1BQO
J. Pola, G. Levin, and M. Szwarc
(15) M. Brousseiy, These doctorat, Poitiers, 1975. (16) J. K. Thomas and R. V. Bensasson, J. Chem. Phys., 46,4147 (1967). (17) P. Debye and E. Huckei, Phys. 2..24, 185 (1923); E. A. Guggenheim, Phil. I%& 19, 588 (1935). (18) P. Pascal, "Nouveau Trait6 de Chimie Minerale", Masson et Cie, Paris, 1956, tome X, p 560; T. W. B. Welsh and H. J. Brcderson, J. Am. Chem. SOC., 37, 816 (1915). (19) K. H. Buschow, J. Dieleman, and G. I. Hoijtinck, Mol. Phys., 7, 1 (1963). (20) G. I. Hoijtinck, Chem. Phys. Lett, 26, 318 (1974). (21) J. P. Keene, E. J. Land, and A. J. Swallow, J. Am. Chem. Soc., 87, 5284 (1965). (22) J. K. Thomas, K. Johnson, T. Kiippert, and R. Lowers, J. Chem. Phys., 48, 1608 (1968). (23) J. H.Fendler, H. A. Giilis, and N. V. Kiassen, J. Chem. SOC.,Faraday Trans. 1, 70, 145 (1974).
(24) J. M. Warman, M. P. de Haas, E. Zador, and A. Hummel, Chem. Phys. Left., 35, 383 (1975). (25) D. Gill, J. Jagur-Orodzinski,and M. Szwarc, Trans. Faraday SOC.,60, 1424 (1964). (26) K. L. Schmidt and W. L. Buck, Science, 151, 70 (1966). (27) E. C. Evers and P. W. Frank, J. Chem. Phys., 30, 61 (1959). (28) L. M. Dorfman and F. Y. Jou. "Electrons in Fluids", J. Jortner and N. R. Kestner, Ed.. Springer, Berlin, 1973, p 447. (29) J. Beiloni and M. Haissinsky, lnt. J. Radiat. Phys. Chem., 1,519 (1969); E. Hayon and M. Simic, J. Am. Chem. SOC.,94,42 (1972). (30) M. Schiavello and G. G. Voipi, J. Chem. Phys., 37, 1510 (1962). (31) R. K. Wolff, J. E. Aldrich, T. L. Penner, and J. W. Hunt, J. Phys. Chem., 79, 210 (1975). (32) C. D. Jonah, M. S. Matheson, J. R. Miller, and E. J. Hart, to be submitted for publication.
Equilibrium and Kinetic Studies of Disproportionation of Sodium Tetracenide in Benzene. The Effect of Added Tetrahydrofuran J. Pola, G. Levin, and M. Stwarc* Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210 (Received February 17, 1976) Publication costs assisted by the National Science Foundation
The equilibrium and the kinetics of disproportionation of sodium tetracenide (Te.-,Na+) in benzene containing small amounts of tetrahydrofuran (THF) was investigated. It was shown that the equilibrium is represented by the stoichiometric equation, 2Te.-,Na+,(THF), @ Te Te2-,2Na+,(THF)2,-2 2THF and the formal equilibrium constant Kdispr = [Te][Te2-,2Na+]/[Te.-,Na+l2 varies from 400 at very low concentration of T H F to in bulk THF. The kinetics of disproportionation was investigated by flash-photolytic technique leading to the value of k-dispr of 1.5 X lo9 M-l s-l for the reaction Te2-,2Na+,(THF)zn-2 Te 2Te--,Na+, (THF),- 1.
