[CONTRIBUTION FROM
THE
DBPARTMENT OF CHEMISTRY OF
THE
UNIVERSITY
O F BUFFALO]
STUDIES IN SILICO-ORGANIC COMPOUNDS. 111. THE PREPARATIONS AND REACTIONS OF SILICON ANALOGS O F CERTAIN ALIPHATIC ORTHOESTERS' HOWARD W. POST
AND
CHARLES H. HOFRICHTER, JR.
Received April 26, 1940
The primary purpose of this investigation was to conduct a more detailed study of the action of the Grignard reagent on ethyl orthosilicate. It was of further interest, however, to study the interchange of alkoxy1 groups between two alkyl orthosilicates and the mechanism of the reaction previously presented (1) between silicoorthoesters and the homologous aliphatic alcohols. In 1908, Khotinsky and Seregenkoff carried out a reaction between phenylmagnesium bromide and ethyl orthosilicate, obtaining ethyl benzeneorthosiliconate (2). These authors also obtained the corresponding products from the Grignard reagents of parabromo-m-xylene and CY- and 6-bromonaphthalene. As a result of their investigation, Khotinsky and Seregenkoff stated that not more than one ethoxyl of any one molecule of ethyl orthosilicate could be replaced by the Grignard. Although the same Grignard reagents were not used in this investigation, it was found both with methylmagnesium iodide and butylmagnesium bromide that more than one ethoxyl could be replaced. Dimethyldiethoxysilicane and tetrabutylsilicane were prepared by this method. Replacement-products of ethyl orthosilicate are difficult to obtain from its reaction with methylmagnesium iodide. Several attempts were made before any pure products were isolated. It was interesting to note that three products were obtained from the reaction-mixture ; ethyl methaneorthosiliconate, dimethyldiethoxysilicane, and ethyl methanesiliconate. The yields of all products were poor.
+ CHsMgI = CsH60MgI + CH3Si(OGH& 11. CHaSi(0CzHs)r + CHsMgI = C2H60MgI + (CHs)2Si(OC2Hs)z 111. CHsSi(OGH& = CZHSOC~H~ + CHaSiOOCzHs I.
Si(0CzHs)d
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.
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SILICON ANALOGS OF ORTHOESTERS
673
Ladenburg has twice reported the preparation of ethyl methaneorthosiliconate by the reaction in a closed tube between zinc dimethyl and ethyl orthosilicate in the presence of metallic sodium (3, 4). Tseng and Chao reported a 44.5% yield of tetrabutylsilicane from the reaction between butylmagnesium bromide and silicon tetrachloride (5). The action of the butyl Grignard on ethyl orthosilicate is so vigorous that the former must be added drop by drop if only the mono replacementproduct is desired. If the ethyl silicate is added dropwise to an excess of butyl Grignard, the replacement goes still further and gives a 56% yield of tetrabutylsilicane. It has been repo+&d previously (1) that the higher homologous alcohols exchange alkoxyl groups with ethyl ethaneorthosiliconate. However, experimental evidence shows that when the alkyl group attached to the silicon is larger than ethyl, exchange takes place only slightly and the silicoorthoesters react to form compounds of higher molecular weights. In order to devise and propose a mechanism that would take into consideration the known facts concerning the exchange reaction, it is necessary to consider the work of Post and Erickson on the carbon orthoformates and orthoacetates (6). With the orthoformates, exchange of alkoxyl groups with the homologous alcohols would be expected to take place through a mechanism involving the acidic character of the alcohol. The promotion of the ionic nature, as proposed by the above authors, is largely due to the polar nature of the orthoformates. A structure such as that of the orthoformates would allow coordination with an alcohol. With the alkyl group of the alcohol, methyl and successively larger, it would be expected that if the reaction is to continue to occur a t all, it should gradually change from an ionic to a coordination mechanism. It is assumed that an investigation of the silicoorthoformates and methane orthosiliconates will prove that the same type of change will occur with the corresponding silicon compounds. That a change does occur is obvious from a comparison of the ethaneorthosiliconates and the propaneorthosiliconates. Ninety-five per cent of the products from the reaction between ethyl propaneorthosiliconate and butyl alcohol have a molecular weight higher than 468. Eighty-five per cent of the products have molecular weights between 468 and 530. Formation of compounds of higher molecular weight may occur as a result of a competition between the alcohol oxygen and the ether oxygen of the silicoorthoester. The larger the groups surrounding the central silicon atom the more difficult it is for the alcohol to penetrate for coordination. As the alkyl groups increase in molecular weight, a certain point is reached at which the above competition favors the formation of the heavy compound rather than the simple orthoester. The formation of the silicon-oxygen-silicon linkage also tends toward the forma-
574
H. W. POST AND C. H. HOFRICHTER, JR.
tion of high molecular weight compounds. It is believed, therefore, that the exchange of alkoxy1 groups between the homologous alcohols and the ethane- and propane- orthosiliconates takes place through a coordination mechanism according to the following equation: OCzHs
IV.
