DISPROPORTIONATION OF MONOSODIUM TETRAPHENYLETHYLENE
June 5 , 1964 [CONTRIBUTION FROM
THE
DEPARTMESTS OF CHEMISTRY OF THE UNIVERSITY OF GEORGIA, ATHENS,GA , ASD CALIFORNIA, RIVERSIDE, CALIF.]
THE
2257
UNIVERSITY OF
Disproportionation of Monosodium Tetraphenylethylene BY JOHN F. GARST,*ERNESTR. ZABOLOTNY, A N D RONALD S.COLE RECEIVEDDECEMBER 23, 1963 The extent of disproportionation of monosodium tetraphenylethylene a t room temperature varies with solvent The factors inin the order (diethyl ether, dioxane) > tetrahl-drofuran > diglyme = 1,2-ditnethoxyethane fluencing the equilibrium are discussed
Introduction Examples of hydrocarbons for which the disproportionation equilibrium (I) for the monoalkali adducts lies in any position except far to the left have been rare.3,4 K
2(RH-, M C )
RH
+ RH-2, 2 M +
(1)
Tetraphenylethylene ( T P E ) has long been known as an anomalous example,4--5 monoalkali adducts being unknown until The present work is concerned with the influence of the solvent on the magnitude of K T P E , ~ We ~ . have employed spectrophotometric methods, primarily, and have examined the solvents diethyl ether, dioxane, tetrahydrofuran, 1,2-dimethoxyethane, and diglyme. Experimental Quartz optical cells of 1.00 c m . and 0.119 or 0.020 cm. path lengths were attached to a 10-nil. Pyrex solvent reservoir and a 10-mi. Pyrex reaction vessel. T o the reaction vessel was attached a 20 cm. length of 7 m m . Pyrex tubing, constricted in several places, which acted as a distilling column for the sodium. About a mg. of zone refined T P E was placed in the solvent reservoir. The whole system was evacuated t o mm. and baked periodically during a (2-hr. period. .lfter coating the reaction vessel with a sodium mirror by distilling the sodium, the ethereal solvent was distilled into the solvent reservoir from storage over the disodium adduct of benzophenone. The reaction vessel and contents were cooled in liquid nitrogen and sealed off h o m the vacuum system. The progress of reaction was followed spectrophotometrically, concentrations being adjusted by tipping and distilling. The concentration of T P E was accurately determined vza its absorption spectrum. Frequently the initial disappearance of T P E did not generate a colored solution. By following the initial disappearance spectrophotometrically until a stable, faintly colored solution did result, the extent of “wastage” of T P E through these side reactions, probably with impurities, was estimated. For the more concentrated solutions, ca. 10-3 M, the wastage was about 352 or less. For more dilute solutions, M or less, varying amounts of wastage were observed, but could be accurately measured, as evidenced by the fact t h a t the extinction coefficients calculated for T P E a 2 , by measurement of the spectrum when all T P E was converted to TPE-2, were consistent ( 1 ) J F. G a r s t a n d R . S. Cole, J A m Chrm SOC.,84, 4852 ( l 8 6 2 ) , h a v e previously reported some of t h e preliminary results on t h e present topic. (2) I l e p a r t m e n t of C h e m i s t r y , University of Georgia, Athens, G a . 13) G J H o i j t i n k , E. d e B o e r , P H v a n d e r Meij, a n d W . P. Weijland, Rec 1rau. chim., 75, 487 (1956). ( 4 ) N . S H u s h a n d J Blackledge, J . Chrm. P h y s , 23,514 (1955) ( 5 ) W . S c h l e n k , J A p p e n r o t , A hlichael. a n d A T h a l , Bo., 47, 473 ( 1 9 1 4 ) , W Schlenk a n d E B e r g m a n n , A n n , 463, 1 (1928). ( 6 ) N. B . Keevil a n d H. E. B e n t , J . A m . Ciirm. Soc., 60, 193 (1938). ( 7 ) H G i l m a n a n d R V Y o u n g , J . O i g Chem , 1, 315 (1936). ( 8 ) ( a ) I) W . Ovenall a n d 1) H . W h i f f e n , Special Publication No. 1 2 , T h e Chemical Society, L o n d o n , 1958, p . 1 3 9 , ( h ) discussion of M. J S. D e w a r , i b i d , p. 1 6 4 ; (c) discussion of S . S. H u s h , i b i d . . p 164, ( d ) discussion of P. G r a y , ibid., p. 166. (9) H. P Leftin a n d W K . H a l l , J P h y s C h r i n . , 64, 382 ( 1 8 6 0 ) . (10) (a) A. G . E v a n s , J. C E v a n s , E. D O w e n . B. J . T a h n e r , a n d J E . B e n n e t t , P I W C Chrm. . SOL.,226 (1962); ( b ) J C h m SOL., 3954 (196:3), ( c ) A G E v a n s a n d B. J. T a b n e r , ibid.. 4613 (1963). (11) A . V . Toholsky a n d D B. H a r t l e y , J A m Chrm S O L ,84, 1391 (1962).