+
+
+
It is generally believed that the disproportionation of planar aromatic radical anions, a reaction that yields dianions and the neutral hydrocarbons, is highly endothermic and its equilibrium constants are very l ~ w . l Examples -~ to the contrary are but all of them refer to systems in which the geometry of a t least some of the reacting species is drastically changed; this allows us to understand the preference in these systems for dianions over the radical anions. It was surprising, therefore, to find out7 that the disproportionation of the planar tetracenide radical anions (Te--,Cat+), derived from the planar hydrocarbon, is favored in diethyl ether, e.g., Kdispr of lithium tetracenide in this solvent exceeds 1 2Te--,Cat+
* Te + Te2-,2Cat+
Kdispr
In fact, as shown by the data collected in Table I, the disproportionation equilibrium of Te.-,Cat+ is strongly affected by the nature of cation, and even more by the nature of solvent for reasons discussed elsewhere7 (see also ref 2 and 3). We wish now to report that the disproportionation constant of sodium tetracenide is even larger in benzene and its value exceeds 400. Moreover, we wish also to describe the spectacular changes in the value of Kdispr in benzene resulting from the addition of small amounts of tetrahydrofuran (THF). Results Tetracene in T H F solution was reduced on a sodium mirror. The reduction was not carried out to completion and the The Journal of Physical Chemistry, Voi. SO, No. 15, 1976
-
concentrations of the radical anions and of the unreduced hydrocarbon in the resulting solution were determined spectrophotometrically. Thereafter, T H F was distilled off and an equal volume of benzene was distilled in. All these operations were performed on a high-vacuum line. The change of solvent led to a dramatic result. The concentration of the hydrocarbon in the benzene solvent substantially increased (the increase corresponded to about one-half of the original concentration of Te--,Na+). For example, a prepared T H F solution was 1.6 X 10-5 M with respect to Te and 6.1 X 10-5 M with respect to Te.-,Na+. After replacement of T H F by benzene the concentration of Te increased to 4.8 X M, while the conversion of 2Te--,Na+ to Te Te2-,2Na+ would make the concentration of tetracene equal to 11.6 (0.5)6.1] X 10-5 = 4.65 X M. This implies that the reaction
+
+
2Te.-,Na+
-
Te
+ Te2-,2Na+
converts nearly all the Te.-,Na+ into Te2-,2Na+. Due to its low solubility, only a fraction of the formed Te2-,2Na+ was dissolved, its concentration was found to be 0.22 X M. However, in spite of the large excess of Te, no Te.-,Na+ was detected in the investigated solution, while a concentration -5 X 10-7 M still could be measured. From such spectrophotometric data we conclude that the disproportionation constant in benzene is greater than 400. Reduction of tetracene in benzene containing small
1691
Disproportionationof Sodium Tetracenide in Benzene
2Te.-,Cat+
2 Te + Te2-,2Cat+
Solventa THF THF THF THF DOX DOX DOX DOX DEE .DEE
Counterion
K+ Cs+
Li+ Na+
1.1x 10-2
K+
6.5 x 10-3 1.6 X 10 1.2 x 10-1
Cs+ Li+
Na+
1.3
3.6 X 10
5.5 x 104
-
3.0 x 104 2.5 x 104 6.0 X lo6 2.0 x 107
6.3 x 109 5.5 x 109 6.5 x 109 7.8 x 109 1.1 x 108
3.1 X lo8
c
C
C
C
5.9 x 107 2.4 X lo8
3.6 X lo6 2.0 x 109
TABLE 11: Effect of the Added THF on the Equilibrium and Rate of Disproportionation of Sodium Tetracenide (Te.-,Na+) in Benzene (-20 "C)" k & Te + Te2-,2Na+
k'dispr, Kdispr
0.132
6
(2.9 f 0.4) X
0.216
4
0.378
3
0.49
1
(8.0 f 0.3) X 10-3 (3.9 f 1.0) X 10-3 2.3 x 10-3
M-'
Kdispr
0.8
0.6 0.4
03 -10
- 9
-8
- 7
-6
-5
-4
-3
-e vs.
log [THF].
k-dispr,
M-l
S-1
S-1
1.2 X lo7
(1.5 f 0.1) X 109 (1.7 f 0.2) X 109
10-2
6.6 X lo6
0.7
Figure 1. Plot of log Kdispr= log [Te][Te2-,2Na+]/[Te.-,Na+]2
k-1
No. of [THFI, M expt
in BENZENE + THF
-
-
k 1, M-l s-'
a THF = tetrahydrofuran; DOX = dioxane; DEE = diethyl ether. The low Kdispr are determined by potentiometric titrations. The higher values are obtained spectrophotometrically. c The DOX solutions of these salts are not photobleached.