CzHcSi(OC2Hs)a
+ ROH
I
$ GHaSiOCaHt++
,I ...........
ocas I I
+
GHsS~OCZHS GHsOH
OR The coordinstion of the alcoholic hydrogen results in the creation of a net positive charge on the silicon atom of the molecule, which can then exert an attractive force on the surrounding oxygen atoms. The moving in of any particular oxygen atom aids the splitting off of the other alcohol and results in an exchange. This mechanism makes it possible for heavier compounds to form when the groups are larger. The following equation illuetrates this competition. @ 4 & 6
v.
2csH?si(oc~s)s-t CIHSOH
+
/ CsH+%-o-c~~j \ ................ I(
i-
GH
I
i-dic8B OCaHs
1
OCzHs OCgHs
i
CiHQOH
.:
1
,..-''
CdHs-O-H
+ (C2H6)*Oe
O-CzH6
,,
o-c2Ho;
i / ......................
c~H?si--O-c~Ht+
\
O-CB"
Alkoxy1 interchange occurring between homologs has been reported by Friedel and Crafts (7). These authors stated that from a stoichiometric mixture of ethyl and methyl orthosilicates, dimethyl diethyl orthosilicate was isolated. It was further stated that none of the 1 :3 compounds was formed. It was believed that in order to form the 2: 2 compound the reaction must go through a step where the 1:3 compound is present. The reaction between ethyl and butyl orthosilicates was studied. One-half mole quantities were refluxed for 120 hours. Fractionation a t reduced pressures
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SILICON ANALOGS OF ORTHOESTERS
produced a 22% yield of triethyl butyl orthosilicate, 30.4Oj, yield of diethyl dibutyl orthosilicate as well as a crude yield of ethyl tributyl orthosilicate of ca. 25% which was not pure enough for analysis. Traces of the original orthosilicates were identified by their refractive indices. From this experimental evidence it can be seen that a redistribution reaction such as proposed by Calingaert and Beatty (8) may have taken place. It is worthy of mention that analogous silicon and carbon orthoesters and similar compounds have very nearly the same boiling points. The following table is the result of a search of the literature for exact analogs. SILICON COMPOURD
B.P.,
%.
104-106 (9) 73 (12) 125-126 (4) 110-111 (12) 145-151 (3) 150-151 (12) 158-160 (1) 165.5 (7) 191-192 (20) 235-238 (1) 225-227 (22) 240-242 (20) 302 (20) 245-250 (9)
B.P., %. (CABBON ANALOO)
103-105 (10) 102 (11) 77.1 (13) 126-128 (14) 112 (15) 144-145 (16) 159-160 (18) 159 (19) 196-198 (10) 192-196 (11) 190-191 (6) 239.5-240.5 (14) 224.2 (19) 224-226 (10) 267-269 (10) 225-227 (23)
EXPERIMENTAL PART
Ethyl orthosilicate, Si (OCIHS)~, was purchased from the Carbide and Carbon Chemicals Corp. Constants found: b.p. 165.5" (760mm.); +OD 1.3821. Ethyl propaneorthosiliconate, CIH7Si( O C ~ H S )was ~ , prepared by the method outlined by Post and Hofrichter (1). Constants found: b.p. 179-180" (760 mm.); ~ P1.4076. D Silicon tetrachloride, SiCl4, was a gift of the Niagara Smelting Corp. Constants found: b.p. 57.6" (760mm.). Other chemicals were purchased from the Eastman Kodak Co. and the constants found agreed with those appearing in the literature. Ethyl methaneorthosiliconate,dH8Si(OC1Hs)s. Two and one-half moles of methyl iodide were allowed to react in the presence of anhydrous diethyl ether with 68.5 g. of a copper-magnesium alloy containing 24.3 g. of magnesium to 3.1 g. of copper. This mixture was added dropwise to 1.5 moles of ethyl orthosilicate. After the spontaneous reaction had subsided, the mixture was digested on a hot-plate a t 150" for three hours. The mixture was cooled and the ether layer separated. Several fractionations of this layer produced three homogeneous products, the highest-boiling
576
H.
