J. JAGUR-GRODZINSKI, M. FELD, S. YANG,AND M. SZWARC
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Electron Affinities of Aromatic.Hydrocarbons in Tetrahydrofuran Solution
by J. Jagur-Grodzinski, M. Feld, S. L. Yang, and M. Szwarc Departntent of Chemistry, State University College of Forestry, Syracuse University, Syracuse, New York 1%?iO, and Donnan Laboratories, The University, Liverpool, England (Received November 12, 1964)
The relative electron affinities (= relative reduction potentials) of a series of aromatic hydrocarbons were determined in tetrahydrofuran (THF) solution. Two methods were employed: (1) potentiometric titration and (2) spectrophotometric studies of equilibria, aromaticz. Potentiometric t,itration, originally aromatic1 aromaticz- 2 aromatic17 developed by Hoijtink, was improved. The effect of various factors, not considered previously, was discussed, and special attention was paid to the fact that the radical ions exist essentially as ion pairs, and not as free ions. Hoijtink’s treatment implicitly assumes a complete dissociation of ion pairs into ions. The final results show a good agreement between both methods. The results lead to the electrochemical series reported by Hoijtink, but numerically the potentials found in our study are substantially lower than those reported previously. The interesting case of tetraphenylethylene is discussed. In this compound the two-electron reduction potential (Ez) is higher than the first one (El). The reason for this reverse in the order of reduction potentials is suggested, and its effect on the potentiometric titration curve is elucidated.
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Relative electron affinities of aromatic hydrocarbons were determined by Hoijtink, et a1 .,1 who developed for this purpose a potentiometric technique. In the same year Paul, Lipkin, and Weissman2 reported their spectrophotometric studies of the equilibria of electron-transfer processes, such as phenanthrenenaphthalene phenanthrene naphthalene-, from which they deduced the values of the relative electron affinities for the same series of hydrocarbons. Unfortunately, their data differed greatly from those reported by Hoijtink, thus raising the question of which method is reliable. Both methods were used recently by our group to determine the relative electron affinities of pyrene, anthracene, and 9,lO-dimethylanthracene* in tetrahydrofuran (THF). Self-consistent results were obtained and these were confirmed by independent kinetic studies.3 However, since our reduction potentials somewhat differed from those published by Hoijtink,‘ we decided to reinvestigate the whole problem and to extend our studies to other aromatic hydrocarbons in order to ascertain the reliability of each method.
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The Journal of Physical Chemistry
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Experimental Potentiometric Titrations. Hoijtink’s apparatus and procedure were slightly modified. The unit used by us is shown in Figure 1, which is self-evident. The buret was terminated by a 2-mm. bore capillary, its lower tip being sealed and then punched with a needle to form six or seven tiny, parallel capillaries. This arrangement considerably slowed down the diffusion of the liquid from reactor R to the upper platinum wire electrode which was touching the sealed tip. The resistance of the unit, when filled with a 0.016 M THF solution of sodium biphenyl, was about 10 megohms. The potential between electrodes was measured by a valve voltmeter4 which was described by Scroggie.6 (1) G. J. Hoijtink, E. De Boer, P. H. van der Meij, and W. P. Weijland, Rec. trav. chim., 75, 487 (1956). (2) D. E. Paul, D. Lipkin, and S. I. Weissman, J. A m . Chem. SOC., 78, 116 (1956). (3) D. Gill, J. Jagur-Grodzinski, and M. Szwarc, Trans. Faraday Soc., 60, 1424 (1964). (4) wish to thank Dr. A, Hickling of Liverpool University for provlding us with this device and for his many valuable comments. ( 5 ) M. G. Scroggie, Wireless World, 14 (1952).
ELECTRON AFFINITIESOF AROMATIC HYDROCARBONS IN TETRAHYDROFURAN
UM ,_PURIFIED HELIUM
IOml. BURETTE
PILLARY RY TIP
Figure 1. Apparatus used in the potentiometric titration of aromatic hydrocarbons.
