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Fig. 1 --Alternative dimerization mechanisms in electrooxidation of tetraphenylborate ion.
initial mole ratios of B(C6D6)4-/B(CBHS)4-were 0.91 and 0.30, the observed mole ratios of (C6D&/ (C6H& mere 1.0 and 0.38, respectively. It seems entirely reasonable to reject the suggestion that there are certain electrode sites on which the perdeuteriotetraphenylborate ion oxidation occurs and other sites 311which ordinary tetraphenylborate ion oxidizes. On this basis the evidence points unequivocally to the reaction scheme in Fig. l a as the correct one, i e . , the dimerization is intramolecular. Tetraphenylborate ion also is oxidized chemically in a two-electron process by ceric ammonium nitrate in acetonitrile. Analysis of the oxidized solutions shows that the dimerization mechanism is the same as for the electrooxidation.
little reason t o suppose that the electrode process is altered by variation between the cationic species mentioned above. The biphenyl analysis was completed in the following manner. A 5-ml. portion of the electrolyzed acetonitrile solution was equilibrated with 12 ml. of aqueous saturated mercuric chloride solution and then extracted with cyclohexane. The cyclohexane extract was dried over calcium chloride and transferred to a sample tube. The cyclohexane was evaporated on a steam bath leaving a small quantity of biphenyl which was subjected to mass spectrometric analysis. The molecular ions at mass 164 and 154 were used to establish the ratio of perdeuteriobiphenyl to biphenyl. The mass spectrum of an authentic sample of perdeuteriobiphenyl (hlerck, Sharp and Dohme) was obtained for comparison.
Acknowledgment.-The author is indebted to Professor Richard Porter for performing the mass spectrometric analyses. The technical assistance of J. Taylor is acknowledged. Fiiiaiicial support was provided under grant AFOSR 61-18 from the Directorate of Chemical Sciences, Air Force Office of Scientific Research. FARADAIC RECTIFICATION AN11 ELECTRODE PROCESSES. T V BY HIDEO1 x 4 1 ‘ (‘oates ChemLcal Labu,iaio, y, Louzsiana State Unzieisity, Baton JZouve 9 , Louss.lana
Experimental Recemed Aprsl 9,1961 Reagents.-Tetramethylammonium perdeuteriotetraTwo methods have been applied in faradaic phenylborate, (CH3)4+B(CGD,)~-, was synthesized by modification of the procedure of Nesmeyanov and Sazonova2 for rectification measurements, namely direct measureordinary sodium tetraphenylborate. Bromobenzene-ds pre- ment of rectification voltage^^^^ and compensation pared from benzene-& (99% isotopic purity, Volk Radio- of the rectification voltage by a voltage step.4 chemical Co., Chicago, Ill.) by the bromination procedure of Best and Wilson3 was used t o prepare the corresponding These methods allow the study of variations of the Grignard reagent. Sodium tetrafluoroborate was added to rectification voltage with frequency but have the the Grignard reagent and the solution was refluxed for 90 disadyantage of requiring determination of the min. and then poured into M-ater and filtered. Addition of voltage applied to the faradaic impedance. This tetramethylammonium iodide to the aqueous filtrate predetermination is not as easy as it might appear a t cipitated tetramethylammonium perdeuteriotetraphenylhigh frequencies ( f 1Mc.) because of stray capacborate. Deuterium analysis4 of the tetramethylammonium perdeuteriotetraphenylborate gave 58.1 atom % excess ity and the inductance of the cell circuit. X simple deuterium compared with the calculated value of 62.5y0. method is described here in which this difficulty is The acetonitrile used as solvent in this work was purified as previously described.1 The sodium perchlorate used as eliminated. The method vests 011 measurement of supporting electrolyte was purified by recrystallization from the frequency at which the r ificatioii yoltage is water followed by drying in vacuo a t 130”. equal to zero. lleasurement of th Electrolyses and Analyses.