for experiments in which the final concentration of T P E - 2 was taken to be the same as T P E concentration after the wastage period. The use of t w optical cells o n the same reaction vessel permitted the accurate determination of widely varying optical densities. A11 spectra were taken on a Carp Model 14 recording spectrophotometer a t room temperature ( c a . 23’). Examination of the temperature coefficients, t o be reported later with results on other alkali metals, indicated t h a t ambient temperature fluctuations about room temperature have only slight effects on the spectra of the solutions, although part of the experimental scatter may & be due to these fluctuations. in K ~ p s , %
Results The dashed curves of Fig. 1 show the experimentally determined shapes of the absorption bands of T P E and TPE-2. No distinct solvent dependence of the positions, shapes, or extinction coefficients of these bands was noted. The extinction coefficient of T P E in 1,2dimethoxyethane, tetrahydrofuran, diethy: ether, and diglyme is (1.35 f 0.04) X 104 a t 3070 A . The extinction coefficient of T P E - * is (3.4 0.1) X lo4 a t 4900 kl* The wave lengths cited correspond to band maxima. In diethyl ether and dioxane, the dissolution of sodium in a T P E solution leads to spectra consisting of the superimposed bands of T P E and T P E P 2 . I n 1,2dimethoxyethane, diglyme, or t5trahydrofuran additional bands a t 3700 and 0600 A. are present. The bulk of our data are from experiments in 1,2-dimethoxyethane. Until otherwise stated, the following summary of results will refer to that solvent. The solid line of Fig. 1 is a typical spectrum containing all four bands. As sodium is dissolved, initial growth of the 3700 and 6600 A. bands is followed by their a t their expense. decline, the 4900 A. band growi!g The intensities a t 3700 and 6000 A. vary linearly with one another (after allowance for overlapping bands).
*
(12) Results reported recently b y E v a n s a n d co-workers10 differ f r o m o u r s
in several respects T h e y r e p o r t extinction coeficients of S a 2 T P E a n d I,i?TPE in t e t r a h y d r o f u r a n as 1 . 8 a n d 2 7 X 104, respectively W e consider o u r observed variations in t h e extinction coefficient of T P E - 2 with m e t a l ion a n d solvent t o he within experimental e r r o r , a n d in no case d i d we o h t a i n a value less t h a n 3 1 X 101. While it is easy t o imagine reactions of t h e T P E anions. probably d u r i n g their initial f o r m a t i o n period (see E x p e r i m e n t a l ) , which m i g h t lead to low estimates of t h e extinction coefficient of T P E - 2 , i t is more difficult t o imagine such processes leading t o high e s t i m a t e s The f a c t t h a t t h e direct d e t e r m i n a t i o n of each s u b s t a n c e t h r o u g h t h e use nf extinction coefficients g a v e good a c c o u n t s of mass balance ovei- a wide r a n g e of relative concentrates s u b s t a n t i a t e s t h e values we employ E v a n s a n d co-workel-s also report variations in t h e h a n d m a x i m u m of T P E - * with metal ion. their m a x i m a being u p t o 250 .$ o n t h e high energy Variations of a h o u t 100 A . were side of those obtained in our laboratories observed in o u r work, a n d we consider such va:-iations t o be of d o u b t f u l significance. in view of t h e b r e a d t h of t h e band W e observed t h e a p p e a r a n c e of a new band a t 4:300 f , in preparations in which extensive decomposition h a d obviously occui-red. Some experiments which led t o h a n d m a x i m a a t energies highei- t h a n 4900 f , . showed distinct evidence of t h e pi-esence o f b o t h b a n d s . being broader t h a n no1 mal and having di5cernihle shouldel-s Such r u n s were discarded T h e y we#-em e t steadily decreasing fl-eiluency as o u r technirjues wei-e perfected I n s p i t e of these diffei-ences, t h e chemical regularities which emerge from t h e work of t h e t w o groups a r e in agreement
