PACIFIC SOUTHWEST ASSOCIATION O F CHEMISTRY TEACHERS COMPARATIVE ORGANIC CHEMISTRY: CARBON AND SILICON
0
I. J. WILK' Stanford University, Stanford, California
MANYarticles have been written regarding the huge expansion of the silicone industry. "Silicones stand at the threshold of another tremendous growth period" is the opinion of a recent journal report (I). Since many publications emphasize the commercial uses of silicones, a brief consideration of the chemistry of organic silicon compounds ( ) in cont.rast to regular organic compounds, should be of value. The periodic chart predicts certain similarities in the chemical properties of carbon and silicon, for both are in Group IV, silicon being directly under carbon.
9.5 p, is about five times that of the C 4 band, a t 9.0 p. Since infrared transitions are dipole transitions, their intensities are proportional to an increase in charge separation. This, in turn, is proportional to the per cent ionic character of the bond. A comparison of ionic character of bonds involving silicon and carbon atoms is given below (8). The table gives average values. Comparison of Carbon and Silicon Bonds %Ionic character Bond %Ionic character
Bond CHEMICAL BONDING
Bond Hybnclizatim. This similarity in behavior of two elements may be explained by critically examining the electronic configuration of the elements:
6+ 6-
Si-C
at
C-C
12 6-
(CHrCHZ)
0
6+ 6-
Si-Br
C B r
22
2.5
6+ 6-
6+ 6-
Si-CI
C-C1
30
6
Carbon: Isa 2s2 2pP Silicon: IsZ2s2 2pa 39%3p2 ( 3 d )
The same number of bonding electrons are available for the elements, in the corresponding suhshells, so that single bond formations of these two elements should be very similar in nature. Carbon shows a covalency of four, and spa hybridization in forming these bonds. Silicon exhibits these same properties. However, due to the presence of an available d orbital in the M quantum level, silicon may go beyond tetracovalency, and achieve a covalency of six, with a hybridization of spad2 (3), as shown by SiF6-- (4). Whereas carbon may form stable double bonds with other atoms, silicon is incapable of undergoing this type of bond formation (5). I m i c Character of Bonds. Although it has been shown that both carbon and silicon form comparable covalent bonds, a closer look at these bonds will evince the fact that the ionic character is much more pronounced in the bonds of silicon. This mav be demonstrated in various ways. The interatomic distance of the Si-0 linkage is 1:66 A. Yet the sum of the covalent radii of silicon and oxygen is 1.87 A. (6). This "shrinkage" in Si-0 distance is evidence of a large prouortion of ionic character in the bond. The same conclusion is suggested by the results of infrared spectroscopy (7). The intensity of the Si-0 band, at
As is shown in the table, silicon, even when linked to carhon, will carry a partial positive charge. This may be explained by recalling that the valence electrons of silicon are farther removed from the nuclear charge than are those of carbon. Hence, silicon may lose an electron more easily than carbon, or even hydrogen. Pauling's electronegativity values for these elements are given as follows: Si: 1.8, H: 2.1, C: 2.5. i+ 8 -
Thus a polarization of type ~ i - H might be expected. REACTION MECHANISMS
The postulate of a siliconium ion being more stable than a carbonium ion, based on the evidence cited above, may be invoked to propose a mechanism for a Wagner-Meerwein type rearrangement, found in silicon chemistry (IOa): xr-
1-CHGI
xr."LC
AlCl
I
Me-Si-CH.
+
Me Public Health Service Research Fellow of the National Cancer Institute.
