HYDROLYSIS OF CHLOROSILANES. 11. RATESAND MECHANISM
Dec., 1957
1595
the same hydrolysis products a t comparable termediates were identified by chemical analysis or by molecular weight and wt.% C1. The data temperatures. C1Isolation of Products Predicted from Conduc- obtained on C1 [(C2H6)zSiO]Si(C2H5)2C1, tivity Curves.-In order to verify the interpreta- [(C,H5),SiO]2Si(C,H~)zCl, (C6H6SiCI2),Oand C6H5tion of the conductometric titration curves, several biC1,0SiC12CH=CHz are summarized below. C1[(C2H5),SiO]Si(CzH5)zC15: b.p. 103" a t 15 mm.: of the predicted hydrolysis intermediates were precryoscopic molecular weight, theory ~ pared under the same hydrolysis conditions used n 2 51.4344; in obtaining the conductometric titration curves. 259, found 253; wt.% C1 theory 27.3, found 26.9. Intermediates of the type CI(R2SiO),R,SiC1, C1[(CzH5)zSiO]zSi(CzH5)zC15: b.p. 115" a t 1 mm.; ~ wt.% C1 theory 19.6, found 19.5. where x = 1, 2, 3, mere recovered by distillation n 2 51.4372; ~)ZO 135-136" : a t 1 mm.; '?&25D from the hydrolysis of (CHs)zSiC12and (C2H5)zSiC12 ( C ~ H ~ S ~ C ~ b.p. in ethylene glycol4imethyl ether.3 The methyl 1.5348; wt.% C, theory 39.1, found 40.1; wt.% intermediates were identified by boiling point and C1, theory 38.6, found 37.3. index of refraction. Some additional confirmatory CsH5SiC120SiC1zCH=CHz:b.p. 68" a t 0.18 mm. ; ~ density 1.30 g./cc. a t 25"; cryot8ests,reported below were made on the two leading n Z 5 1.4950; scopic molecular weight theory 319, found 328; members of the ethyl series. I n the C6K6F.iCl3,CH2=CHSiCb and the C6H5- wt.% C1 theory 44.5, found 43.5. Acknowledgment.-We are indebted to our asSiCl3--CHz=CHSiCL systems, (RE,iCl,),O and RLiC1zOR'SiC1, were prepared by low temperature sociates in the newly formed Silicones Division of hydrolysis in homogeneous solution. These in- Union Carbide Corporation for permission to publish this paper. (3) T h e methyl compounds previously have been prepared and reaction of appropriate Grignard reagents with (SiCldzO. H. J. characterized by W. Patnode and D. F. Wilcock, J. Am. Chem. Soc., '
Emeleus and D. 6. Payne, J . Chem. Soc.. 1590 (1947); D. S. Payns, ibid., 2143 (1954). 5) We are indebted to Dr. R. A. Pike for assistance in preparing
68, 358 (1946). The leading member of the similar series of ethyl silicones has been reported previously by Philip D. George, et al., tbid.,
76, 1585 (1953).
and characterizing these substances.
(4) [C~HsSiClzlzO and [CsH~SiCIzIzOhave been prepared b y the
THE HYDROLYSIS OF ALKYL AND ARYL CHLOROSILANES. 11. RATES AND MECHANISM OF HYDROLYSIS IN HOMOGENEOUS SOLUTION BY L. H. SHAFFER"AND E. M. FLANIGEN Contribution from the Research Laboratory of Linde Company, A Division of Union Carbide Corporalion, Tonawanda, New York Received April 16, 1967
The rates, activation energies and differences in the entropies of activation for the hydrolysis of alkyl and aryl chlorosilanee were determined in homogeneous solution in the presence of excess HCI. The rate of reaction was determined by conduotometric measurements on the reaction mixture. The hydrolysis is first order with respect to water and, a t least for the trifunctional chlorosilanes, is second order with respect to chlorosilanes. In the case of difunctional chlorosilanes, an anomalous negative fractional order was observed. A mechanism for the hydrolysis of trifunctional chlorosilanes consistent with the data obtained involves two principal steps: RSiC13 HzO-+ RSiC120H HCl, followed by RSiCllOH RSiClr + RSiC120C12SiR HCl. The latter reaction is the rate-controlling step. The rate of hydrolysis varied considerably with solvent system but there was no change in activation energy for the hydrolysis reaction in two different homogeneous solvent systems.
