Redistribution of Organochlorosilanes
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B. A. BLUESTEIN and H. R. McENTEE Silicone Products Department, General Electric Co.,Watertord,Ν. Y.
The chemical groups bonded to silicon can be caused to redistribute to form new substituted silane species. Prior work, describing such reactions of inorgano-, organo-, and mixed silanes, is reviewed. Some of these redistributions can be of interest for the im portant commercial methods of manufacturing or ganochlorosilanes. The specific example of the methylchlorosilanes is discussed in this connection. Laboratory and pilot plant data obtained using a new catalyst, sodium chloroaluminate, for this reac tion are presented. The advantages, preparation, activity, and life of the sodium chloroaluminate catalyst are also discussed.
M o s t organometallic compounds (of polyvalent metals) and many inorganic com pounds undergo redistribution reactions—that is, the groups bonded to the central metal atom are capable of interchanging with substituents on another metal atom. Symbolically, this may be represented by R — M + R ' — M ' R — M ' + R ' — M
(1)
This transfer or redistribution can occur between compounds containing metals of the same or of different species. Calingaert (3) has described a specific type of "redis tribution reaction." Such reactions are characterized by zero heats of reaction and equilibrium constants which vary little with temperature and which are essentially determined by entropy changes. Only a limited number of organic reactions are known to fulfill these conditions. The redistribution of groups attached to silicon is a very general phenomenon. Only a few types of these redistributions have been demonstrated to fall into Calingaert's classification. Most substituted silane redistributions appear to be equilibrium reactions, but usually the equilibrium is displaced and is not determined by entropy considerations alone. Interchanges are known to occur among organic, inorganic, and hydrogen groups bonded to silicon and the extant literature is discussed from this point of view. It is not the purpose of this paper to discuss silane redistributions involving other metal compounds (such as the Grignard or organozinc alkylations of chlorosilanes). The application of the redistribution of organochlorosilanes to their commercial synthesis will be described later. Inorganic Redistributions Although the halides of silicon have been known for many decades, it is only in recent years that a few studies have been undertaken with respect to their redistri233
ADVANCES IN CHEMISTRY SERIES
234
bution chemistry. Forbes and Anderson (29) and Besson (15) have shown that mixed halogenosilanes can be made to undergo disproportionation at elevated temperatures. Thus, in the absence of catalysts : I S i C l -> I S i C l . + I S i C l 3
2
2
+ IaSiCl + I S i + C l S i 4
4
(2)
The products formed in this reaction were in the correct statistical ratios to fall within the scope of the Calingaert redistribution reaction. A similar situation was found to prevail with the chloroisocyanates of silicon (1). However, chlorothiocyanates of silicon behaved differently (2, 80) in that the reaction 600°C
Cl Si(NCS)
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3
>' S i C U + S i ( N C S )
(3)
4
has an equilibrium constant of only about 0.1. This reaction was also found to pro ceed slowly at room temperature. Halogenosilanes also undergo interchanges with alkoxy silanes. Thus triethoxychlorosilane and ethoxytrichlorosilane disproportionate readily at 1 0 0 ° C . (49). How ever, diethoxydichlorosilane is not changed under these conditions. Ethyl silicate and silicon tetrachloride redistribute (23) to form all possible chloroethoxysilanes. Dif ferent results are reported (23) with fluoroethoxysilanes in that triethoxyfluorosilane seems stable whereas the others disproportionate at room temperature. The easy disproportionation of diethoxydifluorosilane was not noted by Peppard, Brown, and Johnson (89), who were able to isolate this compound. Some of the results de scribed above are beclouded by the fact that acids act as catalysts for these redistri butions and that the side reaction = S i - — Ο — C 2 Η 5 + = S i — C l -> = S i — O S i = + C H C 1 2
5
(4)