VOLUME
34,
NO. 9, SEPTEMBER. 1957
=
Me I
Me
Investigating the fission o f . a carbon-silicon bond, 463
Eaborn (lob) explained the course of the acid-catalyzed cleavage of p-methoxyphe~~yltrimethylsilane by sug~- * * u
0
gesting that the polarity C-Si intermediate
aided in forming the
which contributes to the merhanism as follows:
pH of the solution had no effect on the hydrolysis rate of Ph3CF, the presence of 1 11f hydroxide ions increased the hydrolysis rate of PhaSiF by a factor of 10Qver that in ac,etone-water alone. Furthermore, para alkyl substitut,ion, in PhaSiF, caused a five-fold decrease in the rate, but the same substitution increased the hydrolysis rate in PhBCF. Since alkyl groups are electron-releasing, the retarding effect observed with Ph,SiF would indicate that the charge on the silicon atom, in the transition state, is less positive than in the ground state. After evaluating all the experimental evidence, a pentaeovalent silicon intermediate was postulated in the hydrolysis of PhaSiF. This is how Swain visualized the mechanism: Ph
+ HOH
2 h%er~ibHS+ PhOMe
fast (3: ~ e ~ ~ i i ) ~ M% ~ ~+S ~ O H H+
+
The "Silicon" Effect. When silicon is linked to an electronegative group by means of a carbon chain, the silicon atoms exert a peculiar influence on the electronegative group. In the compounds C18iCHGl (a), CIISiCH2CH2C1(b), and CI3SiCH2CH2CH2CI(c), the reactivities of the chlorines vary considerably, depending on how many carbon atoms are inserted between the silicon atom and the halogen atom (11). The chlorine in C13SiCHzCH2CIis so reactive that it may be titrated with 0.5 N aqueous alkali. Compounds a, c, and hexyl chloride require treatment with alcoholic base for removal of the halogen. As an indication of the relative reaction rates, the following observations are listed with hexyl chloride as the reference compound:
H>o H
I
+ a+Si-Fa/\
Ph
+ HOH
Ph Ph
Ph
Ph
Free Radicals. In going from the tetraphenyl methanes and silanes to the hexaphenyl ethanes and disilanes the possibility of formation of free radicals must he considered. The preFence of P h G free radicals has been established, the stability of the radical having been explained as being due t o ample resonance stabilization. However, in silicon chemistry, PhSiSi-Phi is a stable compound (16). In order t o provide additional resonance stabilization,
SiCH2CH1Cl> BCHL!H,CH,Cl > SiCH&l > C6H,,CI
The extreme reactivity of substituents attached to a carbon 8 to the silicon has brought forth several explanations. Sommer (12) suggested that a nucleophile, such as the hydroxide ion, mould attack the most electropositive element present, in this case silicon, and not hydrogen. The compound would then undergo a typical E, elimination, as shown below:
Fajans (IS), however, -proposed polarization of the 6+
6-
6+
6-
silicon-carbon chain as follows: B-CH,CH,CI. If this were true, the 8-carbon, carrying a partial positive charge, would be more susceptible to nueleophilie attack than either the a-or ycarhons, thereby accounting for the high reactivity of the 8-suhstitnent. It has been found that the rate of reaction of Me3SiCH2CI with I- is twenty times that of butyl chloride with I- (14). Since the Me& group, similar to the vinyl group, may act as an electron donor or acceptor, depending on the occasion, this property may explain the activating silicon effect. Yet a neighboring group type interahon is not excluded. Mechanism of Halide Hydrolysis. Halogen attached to silicon is removed by hydrolysis by a different mechanism than halogen attached to carbon, as has been shown by Swain (15). He studied the hydrolysis of PhaSiF and Ph&F in 50% acetone-water. While the
TTas synthesized. But it, too, failed to dissociate into stable free radicals. This mas explained by Gilman (16) as due to the smallness of resonance stabilization contributed by structures containing carbon silicon double bonds ( 5 ) . Short-lived silicon free radicals exist, and undergo an abstraction reaction which is peculiar to them (17a, b). A triphenylsilyl free radical, PhBi., in chlorobenzene, will abstract the chlorine from the aromat,ic compound, and generate a phenyl free radical : PhaSi.
+ Ph-CI
-
PhsSi-CI
+ I'h.
No such reaction is found in carbon chemistry. Hammond (Ira) proposed that since t,he Si-Cl bond is more stable than the C-C1 bond (90 keal. vs. 70 kcal.), this type of abstraction may be expected to take place. STEREOCHEMICAL PROPERTIES
A penta- or hexacovalent silicon intermediate was also postulated as occurring during the racemization in MeOH-MeONa or polar solvents such as piperidine of an optically active silicon compound (18) of structure C H ~ ~I ~ - C H ~ C H ~ C H ~ N ( C H ~ C H & OMc
Other stereochemical -properties of organic silicon JOURNAL O F CHEMICAL EDUCATION
compounds depend on the size of the atoms. The covalent radius of silicon is 1.17 A,, compared t o 0.77 A. for carbon (19). This would imply less crowding of groups around the silicon atom, thus making easier bimolecular nucleophilic substitution (SN2), i.e., an attack by the OH- group on the carbon holding the halogen, as seems t o be demonstrated in the following reactions ($0). CH, ..-"
I
CHx-C-CH*X
1
f KOH
I
I
2 hrs. reflux
no reaction
EtOH
CH.