+
Introduction I n the first paper of this series' we described a conductometric technique for determining the products of the hydrolysis of chlorosilanes. In this paper we will present rate data and some information on hydrolysis mechanism which can be obtained by a slight modification of the procedure previously described. The principal result8 are summarized in Table I.2 Trifunctional chlorosilanes, RSiC13, were explored more thoroughly than any others. A few data on RzSiCI, are also included in Table I. Crude measurements were made on R3SiC1 and Sic14 systems but the data obtained are not sufficient to warrant publication at this time, The following generalizations about * Central Research Laboratory, American Machine and Foundry Co., Stamford, Conn. (1) L. H. Shaffer and E. XI. Flanigen, THIS JOURNAL,61, 1591
(1957). (2) We are indebted to Mr. A. J. Petro for completing many of the measurements required in assembling this body of data.
+
+
+
chlorosilane hydrolysis rates can, however, be made. First, the hydrolysis rates of the members of the series SiC14-R3SiC1are related Sic14 > RSiC13 >> RzSiC12 > RaSiCl
And second, for any chlorosilane containing two or more chlorines on the same silicon, the first chlorine reacts very much faster than those remaining? Experimental The apparatus previously describedl was used to obtain hydrolysis rates by conductivity measurements. As is illustrated in Fig. 1, there is a linear relation between water concentration and conductivity when water is added to et,hylene glycol-dimethyl ether-HC1 and dioxane-HC1. Therefore, the hydrolysis rate can be determined by measuring the rate of change of the resistance of a chlorosilaneHC1 solution after an addition of water. The fraction of unreacted water remaining after any time interval is given by ( R L - Itm)/(Ra - R m ) . R, is the resistance a t the time the first measurement is made; R Lis the resistance a t time t (3) See also W. C. Schumb and A. J. Stevens, J . Am. Chem. 78,3178 (1950).
,%e..
L. H. SHAFFER AND E. M. PLANIGEN
1596
* i
$1
Y
Y
$I
$1
0
0
Vol. 61
9 ri
09
2
YI
N
2
2
X
Y
I
X ln
W
i
HYDROLYSIS OF CHLOROSILANES. 11. RATESAND MECHANISM
Dec., 1957
after the fist observation; and R , is the resistance after a very long time. Typical results are shown in Figs. 2 and 3. Most of the measurements were made a t low temperature and in systems that had been previously saturated with HCI at 0". This was done to make the reactions slow enough to be foilowed easily using a stopwatch and the available resistance bridges.
TOTAL RANGE OF HzO CONCENTRATION USED IN ALL HYDROLYSIS / RATE STUDIES
Results The kinetics of the reaction of water with chlorosilanes can be expressed as -= -d(Hzo)
dt
k(H20)m(=SiC1)n(HC1)p
1597
I let
_.
/
fn
I
- 2.0 $ P
(1)
Values for m and n were obtained in this work. The effect of HCl was investigated but not in sufficient detail t o yield a value for p . Order of Reaction with Respect to Water.-The order of reaction with respect t o water m was obtained by measuring the rate at which approximately 0.01 mole of water reacted with approximately 0.1 mole of chlorosilane. Under these conditions, with excess HC1 present, the only materials whose concentrations change are water and the hydrolysis product, and the only products likely to be formed are chlorodisiloxanes. Log-log plots of reaction half times, &, us. initial mater concentrations showed that the hydrolysis was first order with respect t o water and plots of log (Rt-R,)/(Ra-R,) vs. time for reaction of the first 0.01 to 0.02 of a mole of chlorosilane in all runs were linear as is required by first- order kinetics. Figure 3 is typical. Order of Reaction with Respect to Chlorosilane. -The conductometric rate studies described above show that m in eq. 1 is equal to 1. In order to determine the over-all rate equation the values of n and p also must be determined. An attempt to extract n from the available rate data was made. During the conductometric studies, the rates of hydrolysis of several chlorosilanes were determined for successive additions of small equal increments of water solution; thus, the half-times for several initial concentrations of chlorosilane were obtained assuming one mole of water removes two moles of' chlorosilane. This assumption is supported by the first paper of this series.l The empirical order of the reaction with respect to chlorosilane was calculated from the slope of the straight line obtained by plotting log t y , us. log (=SiCl). With the conditions specified above
L" 0-
a
d,0-6.0-7~0-8e.0-b
1.0-2.0-310-4.0-
MOLE RATIO: H,O/
HCL.