is reported to occur (49). These types of redistributions have also been demon strated to occur where the alkoxychlorosilanes have organic or hydrogen groups bonded to the silicon (3, 35). It was concluded that such organo groups increase the stability of the corresponding alkoxychlorosilanes. Very little has been done on the interchange of alkoxy groups bonded to silicon. It has been found that different alkyl silicates can react with each other to form all possible combinations of silicates (40, 4%)· These alkoxy redistributions are cata lyzed by both aluminum chloride and aluminum ethoxide. The redistribution of hydrogen and halogen atoms attached to silicon is among the most facile to effect. Trifluorosilane disproportionates even at liquid nitrogen temperature (17) and the other fluorohydrogensilanes are also readily redistributed (24). No added catalyst was found necessary to redistribute diiodosilane (25). However, catalytic techniques are necessary for hydrogen-chlorine interchanges. Alu minum chloride (26, 48, 51), organic nitriles (9), and dialkylcyanamides with (11) or without (10) metal halide promoters have been used successfully. All of these catalysts have also been used where organic groups are bonded to the silicon. The reactions involved in this type of redistribution do not lead to a statistical distribu tion of all the possible products. Alkoxy groups are similar to halogens in their ability to interchange with hydrogen atoms in silanes. Basic types of catalysts such as sodium metal (31) and alkali metal alkoxides (4, 7, 34) have been generally used, although metal halides are also applicable (26, 27). All the work done has been on the disproportionation type of reaction and no attempts have been made to establish the existence of an equilibrium state. Organic Redistributions The reactions discussed heretofore have not been concerned with the transfer of organic groups from one silicon compound to another where such transfers involve carbon-silicon bonds. That such rearrangements can occur was demonstrated in 1874
BLUESTEIN AND McENTEE—REDISTRIBUTION OF ORGANOCHLOROSILANES b y L a d e n b u r g (37). in a diphenylsilicon C H SiCl 6
A
6
H e showed
235
t h a t the a l k y l a t i o n of phenyltrichlorosilane resulted
compound
+ Z n ( C H ) -> C H S i ( C H 5 ) 3 + ( C H ) S i ( C H ) + S i ( C H )
3
2
5
2
6
similar type of phenomenon
5
6
2
was also o b s e r v e d
5
2
2
5
2
2
5
(5)
4
(22) w h e n i n t h e course o f some
hydrogénation studies a t 3 0 0 ° C . rearrangements of silicon-bonded organic groups
were
found to occur. 2C H Si(CH ) - * (C H ) Si(CH ) + Si(CH ) 2
5
3
3
2
6
2
3
2
3
(6)
4
These i n c i d e n t a l observations were the forerunners of the w o r k done b y C a l i n g a e r t a n d coworkers
(19, 20, 83), c o n c e r n i n g
t h e r e d i s t r i b u t i o n reaction of organosilanes.
T h e y s t u d i e d t h e spécifie r e a c t i o n o f t e t r a e t h y l s i l a n e a n d t e t r a p r o p y l s i l a n e
catalyzed
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b y a l u m i n u m chloride a t 1 8 0 ° C , a n d showed the f o r m a t i o n of the statistically c a l c u l a t e d a m o u n t s of a l l t h e possible e t h y l p r o p y l s i l a n e s .
Others, w o r k i n g w i t h this reac
t i o n (32), h a v e o b t a i n e d n o e v i d e n c e f o r t h e i s o m e r i z a t i o n o f t h e p r o p y l g r o u p s d u r i n g their transfer. The organohydrogen phenyl groups.
silane r e d i s t r i b u t i o n s i n v e s t i g a t e d t o d a t e h a v e a l l i n v o l v e d
T h u s phenylsilane redistributes a t room temperature, w i t h a l u m i n u m
chloride, t o give good yields of tetraphenylsilane a n d silane 4C H SiH - » (C H ) Si + 3SiH 6
6
3
6
Phenylmethylsilane,
(C H ) (CH )SiH ,
the same conditions
yield products
6
5
3
5
4
(4?)(7)
4
a n d phenylchlorosilane,
2
which indicate
C H ClSiH , 6
experimental conditions,
however,
under
2
that the phenyl, hydrogen, a n d
m e t h y l o r chloro groups a l l shift a r o u n d d u r i n g the redistribution. modified
5
phenyldichlorosilane
Under
somewhat
is reported
(SO) t o
yield only dichlorosilane a n d diphenyldichlorosilane : 2C H HSiCl 6
that is, the chloro groups
5
2
-> ( C H ) S i C l 6
5
2
2
+ H SiCl 2
do not redistribute at a l l .
(8)
2
Benkeser and Foster
(18, 14)
h a v e s h o w n t h a t w i t h a s o d i u m - p o t a s s i u m a l l o y as a c a t a l y s t a l l t h e p h e n y l
hydrogen
silanes c a n b e c o n v e r t e d t o t e t r a p h e n y l s i l a n e . groups
d i d n o t redistribute.