In function it is essentially a very stable impedance converter by means of which an input d.c. voltage in a high resistance circuit is converted into an identical output d.c. voltage in a very low resistance circuit. The latter can be measured by any conventional voltmeter with a resistance exceeding 400 ohms. The current drawn from the source by the converter is less than 10-10 amp., and therefore its reading is reliable even if the resistance of the external circuit exceeds 100 megohms. To secure the maximum stability of the device, the input terminals are bridged by a 0.01-pf. capacitor, and the negative input terminal is grounded through the case of the instrument. The voltmeter was calibrated, and the results seem to be reliable within 1%. For a low potential of about 0.04 v. the accuracy is about 0.002 v. All the aromatic hydrocarbons used in this work were carefully crystallized and then sublimed under high vacuum. Their solutions were prepared on a high-vacuum line and sealed in small tubular ampoules equipped with break-seals. The purification of THF is described in ref. 6 . The sodium biphenyl solution, used in the titration, was prepared by overnight reaction at room temperature of a 0.1 M solution of biphenyl in THF with a sodium mirror. The reaction proceeds to 16% conversion, and at this stage the system seems to reach its equilibrium. The concentration of sodium biphenyl was determined by titrating aliquots of the solution with HCI or with methyl iodide. Both methods gave concordant results. The prepared solution was
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stored in an ampoule equipped with a break-seal, the latter eventually being sealed to the titration unit (ampoule AI in Figure 1). The following procedure was used during the titration. After sealing ampoule AI containing the biphenylsodium biphenyl mixture (in 5 : l mole proportions) and the six P ampoules containing the investigated hydrocarbons, the whole unit was evacuated through the three-way stopcock TO. Thereafter, by turning this stopcock, purified helium, bubbling through a sodium biphenyl solution, was admitted. The unit was again evacuated, the break-seal on ampoule AI crushed, and its contents quantitatively transferred into ampoule Az. The whole unit was then repressurized with helium, and the titrating solution was introduced into the buret of 10-cc. capacity. About 10 cc. of the solution was introduced into reactor R, stirred magnetically, and eventually sucked out through stopcock Ta. Thus, the residual moisture adsorbed on the walls of the reactor was purged. Next, 10 cc. of solution were introduced into the reactor, and the potential was measured. It was found that in all the blank runs no potential difference was detected, indicating the absence of any polarization of the electrodes. The second batch of sodium biphenyl solution was then sucked out, and one of the investigated solutions of the aromatic hydrocarbons was transferred into the reactor by crushing the appropriate break-seal. This was titrated by adding, under stirring, the sodium biphenyl solution in 0.5- or l-cc. increments. About 1 sec. after each addition, the potential, which became constant, was read, and its value was plotted
Figure 2. Potentiometric titration: x, tetraphenylethylene; , anthracene. (6) J. J a m , M. Levy, M. Feld, and M. Szwarc, Trans. Faraday SOC., 58, 2168 (1962).
Volume 69, Number I February 1966
J. JAGUR-GRODZINSKI, M. FELD, S. YANG,AND M. SZWARC
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Table I: Reduction Potentiah” for Aromatic Hydrocarbons in T H F Solution a t 25O. Biphenyl Mixt,urein Molar Ratio 5:1; Concentration of Sodium Biphenyl, 0.016 M -Our Aromatic hydrocarbon
Expt. no.
Biphenyl Naphthalene Triphenylene Phenanthrene Pyrene 9,lO-Dimethylanthracene Anthracene Perylene Tetracene
10 5 4 4 2 4 3 3
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El,V.
reduction potentials---E%,v.
(0.0) 0.066 f 0.02c 0.113 f 0.01 0.124 f 0.005 0.505 f 0.005 0.607 f 0.005 0.624 f 0.005 0.917 f 0.005 1.025 f 0.01
Titrated by BiphenylSodium
Hoijtink’s reduction potentialsb
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... ... ... ... 0.34 f 0.01 0.33 f 0.01 0.56 f 0.01
Ei, v.
Ez,v.
(0.0) 0.09 0.19 0.17 0.60
... ... ...
...
... ...
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0.78 (in THF0.74) 1.09 1.28 (in THF 1.28)
0.20 (in T H F 0.40) 0.46 0.66 (in THF 0.82)
To all the observed potentials 0.04 v. was added to correct for the standard potential corresponding to a 1: 1 mi.rture of biphenylsodium biphenyl. Hoijtink’s potentials were determined in dimethoxyethane solution and not in THF. He assumed also that the ratio biphenyl to sodium biphenyl is 1:I. This seems to be erroneous. Corrected for the equilibrium biphenylnaphthalene e biphenyl naphthalene-; Le., 80% more than 0.5 equivalent of B - has to be added to give N - / N = 1. Q
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against the volume of the added liquid. Such plots are shown in Figures 2 and 3. After completion of the titration, the contents of the reactor were sucked out, and the next sample of the aromatic hydrocarbon was introduced and titrated. This arrangement has a twofold advantage. (1) Three different aromatic hydrocarbons could be titrated in duplicates without exposing the electrodes to the air. (2) The same solution of sodium biphenyl was used for all these titrations. It is believed, therefore, that for such a series of experiments the differences in the reduction potentials should be very reliable. To ascertain the reliability of this setup, the experiments were staggered; viz., any investigated hydrocarbon was retitrated after the others were investigated.