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F. GARST,ERNESTK. ZABOLOTNY, A N D RONALD S. COLE
VOl. 86
1.0p I -
O7I
A 06
A
w a
05 CI \
z
Ea -
04 03
02 01
01
02 0 3 04 0 5
06
CT PEI / {T PE
Fig. 1.-Typical spectrum of the sodium adducts of T P E in l,"dirnethoxyetha~ie (solid curve) ; dashed curves, bands due to T P E (3100 A , ) arid T P W 2 (4900 A , ) , actual shapes; das& dotted curves, speculated shapes of h n d s a t 3 i 0 0 and 6600 A , due, presumably, to TPE- (see text).
Clearly, a t least two species are being formed initially, with the rnaterial(s) absorbing a t 3700 and 0600 A. being ultimately converted to TPE-?. Spectra obtained by converting all T P E to T P E - ? , then admitting more T P E by means of a break seal, were identical in form with those obtained by dissolution of sodium in a T P E solution, equilibrium being achieved immediately upon a mixture of T P E and T P E P 2 . Assuming that T P E , T P E - ? , and a third material (X), absorbing only a t 3700 and 6600 A,, are the only species present, knowledge of the extinction coefficients of T P E and T P E - ? permits the calculation of the extinction coeflicients of X a t 3700 and GGOO A. by mass balancing. This procedure involves uncertainty in curve analysis (see Fig. l ) , but fortunately the bands under consideration are spaced so that the problem is not extremely serious. Extinction coefficients of c o n pound X obtained in this manner are 1.3 X IO4 and 2 8 X 1 0 4 a t (Xi00 and 3700 A , , respectively. Compound X is logically T P E - . If so, solutions containing X should be paramagnetic, as indeed they are. Solutions of T P E - ? in diethyl ether and dioxane gave no e.s.r. signals in the g = 2 region.'8a Electron spin resonance spectra of TPE- have been reported and a coupling constant analysis in fair agreement with both the e.s.r.spectrum and a SIcLachlan calculation of spin tlensit.ies has been given. If a simple equilibrium of the type I is assumed, differences in the degrees of ionic dissociation not being 113) A . I ) . hIcl.achlan, \lo1 P h y i . 3 , 233 (l!)fiO) Inourcalculationhrvas emplriyed as calculated, A = 1 14, a n d was n o t treated as adjustahle The resr,luti + 4(w -
2))
(2)
and
w = x + y + z
(3)
where w = ( T P E ) ,x = [TPE-?I, y = [TPE-1, and z = [ T P E ] . The circles of Fig. 2 show the conformance of some typical data to these relationships. They correspond to a single run (experiment 95) in which ( T P E } , formal concentration of T P E , remained constant. For reasons which will appear later, all concentrations were ,M, before divided by ( T P E } ,in this case, 3.35 x plotting. The points closely fit the theoretical curve for a value of near 1.00. As a check on the extinction coefficients and curve analyses, ( T P E ) can be calculated from the measured concentrations of T P E , TPE-, and T P E - ? For the four points (of experiment 95) shown, { T P E } is calculated in this fashion to be 3.35, 3.86, 3.47, and 3.41 x 1[1-4 ,M, while the end points (all T P E or TPE-*) indicate 3.33 X I O P 4 .If. This scatter gives an indication of the magnitude of the errors involved in our procedure. Table I gives the values of K T p B , N a calculated directly from the equilibrium expression. Employing a method previously described,I5 Beer's law was tested for all four bands a t typical concentration ratios and absolute concentrations in the ranges about 10V-10-5 .\I, with the result that all bands appear to obey Beer's law. However, when the concentration of a given solution containing the equilibrium mixture of hydrocarbon and anions was varied [I.?J J I' Gal-st. I ) Walmsley, C. Hewitt, W Zabolotny. J . A i n C k m S o c , 86, 4 1 2 (1964).