W. POST AND C . . E HOFRICHTER, JR.
of which was identified as the silicon analog of ethyl orthoacetate. Constanta found: b.p. 150-151' (760 mm.); c 0 . 9 3 8 ; n% 1.3869; Ladenburg (3), b.p. 145-151', do 0.9283. Anal. Calc'd: M. w., 178. Found (cryoscopic in benzene) :M. w., 174. Si, 15.74. Found: Si, 15.58, 15.76. Dimethyldiethoxysilicane, (CHa)2Si(OC2H&, was also isolated from the above ~ reaction-mixture. Constants found: b.p. 110-111" (760 mm.); d: 0.890; n 4 01.3839. Anal. Calc'd: M. w., 148.2. Found (cryoscopic in benzene): M. w., 146.7. Si, 18.88. Found: Si, 18.55, 18.58. EthyE methanesiliconate, CH8SiOOC2Hs,was also isolated from the above reactionmixture, but waa not conclusively identified, since the nature of the compound made 1.3696. analysis difficult. Constants found: b.p. 73' (760 mm.); d: 0.891; PI) Anal. Calc'd: M. w., 104. Found (cryoscopic in benzene): M. w., 101. n-Butyl orthosilicate, Si ( O C ~ H Q was ) ~ , prepared by adding 6.56 m'oles of n-butyl alcohol dropwise t o 1.5 moles of silicon tetrachloride. An 84.07% yield of n-butyl orthosilicate was obtained. Kalinin (24) has reported a similar preparation both with and without benzene as solvent. The yield with benzene as solvent was 82.5% and without the benzene the yield was 61.2%. The table of yields in the Kalinin paper appears in the reverse order to that stated in the text. The simple physical constants found were: b.p. 142-144" (3 mm.); d: 0.899; n% 1.4128. Anal. Calc'd: M. w., 320. Found (cryoscopic in benzene): M. w., 320. Si, 8.76. Found: Si, 8.88, 9.01. Ethyl butaneorthosiliconate, C4H&i(OCiHs)s. One mole of butylmagnesium bromide was added dropwise to one mole of ethyl orthosilicate. A 27% yield of the above compound was obtained as well as other higher-boiling products. Constants found: b.p. 190-193" (740 mm.); d 7 0 . 8 9 5 ; n " ~ 1.3976. Anal. Calc'd: M. w., 220. Found (cryoscopic in benzene): M. w., 219. Si, 12.74. Found: Si, 12.58, 12.62. Tetrabutylsilicane, Si(CpHp)(, was prepared by adding dropwise 0.825 mole of ethyl orthosilicate t o 4 moles of butylmagnesium bromide. Constants found: b.p. 231' (760 mm.); d: 0.822; 7 ~ 2 1.4332; 2 ~ yield 56%. Anal. Calc'd: M. w., 256.4. Found (cryoscopic in benzene): M. w., 253.3. Triethyl butyl crthosilicate, C ~ H ~ O S i ( 0 C ~ H s )One-half s. mole quantities of ethyl and butyl orthosilicates were allowed t o reflux together for 120 hours. Fractionation at reduced pressures produced a 22% yield of the triethyl butyl compound as well as 1.3935. other products. Constants found: b.p. 82.5" (15 mm.); d: 0.920; Anal. Calc'd: Si, 11.89. Found: Si, 11.92, 11.93. Diethyl dibutyl orthosiEicate, (C2H,O)*Si(OC4Ho)2. A 30.4% yield of this product was also obtained from the above reaction-mixture. Constants found: b.p. 100' (15 mm.); d: 0.909; POD 1.4008. Each of the two compounds from this reaction disproportionated on heating to its boiling point at atmospheric pressure. And. Calc'd: Si, 10.62. Found: Si, 10.58, 10.69. An attempt t o prepare butyl propaneorthosiliconate from the reaction between ethyl propaneorthosiliconate and butyl alcohol was unsuccessful. Ethyl propaneorthosiliconate (0.925 mole) was allowed to reflux for 96 hours with 5.05 moles of butyl alcohol. A distillate was obtained, boiling from 30' t o 79" at atmospheric pressure, probably containing ethyl ether and ethyl alcohol. Further fractionation produced excess butyl alcohol. After the butyl alcohol had been removed, the mixture was cooled and subsequently fractionated at 7 mm. Four fractions were taken: (a) 105-BO", ea. 5%; (b) 180-185", ca. 40%; (c) 185-210", ca. 5%; (d) 210-215', ca. 40%. Obviously little if any of thc monomeric exchange-compound was present.