Results The reproducibility in each series of titrations was better than =k0.005v., and on repetition of the whole experiment the measured voltages were reproduced within 10.01 v. I n some experiments the duplicate titrations were performed with doubled concentrations of the investigated hydrocarbons-the results being unaltered by such a change of conditions. The potential, measured at the stage of an experiment for which the ratio H y : H y - = 1, is recorded in Table I as the respective relative reduction potential, El, Hy and Hy- denoting the concentrations of the investigated hydrocarbon and of its radical anion, respectively. In most titrations this stage is attained when the amount of the added biphenyl- is equal to one-half of the titrated hydrocarbon, and, as seen in Figures 2 and 3, the relevant point is the center of the plateau of the respective experimental titration The Journal of Physical Chemistry
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Figure 3. Potentiometric titration: perylene.
curve. However, in titration of naphthalene a correction is introduced to account for the equilibrium,’ naphthalene naphthalenebiphenylbiphenyl .. .K. Let us denote by the amount of the biphenyl- necessary to convert one-half of the, titrated naphthalene into naphthalene-; by N-l,2 the amount of resulting naphthalene?, and by f, the ratio BIB- in the original solution of biphenyl-. Then (!BO N-i/z)/(B-o - N-i/J = K ; and therefore 1 x = B - o / N - I / ~= ( K l)/(K - f), where x is the required excess of B-. In our titrations f = 5, and from the titration curves one calculates K = 12.6 and x = 0.8. A further check for our data was provided by the titrations of some hydrocarbons with 1:l solutions of
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(7) Those corrections were not considered by Hoijtink.
ELECTRON AFFINITIESOF AROMATIC HYDROCARBONS IN TETRAHYDROFURAN
napht,halene--naphthalene or pyrene--pyrene. The relevant results are collected in Table 11. To evaluate the titration of terphenylene and phenanthrene with napht,halene- solution, it was again necessary to introduce the relevant corrections. For terphenylene, K = 3 (a value derived from the spectrophotometric studies, see Table V), and since f = 1, one finds z = 1.0. For phenanthrene, K = 10 (derived from the data given in Table I), and therefore x = 0.6.
Table JI : Reduction Potentials for the Aromatic Hydrocarbons in T H F Solution a t 25' AE1 from biphenyl-
Aromatic hydrocarbon
El'
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titration, v.
Titrated by 0.0082 N solution of naphthalene- ( N - / N = 1:1) Triphenylene 0. 062b 0,047 Phenanthrene 0. 075b 0.058 Titrated bv 0.0088 N solution of p'yrene- ( T - / T = 1:1) Anthracene 0.119 0.09010.081 9,10-Dimethylanthracene 0.102 0.070 a A E is ~ derived from Table I by subtracting E of the hydrocarbon used in the standard solution (naphthalene or pyreae) from the E of the titrated hydrocarbon. Corrected for the equilibrium between the titrating ion and the formed ion. This means that a 100% excess of naphthalene- solution has to be used in titrating terphenylene and 60% in titrating phenanthrene.
No corrections are necessary in evaluating the results of the titration of anthracene and dimethylanthracene by the solution of pyrene-. The agreement between the data given in Tables I and I1 is fair. The titrations with the solution of naphthalene- led to reduction potentials for terphenylene and phenanthrene which are -0.025 v. lower than those derived from the titrations with biphenyl-. The titrations of anthracene and dimethylanthracene with the solution of pyrene- yielded results 0.03 v. higher than the titration with biphenyl-. Various factors may contribute to these small discrepancies, and in the following section some of them wiU be considered. Some General Problems Concerned with the Potentiometric Titration of Aromatic Hydrocarbons. Most of the aromatic hydrocarbons may acquire either one electron, giving the respective radical anions, or two, being, thus, converted into dianions. Let us denote the relevant hydrocarbon by A and the products of its reduction by A- and A-2. The reductions are repree 2 A- . . .El0 and sented by the equations, A AT e 2 A-2 . . .Ez', where e denotes an electron and
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El0 and E20 are the respective standard reduction potentials. Let us denote by x, y , and x the mole fractions of A, A-, and A-2 in an equilibrated mixture, and by g the ratio of the added electron donor to the total amount of the titrated hydrocarbon. Of course, x y x = 1, and, if the electron transfer is quantitative, then y 22 = g. The system is in equilibrium with respect to the disproportionation A A-2 2A-. . .Kd, and therefore K d = y2/xx. Hence, y = { K d [Kd2 - Kd(Kd - 4)g(2 - g>]'/']/(Kd - 4), z = 1y ) , andz = '/dg - y ) . ForalargeKdandg