-Acetonitrile solutions 0.2 AI to the faradaic impedance is not in sodium perchlorate as supporting electrolyte and approximately 5 m M in total tetramethylammonium tetra- attending experimental difficulties arc avoided. e phenylborate were electrolyzed a t a platinum electrode. Results are given for the reaction (k(C‘X)6 The control potential was +l.0 volt va. aqueous saturated Cr(CN)B-4on mercury, calomel electrode. A modified Booman6 potentiostat was Experimental employed. Solutions n-ere degassed before and during the Apparatus and techniques were previously describrd.3 electrolyses. The total solution volume was 20 ml. No attempt was made to obtain absolute biphenyl Special care was taken that the amplitude of the a .c. signal analyses but rather only the mole ratios of various isotopic applied to the faradaic impedance did not exceed 5 mv. bespecies present. When a solution 2.53 mM in (CHah- cause theory did not apply very well for higher voltages (cJ N+B(CBDL)P-and 2.78 mM in (CH&NCB(CeH6)4- was ref. 2 for discussion). Zero rectification voltage could be electrolyzed completely the mole ratio ,of (CGD,),/(C~&)~observed 50 bsec. after application of the a.c. signal since in the electrolyzed solution as determlned mass spectro- transients had died out. &Cr(CN)G was obtained from metrically was found to be 1.0. The value of the ratio for the City Chemical Corp., New York, N.Y., and was used electrolysis of a solution with an initial solution composition (1) Postdoctoral research associate, 1960-1962, on leave from 1.26 m M in ( C H ~ ) J + B ( C G Dand ~ ) ~ -4.17 mM in (CH3)4AIinami College, Hiroshima Unir srsity, Hiroshima, Japan. S’B(CGHs)4- was 0.38. (2) (a) P. Delahay, M. Senda, and C. E. Wels, J . Am. Chem. Soc., In the previous study’ aodium tetraphenylborate was electrolyzed in a solution containbg lithium perchlorate as 8 3 , 312 (1961); (b) for a review, cf. P Delahay in “Advances in Eletlsupporting electrolyte. It is quite likely that these salts troohemistry and Electroohemlcal Engineering,” Val. I, P. Delahay, as well as sodium perchlorate and tetramethylammonium Editor, InOersoience Division, John Wiley and Sons, New York, W. Y , tetraphenylborate are strong electrolytes. Thus there is 1961, pp. 279-300.
>
+
(2) A. N. Nesmeyanov and V. A. Sasonova, h. 4 k a d . hTqukSSSR, Otd. Khim. Naulb, 187 (1955). (3) A. P. Best rtnd C. L. Wilson, f. Chem. Sac., 239 (1946). ( 4 ) Analyses were performed b y Mr. J. D. Nemeth, Urbana, Illinois. ( 6 ) G. L. Booman, Anal. Chem., 29, 213 (1957).
(3) H. Emai and P. Delahay, J . Phys. Chem., 66, 1108 (1962). (4) (a) G. C. Barker, “Transactions of the Symposium on Eleatrode Processes, Philadelphia, 1959,” E. Yeager, Editor, John Wiley and dons, New York, N. Y., 1961, pp. 525-306; (b) M Senda, H. Inial, and P. Delahay, J . Phys. Chem., 65, 1253 (1961); (c) H. Imai and P. Delahay, %bid., 66, 1683 (1902).
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Sept., 1962 without further purification. Other reagents were of analytical grade. Solutions of KsCr( CN)&were prepared and kept in a darkroom because there appeared to be some photochemical decomposition.