Fig. 1.-Conductometric titration of HCI with water in ethylene glycol-dimethyl ether: , curve A, titration curve for 9;5 M HCI in solvent with 2.5 Af water in solventHCI a t 30 ; - - - -, curve B, titration curve for 0.22 M HC1 in solvent with 5.6 Af water in solvent-HC1 a t 0".
-
r
i
/ -
'0-
0-
0
IO
TIME
Values for the order of reaction with respect to chlorosilane obtained by use of this method are shown in Table 11. These data show an order of reaction with respect to chlorosilane of two in RSiC13 systems. The apparent order of - 1/3 for (CH3)2SiC12is not understood. The increase in rate of hydrolysis with decrease in initial chlorosilane concentration was not observed for any other R2SiClzstudied. Two objections to the above calculations can be raised. First of all the accuracy of the rate data on successive additions of water t o the chlorosilane system may not be good enough t o lend itself t o such fine analysis. Secondly, the concentration of hydrolysis product is not the same in successive
- 1.0
-
CCH312 SiCi.2 AT 25'C CH3 SICLJ AT -56'C
I
I
I
20
30
40
- IN MINUTES
I 50
I
80
I 70
AFTER FIRST MEASUREMENT.
Fig. 2.-Resistance as a function of time after addition of water to chlorosilane: solvent, ethylene glycol-dimethgl ether saturated with HCl.
TABLE I1 ORDEROF REACTION WITH RESPECT TO CHLOROSILANE
-d(Hso) = k ( H a O )(z&iCI)"(HCl)r dt Chlorosilane
11
CrHSBiCI,
I .9
CjHIlSiClB
1.8
CH3SiC13 (CHa)2SiC12
1.9 -1/3
Solvent systeun
Ehhylene glycoldimethyl ether-HC1 Ethylene glycoldimethyl ether-HC1 Dioxane-HCI Dioxane-HC1
Temp ("C.1
-78 -31
0
.o
L, H. SHAFFER AND E. M. FLANIGEN
is98
O-CCH3)z A---CH3
0
S I C L ~AT 2 5 4 . S I C L ~AT -WoC
IO 20 30 TIME IN MINUTES AFTER FIRST MEASUREMENT C R g ) .
Fig. 3.-Rate of hydrolysis of chlorosilanes in ethylene glycol-dimethyl ether saturated with HCl: [(Rt - R,)/ (Ro- R,)] us. time after first measurement.