R
Richards. and E R .
June 5, 1964
DISPROPORTIONATION OF MONOSODIUM TETRAPHENYLETHYLENE
2259
could be measured for dioxane or diethyl ether, since no band assignable to TPE- could be detected. We estimate that such a band would be detectable for values of K T p E , N a up to about 5 X lo4, assuming that the extinction coefficient of T P E - derived from the 1,2-dimethoxyethane data is applicable. This assumption is KTPE.S* supported by the fact that the same extinction coeffi2NaTPE T P E + SazTPE (4) cient gives good accounts of mass balance in diglyme the equilibrium, was 0.91 in the dilute solution and 2.1 and tetrahydrofuran. (There are indications from in the more concentrated solution. This direction of work with Li adducts of T P E that the band a t 3700 A. variation is the expected one if the ion pairs (IL’aTPE) may be due to a different material from the band a t are more ionically dissociated than the triple ions 6600 A. If so, the values of K T P E reported ,~~ should be (Na,TPE), as the work of Hoijtink’s group indicates3 raised by a factor of about 2.) We have previously The variation of K T p E , N a with concentration appears estimated K T P E , N = in diethyl ether as being greater than to be insufficient to disturb seriously the plot shown in lo5,from considerations of the sensitivity of e.s.r. This Fig. 2 , even though the formal concentration of sodium estimate is probably conservative, a lower limit of 106 ions varies during experiment 95. Further, experiment being more likely. 95 can be compared with other experiments of rather The values tabulated for tetrahydrofuran and diglyme different over-all concentrations by dividing all constem from experiments considered to be least susceptible centrations by { T P E } before plotting the resulting to errors owing to curve analysis and the anomalies aspoints on the graph of Fig. 2. If Beer’s law were persociated with very large relative excesses of T P E (see fectly obeyed, the resulting data should fit the relationbelow). In each case, the value given is substantiated ship established by experiment 95. The squares of the by several other experiments. figure show that the agreement is surprisingly good, In 1,2-dimethoxyethane the directly determined nearly all points falling between the theoretical curves values of K T P E , N ~scatter between 0.45 and 1.2 for most , ~0.50 ~ and K T p E , N a = 1.00 with more near for K T P E = of the points reported, which are typical of many more the latter value. The agreement of this figure with our actually obtained. At values of [ T P E ] / {T P E } greater previous estimate ( K T P E < , ~1.8 ~ ; the calculated figure, than 0.90, larger equilibrium constants resulted, in assuming no errors in measurement, was ().4),lemployspite of the fact that the simultaneous use of thin and ing a different and independent method (e.s.r. intensity thicker cells permitted the accurate determination of measurements), is gratifying. Data from which the widely different optical densities. Similar variations points of Fig. 2 can be identified and from which the appear in the tetrahydrofuran and diglyme data. original concentrations of all species can be computed, While this scatter seems a t least semisystematic, with the aid of Fig. 2, are given in Table I . it is difficult to account for the trends in any reasonable fashion. The variations are too large to be a result of TABLE I simple curve analysis errors. DISPROPORTIOKATION OF h f O N O S O D I U M TETRAPHENYLETHYLENE One possibility which seems capable of accounting Experi~TPE In ~TPE I/ ment Solvent X 10’ TPE KTPE.N,~ for the l,2-dimethoxyethane data, a t least, is that T P E has a hidden band a t 3900 A. with an extinction coeffi95 1,2-Dimethoxyethane 33.5 0.94 i 33.5 46 1.1 cient comparable to that a t 6600 k . If so, values of 33.5 .22 0 77 K T P E , calculated ~~ for ratios [ T P E ] / (T P E ) near 1.0 33.5 .05 72 would be significantly larger than those calculated from 108 8.56 .62 .71 data corresponding to smaller ratios [ T P E ] { T P E ] , 109 12.5 59 81 which would be more precise. For this reason we have 6.0 31 .so chosen to omit the largest values in our averaging of 113 12.7 74 1 2 K T p E , N a in Table I. This possible factor was also con115 116 94 9.0 sidered in evaluating the reliability of K T P E , N deter~ 10.7 .63 1.1 mined in solvents tetrahydrofuran and diglyme (see 9 88 .39 0 i3 above) from various experiments. 