2Te--,Na+
I
Kdispr
5.8 x 10-9 1.0x 10-5 4.6 X 3.2 X 6.6 X lod2 6.5 X
Li+ Na+
I
2Te', Na* t Te Te2; 2Na+
Kdispr
k-1
I
I
TABLE I: Equilibrium and Rates of Tetracenide (Tea-) Disproportionation (-20 " C )
We suggest, therefore, that T H F solvates the ionic species and the observed disproportionation is given by the stoichiometric equation 2Te.-,Na+,(THF),
* T e + Te2-,2Na+(THF)zn-2 +2THF K
We imply, also, that the concentration of the unsolvated Te.-,Na+ and Te2-,2Na+ is vanishingly small in solutions containing small amounts of THF, an assumption justified a Note for the sake of comparison that Kdispr = 1.0 X in by the observed low concentration of Te2-,2Na+, and the pure THF and >400 in benzene. undetectable concentration of Te--,Na+, in benzene free of THF.11 amounts of T H F yields a mixture of Te, Te.-,Na+, and By applying the flash-photolysis technique described in Te2-,2Na+. The concentrations of these reagents were deearlier papers,g we investigated the kinetics of disproportermined spectrophotometrically, and the values of the formal tionation. A flash of visible light ejects electrons from K d j s p r were calculated as [Te] [Te2-,2Na+]/[Te.-,Na+I2. For Te2-,2Na+ and subsequently some of them (e-,Na+) are a constant concentration of T H F the computed, Kdispr)S were captured by tetracene yielding Te.-,Na+. Thus the concenfound to be unaffected by dilution. For example, in benzene tration of Te2-,2Na+ and of T e decreased after a flash while containing 0.216 M THF, the concentrations of Te, Te.-,Na+, the concentration of Te.-,Na+ increased accordingly. The and Te2-,2Na+ were determined in a 0.4-mm cell to be 1.23 equilibrium 2Te--,Na+ e Te Te2-,2Na+ is upset and the X 1.48 X and 1.54 X M, respectively. By using return to the equilibrium, monitored in the dark period after the technique described elsewhere,s the investigated solution the flash, allows us to determine the kinetics of disproporwas 200-fold diluted, without adding any fresh solvent, and tionation. The results are included in Table 11. The rate conthe concentrations of the reagents were redetermined in a 10 stant k-dispr refers to the reaction cm-long cell. They were found to be 5.8 X IO-?, 5.3 X 10-6, and k-dispr 2.6 X M. Thus, the latter data give K d i s p r = 5.3 X Te Te2-,2Na+(THF)zn-2 --+ 2Te.-,Na+(THF),-l while the former lead to the value of 8.6 X M, both values being substantially smaller than the K d i s p r obtained in the while the calculated rate constant h'dlspr is based on the values absence of added THF. of Kdlspr given in Table 11.Thus, h'dlspr = k d i s p r / K ~ 2 [ T H F ] z , The results of such studies collected in Table I1 demonwhere kdispr denotes the rate constant of the reaction strate that the formal K d i s p r decreases with increasing concentration of THF. The plot of log K d i s p r vs. log [THF] is 2Te.-,Na+(THF),-l* Te Te2-,2Na+(THF)z,-2 shown in Figure 1; it is linear with slope of -2. This implies that the formal Kdispr is given by a "true" K/[THFI2, Le. and the K s refers to the equilibrium
+
+
+
Kdispr
= [Te2-,2Na+][Te]/[Te--,Na+]2 = K/[THF]2
Te.-,Na+(THF),-l+
T H F G Te.-,Na+(THF),
The Journal of Physical Chemistry, Vol. 80, No. 15, 1976
1692
J. Pola, G.Levln, and M. Srwarc
Discussion Assuming that the linear relation between log Kdispr and log [THF] reported in this paper is valid over a much wider range of concentrations of T H F than explored here, we may calculate the concentration of T H F in our “pure” benzene. The extrapolation to Kdispr = 400 leads to the [THF] = 1 X 10-3 M, and hence the concentration of THF in our “pure” benzene is lese than M. This is a reasonable result. Extrapolation to “pure” T H F ( ~ 1 M) 0 leads to a value of Kdispr = 5 X in a surprisingly good agreement with the experimentally determined7 value of 1X 10-6. This agreement may be fortuitous since the structure of the pertinent ionic species in bulk T H F is probably different than that attained in a dilute solution of T H F in benzene. What is the value of n , the number of T H F molecules solvating Te.-,Na+ in dilute benzene solution of THF? Since the solubility of Te2-,2Na+ in benzene containing T H F is much higher than in the rigorously purified benzenell this aggregate has to be solvated by THF and thus 2n 2 > 0, Le., n > 1. Most probably n = 2 and this assumption gains some support from the studies of solvation of sodium naphthalenide by T H F in diethyl ether.10 In our approach we implicitly assume that the concentration of Te.-,Na+ solvated by less than n molecules of THF, or Te2-,2Na+ solvated by less than (2n - SITHF, is vanishingly small. This seems to be plausible. Moreover, we tacitly assume that the reaction