26 24 30
x
x
28
flRST ADDITIONS OF WATER X SECOND ADDITIONS OF WATER 0
amounts of acid in a solvent for water should decrease the activity of water. These effects can be minimized if the hydrolysis is measured in the presence of a standard large excess of HC1. Since qualitative observations showed that excess anhydrous HC1 slowed the reaction to a convenient rate, most of the measurements were carried out in solvents saturated with HC1 a t 0'. We found later that the hydrolysis rate in ethylene glycoldimethyl ether changed appreciably with HC1 concentration when this substance was about 8 M . Probably this is due to the fact that above this concentration there is more than one HC1 molecule per solvent molecule, and therefore the concentration of non-solvated HC1 increases rapidly. The results of a typical set of measurements on the effect of HCI concentration on rate are shown in Fig. 4.4 A 10% change in HC1 concentration can double the reaction half-time. However, the effect of temperature on reaction rate is so much more pronounced that reasonable activation energies and entropies can be obtained without correcting the data to a common HC1 concentration. For example, the observed curve for the effect of HC1 concentration on the hydrolysis rate of CaH6SiC13 was used to calculate corrected halftimes referred to 8 M HCI. The values for the activation energy and 2.3R log Z derived from these corrected figures are 6.8 kcal./mole and 19.8 cal./deg. while the values derived from the raw data are 6.7 kcal./mole and 18.8 cal./deg., respectively. This being the case, no corrections were applied in treating the data obtained on the other chlorosilanes. Rates of Hydrolysis and Determination of Activation Energy.-The apparent first-order specific rate constants for the hydrolysis of chlorosilanes were calculated using the relation
u; 22-
5 f
(3)
2018-
,Since both chlorosilane and HCl were present in large excess, ternis in concentrations of these materials are included in k so that
1614L
12-
=
k = Akl(-SiC1)n(HCl)~
IO864-
(M=8.0 AT 2 5 % )
2I
0
Vol. 61
1
*
I
2
3
4
5
6
7
8
9
(4)
where n is the order of reaction with respect to SiCI, p is the order of reaction with respect to HCI, A is a numerical ons st ant,^ and kl is the true specific rate constant for the reaction. The values of k and half-times for the hydrolysis of various alkyl and arylchlorosilanes studied are shown in Table I. The data in all cases pertain t o reaction in homogeneous solution in the presence of excess HCI and chlorosilane. Activation energies were determined by plotting log t l / , vs. 1 / T as is illustrated by Fig. 5. The slopes of such plots are equal to -E/2.3R and the activation energy, E, for each chlorosilane studied is recorded in Table I. Using the Arrhenius equa(4) We are indebted to Dr. H. A . Hartung of this Laboratory for obtaining the data plotted il? Fig. 4 and for making a brief study of the effect of HC1 concentration on hydrolysis rate in two other chlorosilane systems. ( 5 ) I n principle, A can be determined froin tne theory of absolute reaction rates b u t in general its value will depend on activity coef ficients and the relationship of the various equilibria in a chain of consecutive reactions.
HYDROLYSIS OF CHLOROSILANES. 11. RATESAND MECHANISM
Dec., 1957
tion, k = Z exp ( - E I R T ) , and the tabulated values of IC, individual values of the frequency factor, 2, were calculated for each set of rate data. These values, recorded in Table I, are quite constant for each chlorosilane, and this high degree of constancy provides a good internal check on the body of data presented here. The frequency factor, 2, is a function of the entropy of activation and several other terms. The other terms are essentially constant for the experimental conditions used and, therefore, the Z values given are related to the entropies of activation for hydrolysis in such a way that 2.3R log Z1 -2.3R log Zz = AS1++ - AS2++. Values of 2.3R log Z ( A s + + a constant) are recorded in the last column of Table I. When additional data on the factors influencing hydrolysis become available, it should be possible to calculate the individual entropies of activation. Effect of Solvent.-The majority of the hydrolysis experiments were carried out in ethylene glycol-dimethyl ether saturated with HC1. The rates of hydrolysis of CH3SiC13, C6H5SiC13 and (CH3)2SiC12were also measured in dioxane-HC1. The rate data recorded in part B of Table I show that the rates of hydrolysis of (CH3)2SiC12 and CH3SiC13are approximately three times as fast in dioxane. This effect may be due in part to the lower acid concentration (-5 M ) obtained when dioxane was saturated with HCl. I n the case of C6H5SiC13,dioxane-HC1 caused slower rates of hydrolysis than ethylene glycol-dimethyl ether a t the same temperature. The activation energy for the hydrolysis of CH39iCl3was found experimentally to be the same in either solvent-HC1 system.6 The activation energies for the hydrolysis of (CH3)zSiC1zand C6HsSiC13 were not determined in dioxane.