SILICON ANALOGS OF ORTHOESTERS
577
Examination of the major fractions showed them to be fairly simple polymers although their specific identity was not determined. Constants found: (b) d: 0.911; n20D 1.4268. Anal. Found (cryoscopic in benzene): M. w., 468. Si, 13.63. Fraction (d) showed d: 0.925;n20D 1.4300. Anal. Found (cryoscopic in benzene): M. w., 530. Si, 12.33. CONCLUSIONS
The statement that no more than one ethoxyl of ethyl orthosilicate can be replaced by means of the Grignard reagent is untenable. The nature of the reaction between ethane- and propane- orthosiliconates and the homologous alcohols can be explained on the basis of a coordination mechanism, which seems to be the result of a gradual shift from an ionic mechanism due to the shielding of the permanent dipole by larger alkyl groups attached to silicon. Interchange of alkoxyls between two alkyl orthosilicates necessarily produces all isomers. There is no reason why one isomer should be more stable than another. A random radical distribution would appear to approximate a thermodynamic equilibrium. SUMMARY
1. The action of the Grignard reagent on ethyl orthosilicate has been investigated and methyl methaneorthosiliconate, dimethyldiethoxysilicane, ethyl methanesiliconate, ethyl butaneorthosiliconate, and tetrabutylsilicane have been prepared by this method. The data covering their simple physical properties are given. 2. The work on alkoxyl exchange between silicoorthoesters and the aliphatic alcohols has been extended. Based on theoretical and experimental evidence, a mechanism for this reaction has been suggested. 3. The alkoxyl interchange between homologous alkyl orthosilicates has been investigated and the formation of the 1:3 compounds definitely ascertained. 4. The yields of the preparations of butyl orthosilicate and tetrabutylsilicane have been improved. BUFFALO, N. Y. REFERENCES
(1) POSTAND HOFRICHTER, J . Org. Chem., 4,363 (1939). (2) KHOTINSKY AND SEREGENKOFF, Ber., 41,2946 (1908). (3) LADENBURG, Ber., 6, 1029 (1873). (4) LADENBURG, Ann., 173, 151 (1874). (5) TSENG AND CHAO,sci. Reports, National University of Peking, 1, No. 4, 21 (1936). (6) POST AND ERICKSON, J . Am. Chem. SOC., 66, 3851 (1933).
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H. W. POST AND C. H. HOFRICHTER, JR.
(7) FRIEDEL AND CRAFTS, Ann. chim. phys., (4) 9, 10 (1866). (8) CALINGAERT AND BBATTY, J . Am. Chem. SOC.,61,2748 (1939). Ber., 41, 3390 (1908). (9) MELCER, (10) SAHAND MA, J. Am. Chem. SOC.,66,2964 (1932). (11) PINNER,Ber., 16, 1643 (1883). (12) This paper. (13) HODQEMAN, Handbook of Chemistry and Physics, (1938). AND WHITE,J . Am. Chem. SOC.,67,2480 (1936). (14) BROOKER (15) POST,J . Am. Chem. Soc., 66, 4176 (1933). (16) SAH,J . Am. Chem. SOC.,60,516 (1928). (17)ANDRIANOV AND GRIBANOVA, J . Gen. Chem., U.S. S.R., 8,568 (1938). (18)SIGMUND AND HER~CHDORFER, Monatsh., 68, 280 (1931). (19) ROSE,Ann., 206, 249 (1880). (20)TAURKE, Ber., 58, 1661 (1905). (21) FRIEDEL AND LADENBURG, Ber., 3, 15 (1870). (22) ABATI,Z.physik. Chem., 26, 353 (1898). (23) SAH,MA, AND CHAO,J . Chem. SOC.,1931, 305. (24) KALININ,Compt. rend. Sei. U.R. S.S.,18,433 (1938).
