an acid anhydride has been presented by Post and Erickson (4). Many other reactions of acid anhydrides can be readily explained on the assumption of a similar reversible, spontaneous decomposition. By combining these two equilibria it is possible to predict for the silicborthoesters, as well as for the carbon compounds, a possible interaction between the silicoorthoester and an acid anhydride according t o the following equations:
+
+
11. 111. IV. V.
+
CaH6Si(OR)s % (CzHsSi(OR)z)+ (OR)(CH&0)20 % (CH3COO)(CH&O)+
+
+
(CzH&3i(OR)z)+ (CH&OO)- % C H ~ C O O S ~ ( C Z H ~ ) ( O C ~ H ~ ) ~ (CH&O)+ (OR)CHiCOOR
+
1 Abstracted from the thesis presented by the second author to the faculty of the University of Buffalo in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
443
444
H.
W. POST A N D
C. H. HOFRECHTER,
JR.
Thk reaction would be expected to take place at room temperature, but was carried out at the refluxing temperature of the particular acetate formed according to the R group in the silicoorthoester. Ethyl, propyl, and butyl ethane orthosiliconates were found to react with acetic anhydride. In all cases the reaction was found to go to completion. Three moles of the respective acetates were obtained when three moles of acetic anhydride were used with one mole of the orthoester. The lowest yield of the alkyl acetate from this reaction was 96.7%. The triacetylated products were unstable and decomposed when fractionation at reduced pressures was attempted. A 23% yield of ethyl diethoxysilicomethyl acetate was obtained when equimolal quantities of acetic anhydride and ethyl ethane orthosiliconate were used. When the reactants were mixed at room temperature, there was no conclusive evidence that the reaction began to take place immediately. However, as soon as the temperature was raised to the boiling point of the alkyl acetate, the simple ester began to distill off immediately. Attempts were made to follow these reactions quantitatively by following the amount of the simple ester which had been removed at specified times after the reaction had been initiated. Large quantities of reactants gave the qualitative general trend, but the data were too erratic to depend upon. Smaller quantities of reactants made it possible to calculate the equilibrium constant and specific reaction velocity constant for the reaction between propyl ethane orthosiliconate and acetic anhydride. The data were obtained by allowing the reaction-mixture to reflux for thirty-eight hours until, as was assumed on the basis of other experimental data appearing later, the mixture had reached equilibrium. In the course of the following ten minutes, all of the propyl acetate that had been formed was removed by distillation. The volume was accurately measured. To ensure that none of the propyl acetate remained, the time was recorded; this time was taken as to and the reaction rate followed at regular intervals thereafter. The data are given for one-half hour later and at one and one-half hour intervals thereafter. The results obtained appear in tabular form. A B C D VI. a t start a t equi-
librium
C*Hd5(OCsH,)s 0.254 moles 0.122 moles
+ 3(CHaCO)zO S 3CHsCOOCsHr + C2H&3i(OOCCH3)r 1.060 moles
0
0
0.663 moles
0.397 moles
0.132 moles
These data correspond to the temperature at which the mixture refluxes, which can be specified as 110" f 5". The final temperature was very
445
RUCTIONS OF SILICOORTHOESTERS
close to the temperature of boiling propyl acetate. The data for the specific reaction velocity constant were taken at a somewhat higher temperature, necessarily in the vicinity of 140'. It was obvious, that in order to remove all the propyl acetate and maintain a proper reflux ratio, the temperature had to be maintained slightly above the boiling point of acetic anhydride (139.6"), the next lower boiling constituent. The specific reaction velocity constant was calculated according to the following equation for a second order reaction:
b(a
- x)
where a equals the initial concentration in moles of acetic anhydride, b equals the initial concentration in moles of propyl ethane orthosiliconate, and z equals the accumulated number of moles of propyl acetate removed at any time t. f
l
0 1800 7200 12600
18000
-
z
0 0.037 0.109 0.172 0.223
a--z
0.663 0.626 0.554 0.491 0.440
I
b--o/3
0.122 0.110 0.086 0.065 0.048
kX
10-6
4.92 4.37 4.85 5.37
When calculated for a first order reaction, the data give constants which deviate from each other by 40%. Therefore, in view of the above data, it can be said that the reaction is more nearly of the second order than the first. Experimental evidence shows that the same type of reaction takes place between the acid anhydrides and ethyl orthosilicate. When equimolar proportions of the reactants were taken, not only were the monoacylation products formed but also the diacylation products. It is assumed that the mechanism postulated in reactions 11-V for the ethane orthosiliconates holds also for the acylation reactions of ethyl orthosilicate. The characteristics of the two reactions are quite similar. In order to obtain a diacetylation product, a second ionization must take place, thus: VII. VIII.