Theory We consider rectification for a simple charge transfer process without complication due to adsorp:jon, coupled che-mica1 reaction, etc. The rectification voltage AEmfor a reaction 0 ne = R involving two soluble species is such that
+
where f, is the frequency at which AE, = 0 for the value of p given by eq. 8. Equation 9 can be applied a t two potentials for which p has the values pi and p 2 , and the resulting system of equations yields
RT AI?- .- ___2a - 1 I
n F V,i2
4 (1 1
where VA is the amplitude of the alternating voltage across the faradaic impedance, a the transfer coefficient, (7's the concentrations, D's the diffusion coefficients, 6' the phase angle between current and voltage, and n, F , R, T are as usual. The term (2a - 1)/4 it3 positive or negative according to a 2 0.5. The second term is positive or negative depending on the value of ctn 0 which depends on frequency. A t sufficiently high frequencies ctn 0 >> 1, and 1 1
+ ctn 8 + ctn2 8
1 ctn 8
_ _ _ I -
(
_IaO _____
CoDo""
2""nF
1 CRDR"~ w""
+ -)
(2)
where Iaois the apparent exchange current and o = 2 ~ f f, being the frequency. Further 1 ~ 0
=
nFka*Col-nCRa
where liaOis the apparent rate constant. condition for AE, = 0 is
or
1log &2 + (1 -
2
a!
SPZ
)log
(a)
=
The parameter 01 is determined from the intersection of the curves representing the left-hand and right-hand members of eq. 11, respectively, as functions of CY. k,O then is calculated from eq. 9. The above procedure-can be simplified when the frequency for which AE, = 0 is measured a t the half-wave potential and one can assume DO = DR = D. One thenhas
kaO = (~j'D)l'~ (12) The parameter a then is calculated from eq. 9. Description and Discussion of Results The above method was applied to C r ( C f l ) ~ - ~ e = Cr(CN)6-4 on mercury under polarographic conditions. Results6 for experimental conditions of Table I viTere as follows: a = 0.53 for each combination 11-113, 111-IV, and 11-IV; k,O = 1.58,
+
TABLE I EXPERIMENTAL^ CONDITIONS FOR 5 mM KEC,Cr(CN)6 IN 0.5
M KCN, 3 M KC1 AT 25' J,
E, v us. s.o.e,
The parameters a and kao can be determined the value of w for which AE- = 0 provided CO and C'R are known. We vonsider the case in which R, is generated in situ by polarography. Thus
eo = C O O [ l
-
(;/id)]
(5)
cOo(DO/DR)l'Z(i/id) (6) where Coo is the bulk concentration of 0, i the polarographic currelit, arid i d the diffusion currcnt. By setting cR =
C'&R'/'/(;10DO1''
= p
(7)
(i/id)]
(8)
with p (i/id)/[l one deduces from eq. 4
-
-1.335' -1.340 -1.345 -1,350 a Half-wave potential
s/td
0.500
552 609 648
Mo. see
-1
0 050 32 95 1 60
1.14, and 1.00cm. set.-' for the data 11, 111,and ITT, respectively. The value of k,O based on eq. 12 was 1.06 cm. sec.-I, and the corresponding a deduced from eq. 11 was 0.54. Results for a are excellent, , varies somewhat from one set of data to whereas kO another. This variation in kao results, among other possible causes of departure from theory, from the approximation made by the use of eq. 2 (see below). It should be noted also that a decrease of kaOas E becomes more negative is to be expected because of the double layer correction. (The potential across ( 5 ) D o = 0.71 X 10-6 cm.2 SIC.-', as calculated from id. This compares quite well with the value 0.65 X 10-6 cm.2 see.-' for 1 M XCN reported by D. N. Hume and I. M. Kolthoff, J . Am. Chem. Soc., 66, 1897 (1943).