U n d e r these a l k a l i n e c o n d i t i o n s m e t h y l
Phenylpotassium
was,
therefore,
postulated
t o be a n
intermediate i n this t y p e of r e d i s t r i b u t i o n : (C H ) SiH 6
5
(C H ) SiH 6
5
2
2
2
+ Κ —> C H K + ( C H ) H S i K
2
6
5
6
6
(9)
2
+ C e H K -> ( C H ) S i H 6
6
5
(10)
3
(C H ) SiH + C H K - * (C H ) Si 6
T h e last m a j o r t y p e or
alkoxylsilicon.
5
3
6
5
6
5
(11)
4
of r e d i s t r i b u t i o n is t h a t between
A l k o x y groups
have
been
shown
organosilicon
t o interchange
a l k e n y l g r o u p s w i t h s o d i u m e t h o x i d e c a t a l y s i s (5, 6, 8).
and
with
chloro-
aryl a n d
U n d e r these m i l d r e f l u x c o n
ditions, the saturated a l k y l groups do not shift. T h e phenylchlorosilanes catalysts proceeds.
(14, 36) t o f o r m A l t h o u g h i t was
have
been found
tetraphenylsilane. earlier stated
to disproportionate T h e reverse
slowly w i t h
reaction apparently
basic also
(41) t h a t t e t r a p h e n y l s i l a n e d i d n o t r e a c t
w i t h silicon tetrachloride a t 3 0 0 ° C , a subsequent
patent
(28) s h o w e d
the formation
of p h e n y l c h l o r o s i l a n e s f r o m these r e a c t a n t s . M o s t o f t h e p r e v i o u s l y discussed r e d i s t r i b u t i o n r e a c t i o n s a r e of a c a d e m i c only. tance.
T h e methylchlorosilane redistributions, however, This
situation prevails
f o r several
reasons.
appear of commercial First,
interest impor
t h e methylchlorosilanes
a r e t h e l a r g e s t g r o u p o f c h e m i c a l s u s e d i n t h e p r e p a r a t i o n of silicones f o r c o m m e r c i a l exploitation.
Secondly, two of the commercially available methods of manufacturing
methylchlorosilanes—(1)
the direct process of m e t h y l chloride w i t h silicon a n d (2) t h e
G r i g n a r d process o f m e t h y l c h l o r i d e w i t h m a g n e s i u m a n d s i l i c o n t e t r a c h l o r i d e (43)— result i n c o m p l e x m i x t u r e s o f all t h e possible
methylchlorosilanes
236
ADVANCES IN CHEMISTRY SERIES CH3CI + Si(Cu)
\ CH3S1CI3 + ( C H ) S i C l 2 + ( C H ) S i C l 3
3
2
3
(12)
/*
CH3CI + M g + S i C l
4
B e c a u s e these species are n o t u s e d i n t h e s a m e r a t i o s i n w h i c h t h e y a r e p r o d u c e d , m e t h o d s of i n t e r c o n v e r t i n g o r r e d i s t r i b u t i n g these b y - p r o d u c t s s h o u l d b e o f e c o n o m i c importance.
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T h e r e d i s t r i b u t i o n of m e t h y l c h l o r o s i l a n e s w a s first r e p o r t e d b y S a u e r a n d H a d s e l l i n 1948 (46). T h e y s h o w e d t h a t t h e r e a c t i o n s p r o c e e d e d a t 250° t o 4 5 0 ° C . u n d e r t h e influence o f a l u m i n u m c h l o r i d e . T h e r e d i s t r i b u t i o n s w e r e a l l e q u i l i b r i u m r e a c t i o n s , but deviated f r o m the r a n d o m nature stipulated b y Calingaert's reactions. I t was also f o u n d p o s s i b l e t o m e t h y l a t e p h e n y l - a n d e t h y l c h l o r o s i l a n e s u s i n g t h e m e t h y l c h l o r o s i l a n e s . Z e m a n y a n d P r i c e (52) h a v e s t u d i e d t h e k i n e t i c s a n d t h e r m o d y n a m i c s o f the reaction. T h e y obtained the e q u i l i b r i u m constants for the reactions: Si(CH ) 3
4
+ (CH ) SiCl 3
2
(CH ) SiCl + CH SiCl 3
3
3
2
( C H ) S i C l 3
3