1599
+
Discussion A series of reactions has been devised to describe the mechanism of hydrolysis. These reactions are based on the data reported above and on the results of the conductometric titration studies described in Part I’ of this series. The reactions proposed are consistent with the mechanism discussed by Swain’ in connection with his comparison of the hydrolysis of trityl fluoride and trisyl fludride, and with the mechanism proposed by Schumb and Stevens3 for the partial hydrolysis of Sic], in moist ether solutions. Reactions (iib) and (ivb) are written specifically for R2SiCl2compounds to illustrate how silanols and hydroxy endblocked siloxanes can ultimately form using the mechanism proposed. We do not believe that these steps are of any significance in the early stages of the hydrolysis reaction. The remaining types of reactions are general and may occur for all values of x from 0 to 3. “S” in the series of reactions listed stands for one or more molecules of the solvent in which these reactions are carried out. HzO + HC1 + S +H20.HCI.S (9 ( G ) The activation energy should be constant in any polar solvent, b u t t h e entropy of activation is expected to be more negative in less polar solvents. This, of course, results in slower reactions in lpss polar me ia, see R. G . Pearson, J . Chem. P h y s . , 20, 1-178 (1952). (7) C. G . Swain, R. M. Esteve and R. H. Jones, J . A m . Chem. Soc., 71, 065 (194Y),
1x1011
”
38
I
40
I
I
42
I
’
I
44
K+
I
46
I
I
48
’
I
50
’
I
52
I
x IO’.
Fig. 5.-Half times for the hydrolysis of RSiClt in homogeneous solution saturated with HC1: solvent, ethylene glycol-dimethyl ether except where otherwise stated.
+ Hz0.HC18 --+ R,SiC13-.(OH) + 2HCl.S (iia)* RzSiC1(OH) + HzO.HCIS + R2Si(OH)2+ 2HCl.S (iib) R,SiC13- I ( OH) + R,SiC13- OH) + (R,SiCL-,)zO + HzO (iii) R,SiCI,-.(OH) + R,SiCh-, + (R,SiC13-.)z0 + HC1.S (iva) RzSi(OH)z + R,SiCla- +
RsSiC14-.
2(
(HO)RzSiOR,SiC13-. (ivb)
2R,SiC14-.
+ HzO.HC1.S +
(R,SiC13-,)z0
+ 2HCl.S
(v)
In a saturated HC1-solvent system a t low temperatures one may assume that any water added is in the form of the (HzO.HCl.solvent) and that the equilibrium in equation (i) is far to the right. Reactions (ii) provide for the observed formation of =Si(OH)* groups in some cases. A single step reaction t o form =Si(OH)z groups is unlikely because of the low probability of three-body collisions compared to the chance of a two-body collision. The kinetics of hydrolysis have shown uniformly that the reaction is first order with respect to water and this observation means that the rate-controlling step involves only a single water molecule. The occurrence of reactions (ii) as a two-step process is, therefore, consistent with the observed facts. In case products like R,8iCl3-.OR,SiCl~-, are formed, reaction (v) would provide a simple mechanism consistent with the data obtained to date. However, again a rather improbable three-body collision is required; and besides, there is no reason to suppose that the mechanism of the initial attack of water on a chlorosilane should depend on the ultimate reaction product. Therefore, it is most (8) This could also be expressed R$iCIe ..HzO HCl*S.
+
as
RzSiClrz
+
HzO*HCI.S
-+
J. LAVOREL
1600
probable that hydrolysis occurs by a chain of bimolecular reactions such as (i), (ii) and either (iii) or (iv). In the proposed chain of reactions it is impossible to decide whether (iii) or (iva) describes the situation. Only in the case of (C6H5)28iC12 have indications
OH I
Rate =
Vol. 61 ~
-d(HzO) -d(RSiCls) = dt dt -
where k stands for a forward velocity constant and K stands for an equilibrium constant. If (HCI)/ K i i is appreciably greater than (lriv/kii) (RXiCh), then this reduces to
of stable groups like =Sic1 been found and this implies t,hat for the other chlorosilanes studied, either Rate = ki,Kii(HzO)(RSiC13)2/(HC1) (6) reaction (iii) or (iv) is very fast compared to (ii) and (ii) is the rate-determining step, or that the Equation 6 is consistent with the known facts but equilibrium for (ii) is far to the left. The data can only be established conclusively by further exavailable on the order of reaction with respect to perimentation. A further study of the order of chlorosilane demand that two molecules of chloro- reaction with respect to chlorosilanes, particularly silane be involved in the rate-controlling step, and in difunctional systems, should be made, and the this requires (iii) or (iv) to be slower than (ii). A kinetics of the reactions of R3SiOH and R,Si(OH), reasonable rate equation which fits the observed with R,SiC14-. should be determined. facts for the hydrolysis of trifunctional chlorosiAcknowledgments.-We are indebted to our aslanes can be obtained by assuming that reaction sociates in the newly formed Silicones Division of (iia) is a reversible reaction which is then followed Union Carbide Corporation for permission to pubby (iva), Then for RSiC13compounds lish this paper.