4.48 26 0.45 116 158 96 11.5 While there is evidence that N a T P E ion pairs, a t 1.2-Dimethoxyethane average 0.84‘ least, may be to some extent dissociated, the system 80 Diglyme 180 0.64 1 0 comes surprisingly close to obeying the simple equilib107 Tetrahydrofuran 15 8 0.64 4Zd rium law related to eq. 4. We will assume eq. 4 as the Dioxane, I(T~E,N~ too large for measurement‘ basis for our present discussion.I7
by a factor of 34 (from {TPE) = 8.87 X ill and { N a ) = 5.68 X A f to ( T P E ) = 2.98 X 10Y3 Ad and { N a } = 1.87 X lop3M ) , variations in the relative intensities of the bands were observed. The equilibrium “constant,” calculated as if eq. 4 correctly represented
7
Diethyl ether, KTPE,S% too large for measurement’ ( T P E I = formal concentration of T P E . Evaluated directly from the equilibrium expression. e Average omits those points corresponding t o [TPE]/[TPE J greater than 0.90. Reference 1Oc reports K T P E . N at~30’ as 2800, in pronounced disagreement with the value given here.’$ e Estitnated from the sensitivity of t h e spectrophotometric method as being greater than lo5. Estimated from e.s.r. sensitivity’ as being greater than lo5 and probably greater than lo6. a
’
The results in solvents other than 1 ,?-dimethoxyethane are less extensive. No equilibrium constant (16) A . C. Aten. V . IXeleman, a n d G . J . Hoijtink, Discussio~isF Q T ~ Q Y Soc., 29, 182 (1960).
Discussion Four factors have been previously mentioned as possibly contributing to the anomalously large value for K T P B , N ~ : (1) solvation energies could favor the dianion, a corollary being that more polar solvents would lead (2) ionic aggregation to an increase in KTpE,xa4RC; effects could favor the dianion8”; ( 8 ) the difference be( 1 7 ) If X a T P E ion pairs a r e appreciably dissociated, h u t Sa?TPIiti-iple ions a r e not,a”b t h e intei-pretatian of t h e solvent a n d metal ion influences discussed helow follows along nearly identical lines a n d leads t o t h e same conclusions
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F. GARST,ERNEST R. ZABOLOTNY, A N D RONrlLD S. COLE
Vol. 86
tween the electron-electron repulsion energies in planar adopted in the gas phase, and may vary from one solvent TPE-' and those in TPE-' could be less than the simito another. The ionic aggregates of eq. 6 and 7 are lar differences for other hydrocarbons4; and (4) the considered here to have the same interionic geometries energy of TPE-* might be even lower than predicted as those finally adopted by the same aggregates in the for a planar molecule, due to possible changes (not availsolvent in question. If there should be no differences able to other hydrocarbon anions) in g e ~ m e t r y . ~ , * ~ - ~between the geometric dispositions of the ions of aggreThe order of polarities of the ethers employed in the gates of minimum free energy in various solvents, 6'2 present investigation appear to be diethyl ether < will vanish. dioxane < tetrahydrofuran < 1,2-dimethoxyethane < If the ions were completely dissociated in some soldiglyme.'j Thus, the trend of K T P E with , ~ ~solvent is vent, 6'2 would vanish and 6'3 would be predicted, on the opposite of t h a t predicted by hypothesis 1 above. the basis of the Born charging model, to be negative, If solvation effects are important, one may reasonably reflecting solvation energy of a dianion (TPE-') which conclude that i t is the ion pair and triple ion solvation is larger than that of two monoanions (TPE-) of the energies which should be considered, since there is a same size.4 An increase in solvent polarity would distinct metal ion effect on K T p ~ .l@r,ll I I* The facts therefore lead to a larger value of K T P E , This ~ ~ . is~ not are consistent with hypothesis 2 above, and i t is quite in accord with observation. likely that the factors of hypotheses 3 and 4 may also If the ions were completely associated and if there were apply to the consideration of the magnitude of KTPE,BR. no changes in the interionic geometry from one solvent Aggregation and Solvation Effects.