-
Te.-,Na+,(THF),-l
+ THF
Te.-,Na+,(THF),
It seems that the “nonsolvated” Te2-,2Na+ aggregate absorbs at somewhat shorter wavelength than Tez-, 2Na+,(THF)zn-2, because the absorption spectrum of a solution obtained by reducing T e with metallic sodium in our “pure” benzene shows two broad peaks, at 580 and 620 nm, respectively. The latter band is observed in the T H F solution of Te2-,2Na+, and hence i t is attributed to Te2-, 2Na+,(THF)2,-2, while the former apparently arises from the “nonsolvated” Te2-,2Na+. This solution seems to be saturated in respect to the “nonsolvated” Te2-,2Na+, Consequently, the addition of T H F to such a solution if maintained in contact with a sodium mirror leads to an increase in the intensity of the 620-nm band but not of the 580-nm peak. This observation confirms the proposed assignment of the two absorption bands to the “nonsolvated” and THF-solvated Tez-,2Na+. In conclusion, we wish again to emphasize how important is the knowledge of the structure of ionic species and of their modes of solvation in understanding of their behavior and reactivity and how sensitive such systems may be to changes in the degree of solvation. Acknowledgment. The financial support of these studies by the National Science Foundation is gratefully acknowledged. Dr. J. Pola, who was on leave of absence from the Institute of Chemical Processes Fundamentals, Prague, Czechoslovakia, wishes to thank the U S . National Academy of Sciences and the National Academy of Sciences of Czechoslovakia for supporting his stay in Syracuse,
-+
+
is much faster than the reaction Te Te2-,2Na+,(THF)zn-2 that yields ZTe.-,Na+,(THF),-l. This assumption is reasonable in view of the relatively high concentration of THF, and hence the rate-determining step in the observed dark reaction is the disproportionation. What is the value of & & p r of Te.-,Na+ in a truely pure benzene, rigorously free of THF? Unfortunately, our data do not provide information to this question. This disproportionation constant, referred to as &ispr,B, is related to the formal Kdispr measured in our studies by the equation
References and Notes
(1) N. S. Hush and J. Blackledge, J. Chem. Phys., 23, 514 (1955). (2) (a) G. J. HoiJtink,J. van Schovten, E. de Boer, and W. I. Aalbersgerg, Recl. Trav. Chim. Pays-Bas, 73, 355 (1954); (b)G. J. Hoijtink and J. van Schooten, bid., 71, 1089 (1952); 72, 691, 903 (1953); (c) G. J. Hoijtink, E. de Boer, P. H. Van der Meij, and W. P.Weijland, bid., 75, 487 (1956). (3) (a) J. Jagur-Grodzinski, M. Feld, S. L. Yang, and M. Szwarc, J. Phys. Chem., 69, 628 (1965); (b) A. Rainis and M. Szwarc, J. Am. Chem. Soc., 96,3008 (1974); (c) R. S. Jensen and V. D. Parker, bid., 97, 5619 (1975). (4) (a) J. F. Garst and E. Zabolotny, J. Am. Chem. Soc., 67,495 (1965); (b) R. C. Roberts and M. Szwarc, bid., 87, 5542 (1965). (5)E. R. Zabolofnyand J. F. Garst, J. Am. Chem. Soc., 86, 1645(1964); 88, 3872 (1966). (6) (a) T. J. Katz, W.H. Reinmuth, and D.E. Smith, J. Am. Chem. SOC.,84,802 (1962); (b) H. L. Strauss, T. J. Katz, and G. K. Fraenkel, ibid., 85, 2360 (1963); (c) F. J. Smentowski and G. R. Stevenson, J. Phys. Chem., 73, Kdispr,B = K~~~~~Ks,T~~-/KS,T~.-’[THF]’ 340 (1969); (d) G. R. Stevenson and J. G. Concepcion, /bid., 76, 2176 (1972). where Ks,T~.-and K s , T ~ zrefer - to the respective equilibria (7) G. Levin and M. Szwarc, J. Am. Chem. Soc., 96, 421 1 (1976). (81 0.Rarnme. M. Fisher. S. Claesson. and M. Szwarc, Proc. R. SOC.London, Ser.-A, 327, 467 (1972). Te.-,Na+ f n T H F F! Te.-,Na+,(THF), Ks,T*(9) (a) G. Levin, S. Claesson, and M. Szwarc, J. Am. Chem. Soc., 94, 8672 (1972): fb) B. Lundaren. G. Levin, S. Claesson, and M. Szwarc, ibld., 97, and 262 (1975). (IO) L. Lee, R. Adams, J. Jagur-Grodzlnski, and M. Szwarc, J. Am. Chem. SOC., 93, 4149 (1971). Te2-,2Na+ (2n 2)THF (11) In fact, even the THF “free” benzene contained a minute amount of F! Te2-,2Na+(THF)z,-2 KS,TeZTHF. \
+
-
The Journel of Physical Chemlstry, Vcl. 80, No. 16, 1976
,