INFLUENCE OF CONCENTRATION ON THE ABSORPTION SPECTRUM AND THE ACTION SPECTRUM OF FLUORESCENCE OF DYE SOLUTIONS BY J. LAVOREL~ Department o j Botany, University of Illinois, Urbana, Illinois Received May 6, 1967
The action spectrum of the fluorescence of fluorescein in solution is concentration-dependent ; in particular, the longknown drop of efficiency on the long-wave side of the absorption band is practically non-existent at low concentrations, and becomes very marked at high concentrations. These variations in the action spectrum of fluorescence are paralleled by changes of the absorption spectrum. Both can be quantitatively explained by the formation of a non-fluorescent dimer, if one assumes that this dimer has two absorption bands-one on each side of the monomer hand. Similar effects are observed -and the same interpretation appears adequate-also for solutions of thionin. Observations on two chlorophyll-a solutions of different concentration suggest the occnrrence of a similar dimerization effect. The results have a bearing on the interpretation of the concentration quenching of fluorescence, and of the decline observed at the long-wave end in the action spectra of photosynthesis and of the fluorescence in green plants.
Introduction for an intrinsic reason why molecules, excited in the The sharp drop in the efficiency of fluorescence frequency range corresponding to the long-wave excitation, found in many dissolved dyes when side of their absorption band, should dissipate their light is absorbed on the long-wave side of the peak electronic excitation more successfully-in comof the absorption band, has been first observed by petition with fluorescence-than molecules excited Valentiner and Roessiger3s4in the case of fluores- by light of higher frequency. Explanations of this type are somewhat difficult cein. Nichols and Merritt5 had shown previously that, throughout the remainder of the absorption to reconcile with the well-supported idea that, in a spectrum, the energy yield of fluorescence does not condensed phase, thermal equilibration of intravary strongly. Vavilov6 found that t>hequantum molecular vibrations takes a very short time comyield of fluorescence of many dyes remains constant pared to the life-time of the fluorescent state; throughout the spectrum-except a t its long-wave and that accordingly, by the ttime the excited molecules emit fluorescence, they are distribut>ed over end. One can follow two lines of argument in order to a spect,rum of vibrational states which does not deaccount for this phenomenon. First, one can look pend on the way in which they had been originally excited. It has been suggested, however, that (1) Work performed in the Photosynthesis Laboratory of the exceptionally “hot” molecules (i.e., molecules in Department of Botany of the University of Illinois, Urbana. Illinois. during the tenure of a Rockefeller Fellowship, with the assistance of high vibrational states) may lose their electronic the U. S. Office of Naval Research. excitation energy by radiationless transition into (2) Laboratoire de Biologie Physico-chimique, Facult6 de8 Sciences, the ground state before thermal equilibration. Universite de Paris, Paris, France. It, should be noted that such “hot” molecules (3) 9. Valentiner and M. Roessiger. Berlin Bar., 16, 210 (1924); Z . Phyaik. 32, 239 (1925). should contribute to absorption not only on the (4) 8. Valentiner and M . Roessiger, ibid., 36, 81 (1926). long-wave side, but also on the short-wave side of ( 5 ) E. L. Nichols and E. Merritt, Phya. R e v . , 31, 381 (1910). the main peak, causing the efficiency curve of (6) S. I. Vavilov, Phil. Mag., 43, 307 (1922).
..