-The following to another, 6'2 would again vanish from eq. 12, leaving scheme affords one view of the factors influencing the only 6'3 for consideration. Term 3 is zero in the gas equilibrium constant and provides a basis for the disphase and should be positive in solvent, since the solvacussion of the effects involved. The A F o ' s correspondtion energies of higher aggregates (which probably have ing to the following series of hypothetical reactions are no dipole moments) should be less than the solvation viewed as coniponents of A F o 4 . For convenience of energies of ion pairs. Furthermore, if the solvation discussion, parenthetical groupings of terms have been energies are simply proportional to some function reindicated (eq. l l a ) and the resulting four terms have flecting solvent polarity, as is commonly presumed, 6'3 will also be positive, terms 3 becoming monotonically BTPE-(gss) r'TI'E-2(gas) + T P E ( g a s ) (5) more and more positive in more and more polar media. TPE-(gas) MT(gas) MTI'E(gas) (6) Thus it is predicted that KTpE,M should be smaller in more polar solvents, in accord with observation. TPE-2(gas) + 2M*(gas) M?TPE(gas) (7) Situations which might involve incomplete ionic assolvent sociation are inore complex, b u t can be treated in the M T P E ( g a s ) yMTl'E(so1ution) (8) same general fashion outlined here. Simple considerasolvent tions suggest that higher degrees of ionic dissociation M ? T P E ( g a s )7 -MsTPE(solution) (9) ought to correspond to lower values of KTPE,M. The consideration of possible solvent dependent insolvent TI'E(gas) TPE(so1ution) (10) terionic geometry variations in aggregates introduces another complication. "Loosening" of the aggregates been given numerical symbols (eq. 1l b ) . If 6s is a differis the major such variation which might be important. ence operator reflecting a change to a more polar sol"Loosening" will occur only if the accompanying increase vent,Ig eq. 12 applies, 6'1 is zero, and 6"4 is neglected. in solvation energy (due to increased electric moments of the aggregate and to decreased steric hindrance to the AFo4 AF'j (AF",- 2AFotj) solvation of the ions by their partners) compensates for ( A F o 9 - 2AFos) AFolo ( I l a ) it. I t is difficult to estimate rationally the magnitude or sign of 6'2, but its appearance in eq. 12 reflects a AFo,=1+2+3+4 (11b) possible mechanism b y which the direction of the solvent S'AF'4 = 6'2 8'3 (12) effect might be reversed. Since the direction is properly accounted for by the assumption that 6'2 is zero, we While the symbol Fo will be employed in the following assume that its actual value is smaller than and/or of discussion, most of the deductions will actually be based the same sign as 6'3, on considerations of potential energy, the tacit assumpI t should be noted that since term 3 of eq. 11 is tion being that the free energies will reflect these varipositiw, and since AF04 in solution is much less than ations. The analysis may therefore be in error for any predicted from neglecting terms 2, 3, and 4, it is clear situation for which that assumption fails. that either term 2 plays a dominant role in the deterIf eq. C, and 7 are understood to relate to products mination of APoain solution or that the estimation of which adopt geometries of minimurn free energy in the term 1 did not take into account some pertinent factor, gas phase, 6'2 is zero. For the purposes of this discusor both. sion, however, we presume that the ionic configurations Term 2 should be negative, as required from electroof the aggregates may be different in solution from those statics, if the distances of separation of counterions are (18) Unpublished irsults f i o m t h e a u t h d s laboratories about the same in the ion pair as in the triple ion. I t ( ] $ I ) F