CH,-Si-CH,X
-
+ KOH
2 hr?. reflux
99% reacted
EtOH
CHa
One might predict from the above that, while PhiC is somewhat difficultto synthesize, the silicon analogue, Ph& might be prepared with relative ease, since there exists less steric hindrance around the silicon atom. Even
(-)a
/Me
SILICON POLYMERS
A brief mention of the chemistry of silicon polymers is in order. In carbon chemistry halides, such as RCH,CI, upon hydrolysis go to alcohols, and those of type RCHCll go to aldehydes. If the corresponding silicon compounds are hydrolyzed, the resuking hydroxide compounds formed are generally unstable and condense intermolecularly to form water and Si-0-Si linkages. R S C I will go to R8SiOH which will coadense to R 8 - O S i R a . Rut RzSiClz will not form R & = O on hydrolysis. (No substance containing a silicon-oxygen double bond has as yet been isolated (5)). Instead, it will form an intermediate of structure R2Si(OH)2,which mill intercondense to form long chains of structure R
1
R
I
R
R
R
I
I
R
Like silica, the silicone polymers contain silicon-oxygen linkages. Thus, silicone polymers combine properties of organic compounds as well as those of inorganic compounds, their high temperature stability being especially noteworthy. The RzSi-0 groups, due t o the nondirectional nature of the silicon-oxygen bond, rotate with relative freedom, analogous to the action of hall and socket joint ($8). Thus a close approach of silicone chains is made impossible, whereas carbon chain polymers possess large molecular attractions. This is probably primarily responsible for the small temperature dependence of physical properties of silicone polymers. ACKNOWLEDGMENT
its carbon analogue not reported, can be synthesized. It is of interest to note that four isomers of tetra-otolylsilane have been isolated ($1). By way of explanation the authors stated that lack of free rotation about the silicon-carbon bonds established definite spatial arrangements of ortho-tolyl groups. By means of atomic models, eight different configurations were constructed, four meso and four racemic forms. An analogy in carbon chemistry may be found in the stereochemistry of the ortho-ortho' substituted biphenyls.
-?-0-Si-0-Si-O-Si
ing monomers such as RSiCls, which first form RSi(OH)* on hydrolysis, and then condense
I
R
I I
R
etc.
'
Such structures are the basis of the common silicone hydraulic fluids and elastomers, especially where R = CHa. Cross-linked structures may be synthesized by utiliz-
VOLUME 34, NO. 9, SEPTEMBER, 1957
The author desires to express his gratitude t o Professor G. Hammond, of Iowa State College, for a discussion of silicon free radicals and permission t o refer to some of his unpublished findings and to Dr. L. Goodman and Dr. R. I. Mixer, of Stanford Research Institute, for reading the manuscript. LITERATURE CITED (1) Chem. andEng. News, 35,16 (1957). (2) ROCHOW, E. G., "An Introduction to the Chemistry of the Silicones," 2nd ed., John Wiley & Sons, Ino., New York, 1951. Excellent 1,eference. (3) PAULTNG, L., "The Nature of the Chemical Band." 2nd ed., Cornell University Press, Ithaes, New York, 1948, p. 93. (4) SIDGWICK. N. V.. ('The Electronic Theom ' of Valencv." .. Clarendon press. London. 1927. o. 68. (5) PITZER, K. S., J . Am. hem. See.; $0, 2140 (1948). (6) Reference (S), p. 299. (7) WRIGHT, N., A N D If.J. HUNTER, J. Am. Chem. Soe., 69, 803 (1947). (8) Reference (S), p. 70. (9) Ibid., p. 60. (10a) SOWER, L. H., et al., J . Am. Chem. Soe., 76, 801 (1954). (lob) EABORN, C.,J . Chem. Soe., 1953,3148. (11) SOMMER. L. H.. el a/.. J . .4m. Chem. Soc.. 68. 488 (1946). (12j SOMMER: L. H.', el o i , J . Am. Chem. s&.,70, 2869 (19483, 76,1613 (1954). (13) FAJANS, K., Chem. and Enq. News, 27,900 (1949). (14) COOPER, G. K., AND M. PROBER, J. Am. Chem. SOC.,76, 3943 (1954). (15) SWAIN, C. G., et al., J . Am. Chem. Soe. 71, 965 (1949). (16) GILMAN. H.. AND G. E. D u m . J. Am. Chem. Soe.. 73, 5077 (195i). (17a) HA-OND, G., private communicstion, February, 1957. ( l i b ) CURTICE, J. S., I m a Stale College J . Sci., 29,399 (1955). (18) KIRSCHNER, S., ESS. Abslr. 14, 1530 (1954). (19) Reference (S), p. 164. (20) W H I ~ O R F. E , C., A N D L. H. SOMMER, J . Am. Chem. SOC., 68,481 (1946). (21) SMART, G. N. R., et al., J . Am. Cbem. Sm., 77, 5193 (1955). (22) ROTE,W. I,., J . Am. Chem. Soe., 69,474 (1947).