CH&OOSi(OC2H&, (CH3COOSi(OC2Hs)z)+
(CH&OOSi(OC*H&)+
+ (OC2H5)-
+ (CH3COO)- = (CH&OO) ZSi (OCzHs) 2
Furthermore, when heated to their boiling points, these compounds decompose to give ethyl acetate and high-boiling products. However,
446
H.
W. POST AND C. H. HOFRICHTER, JR.
intramolecular decomposition does not take place. Diethyl silicate would be the product in this case. In the number of similar experiments attempted, this compound was never found to be present among the products. Intermolecular decomposition predicts the formation of high molecular weight compounds in accordance with experimental facts. In order that this type of decomposition should take place, a second type of ionisation is necessary, thus:
IX.
X.
CH3COOSi(OCzH& e (CHaCO)+
+ (OSi(OC&I&)-
+ (OSi(0C~l35)~)-+ Products of high
(CHaCOOSi(OC&Is)2)+
molecular weight EXPERIMENTAL PART
Ethyl orthosilicate, Si ( O C Z H ~ )was ~ , purchased from the Carbide and Carbon Chemicals Corp. Constants found: b.p. 165.5" (760 mm.); n% 1.3821. Ethyl ethane orthosiliconate, CZHsSi (OCzH6)8, was prepared according t o the known method of the action of ethylmagnesium bromide on ethyl orthosilicate (5,6). The column used for fractionation has been described (6). Constants found: b.p. 158-160" (760 mm.); n% 1.3853. Propyl ethane orthosiliconate, CZHSS~(OC~H,)~, was prepared by the method outlined by Post and Hofrichter (6). Constants found: b.p. 202-204" (760 mm.); n% 1.4017. Butyl ethane orthosiliconate, C&Si (OC4HQ),,waa prepared in the same manner 1.4128. as the propyl. Constants found: b.p. 235-238' (760 mm.); n*4~ Acetic anhydride, (CH&O)zO, was purchased from the Eastman Kodak Co., redistilled; b.p. 138.5-139.5" (760 mm.). Propionic anhydride, ( c ~ H & o ) ~ was o , purchased from the Eastman Kodak Co., redistilled; b.p. 168' (760 mm.). Benzoic anhydride, (CaH&O)20, was a commercial product; m.p. 42". Ethyl diethosysilicomethyl acetate, CHsCOOSi(C2Hs) (OCZH6)z. Ninety-eight milliliters (1 mole) of acetic anhydride was allowed to reflux for two hours with 208 ml. (1 mole) of ethyl ethane orthosiliconate. The ethyl acetate was then fractionated from the mixture. The temperature of the column was held a t the boiling point of ethyl acetate until the reaction was complete. This procedure removed all ethyl acetate as i t was formed, and the reaction was complete in a few minutes over five hours. Fractionation of the remaining mixture a t 15 mm. gave a 23% yield of the above product, as well as other high-boiling products. The physical properties of the compound were then determined, b.p. 94" (15 mm.); a micro determination at atmospheric pressure gave 191.5". Decomposition took place, and each suc~ M. w. (cryoscopic in benzene) 209; ceeding reading was lower, dto 1.020; n P D1.4048; theoretical 206. Anal. Calc'd for CsHlaOISi: C, 46.56; H, 8.80; Si, 13.63. Found: C, 46.25, 46.04; H, 8.64, 8.59; Si, 14.01, 14.03. The complete acetylation of ethyl, propyl, and butyl ethane orthosiliconates was attempted. The yields of acetate in the respective reactions were 99.3% 96.7'%, and 97.3%. The acetylated ethane orthosiliconate decomposed during fractionation in each case. TriethosysilicomethyZ acetate, CH&OOSi (OCaH6)8. One mole of ethyl ortho-
REACTIONS OF SILICOORTHOESTERS
447
silicate was mixed with one mole of acetic anhydride. A 99.3% yield of ethyl acetate was fractionated away from the mixture in the course of the following four hours. Fractionation of the mixture produced a 45% yield of the above product. Constants found: b.p. 81" (19mm.); di0 1.020; n% 1.3910; literature (l), b.p. 135-140" (52mm.). Anal. Calc'd: Si, 12.63. Found: Si, 12.76, 12.58. Diethozysilicomethyl diacetate, (CHaCO0)zSi (OCzH6)z. In addition to the above product, a 40% yield of this compound was obtained from the above reactionmixture. Constants found: b.p. 100" (19 mm.); d:' 1.076; n% 1.3960. Anal. Calc'd: Si, 11.88. Found: Si, 11.98, 11.95. Triethozysilicomethyz propionate, CZHsCOOSi(OCtH&, was prepared like triethoxysilicomethyl acetate. A yield of 98.5% of ethyl propionate was obtained. The remaining mixture was fractionated at 15 mm., and a 37% yield of the desired product was obtained. Constants found: b.p. 101" (15 mm.); d:' 0,999; n% 1.3946. Anal. Calc'd: Si, 11.90. Found: Si, 11.94, 11.82. Diethozysilicomethyl dipropionate, (C&COO)zSi ( O C Z H ~ ) ~I.n addition to the above product, a 36% yield of this compound was obtained from the above reaction-mixture. Constants found: b.p. 125' (15 mm.); d:' 1.025; nZOD 1.3998. Anal. Calc'd: Si, 10.62. Found: Si, 10.71, 10.68. A reaction similar to those above was attempted with benzoic anhydride. Ethyl benzoate was obtained and identified by its boiling point, 211" (160 mm.). A mixture of silicate products was also obtained but not identified. In compounds where carbon was attached to silicon, the silicon analysis was carried out by using a sodium peroxide fusion. With products such as the lastfour, which were easily hydrolyzable, concentrated (40%) perchloric acid served to oxidize the compounds to silica. The analysis from either point was then carried out according to approved methods. Ethyl diethoxysilicomethyl acetate, however, was spontaneously inflammable with sodium peroxide and was not oxidized by perchloric acid. A dilute mixture of chromic and perchloric acids, concentrated by heating until the temperature of the mixture reaches 200", will oxidize the siliconcarbon linkage. CONCLUSIONS
The experimental work herein presented indicates that the reaction between silicoorthoesters and acid anhydrides follows a mechanism which can be explained on the assumption of an ionic splitting. It is indicated, furthermore, that the monoacylated compound, once formed, may dissociate in two different ways. This fact is shown: (a) by the decomposition of the monoacetate and propionate without the production of diethyl silicate, but with the formation of high molecular weight products; (b) by the interaction with the second molecule of the acid anhydride to form the diacetate or dipropionate. These two types of dissociation are shown in equations VI1 and IX. The determination of the specific reaction velocity constant at the refluxing temperature of the mixture of propyl ethane orthosiliconate and acetic anhydride has served to show that the acetylation reaction is most probably of the second order. This fact is in agreement with an ionic mechanism such as is postulated. It is reasonable to assume that propionylation also follows the same mechanism.
448
H. W. POST AND C. H. HOFRICHTER, JR.
I n the fractionation of the product obtained from the reaction between ethyl orthosilicate and benzoic anhydride, the reaction was forced to the left, since the ethyl orthosilicate is the lowest-boiling fraction. This is undoubtedly one reason why a pure compound could not be isolated. SUMMARY
1. The acylation of alkyl orthosilicates and silicoorthoesters has been extended, and the methods for the preparation of triethoxysilicomethyl acetate and propionate, diethoxysilicomethyl diacetate and dipropionate, and ethyl diethoxysilicomethyl acetate have been described. The data covering their simple physical properties are given. 2. The equilibrium constant and the specific reaction velocity constant under certain conditions for the reaction between propyl ethane orthosiliconate and acetic anhydride have been measured and presented. BUFFALO, N. Y . REFERENCES (1) (2) (3) (4) (5) (6)
FRIEDEL AND CRAFTS,Ann. chim. phys., [4] 9, 10 (1866). FRIEDEL AND LADENBURG, Ber., 3, 15 (1870). DEARING AND REID,J . Am. Chem. Soc., 60,3058 (1928). POSTAND ERICKSON, J . Org. Chem., 2 , 260 (1937). ANDRIANOV AND GRIBANOVA, J . Gen. Chem., U.S.S.R., 8 , 552 (1938). POSTAND HOFRICHTER, J . Org. Chem., 4, 363 (1939).