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the diffuse double layer from the plane of closest approach to solution increases as E becomes more negative.) The above treatment is based on eq. 2, ie., on the assumption that (1 ctne)/(l ctn28) FJ 1/ ctn I!?. The validity of this assumption was determined for the above case (IC = 1 cm. sec.-l) by computing ctn 0 from the theory of the faradaic impedance.6 Results were for f = 0.05, 0.25, 0.5, 1, 2 Mc. see.-’: (1 ctn O ) / ( l 4- ctn20) = 0.54, 0.32, 0.24, 0.17, 0.13; l/ctn 0 = 0.45, 0.27, 0.21, 0.15, 0.11. The assumption embodied in eq. 2 thus is fairly justified in this case, especially a t the higher frequencies. In conclusion, the above method has the merit of simplicity over other methods previously applied in faradaic rectification measurements. It allows indirect verification of the frequency dependence of the rectification voltage since measurements cover a fairly wide range of frequencies (cf. Table I). This method could be extended to electrode processes requiring a more involved frequency dependence than that of eq. 1 (charge transfer coupled with coupled chemical reaction, adsorption of reactants and/or products, etc.). Acknowledgment.-This work was supported by the National Science Foundation. The author is indebted to Professor Paul Delahay for his interest and discussion of this investigation.
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phase was analyzed by GLC, using a 6 mm. o.d. X 6 m. “Ucon” column. Most of the data obtained in this study were gathered from a radiolysis experiment in which hexadiene (90 g.) was irradiated a t 25’ in an aluminum circulating cell8 with a 32 pa. 3 MeV. electron beam for 53 min. The dose was calculated to be 2.85 X 108 rads. Gas evolved was analyzed mass spectrometrically. Extensive gas chromatographic analyses were conducted on a distillate fraction, containibg products < Csplus some of the unreacted feed, and on tihe residue, consisting of the remainder of the feed pius the heavier products. Vractional distillation of the residue provided a Co-C12 fraction for detailed examination, and a yield value for heavier polymer. The CSand CIZcomponents were analyzed by gas chromatography, and some of the trapped fractions were examined by infrared and mass spectrometry. Then in order to obtain clear information on the carbon skeletons a sample Nyas hydrogenated with platinic oxide catalyst in methanolacetic acid. The hydrocarbon was isolated by dilution with water, separation, drying, and decanting. Completeness of hydrogenation was checked by infrared analysis. The hydrogenated products were analyzed by GLC, with identification by a combination of the mass spectral fragmentation patterns and GLC emergence times.
Results and Discussion Yields of products from the irradiation atJ 25’ are given in Table I. The yield of hydrogen, G = 0.45, is lower than that with 1-hexene, 0.8, as might be expected with this higher degree of unsaturation. The product pattern is similar to that from l-hexene, except that products a t Ca (propylene) and CS are much more prominent. Like 1-hexene, the principal heavy products are formed by addition to (6) Cf.ref. 2b, pp. 267-268. one of the carbon atoms attached to a double bond with, again, a preference for joining a t the end to RADIOLYSIS OF LIQUID 1,s-HEXADIEXE produce a straight chain. The comparative rarity of C2branches again indicates a minor role for allylBY H. B. VAN DER HEIJDEAXD C. D. WAGNER type radicals She22 Development Company, Emeryvzlle, Californza Received April 9, 1962
There is nom- considerable evidence that the radiolysis of liquid and solid olefins frequently involves ionic and ion-molecule condensations.8-s It has been shown that the points a t which condensation occurs in terminal olefins are a t the carbons attached to the double bond. The importance of the double bond posed the interesting question of whether a non-conjugated diolefin would cyclize. For this reason the radiation chemistry of 1,5-hexadiene was studied. Experimental The 1,5-hexadiene was obtained from Farchan Chemical Co. After distillation i t was found by gas-liquid chromatography (GLC) analysis to contain O.OS$& I-hexene and 0.03 7%of an unidentified hydrocarbon boiling slightly lower. One irradiation was conducted with bremsstrahlung photons from the gold target, of the 3 MeV. Van de Graaff accelerator.’ To do this a 0.3-g. sample was sealed in, vacuo in a 7 mm. o.d. glass tube, and placed in position in a thin metal tube under the target. Cold nitrogen gas maintained the temperature a t -150’ during the half-hour irradiation a t an intensity of 1.3 x 108 rads/hr. The liquid (1) W. E. T. Davison, S. H. Pinner, and R. Worrsll, Chem. Ind. (London), 1274 (1957). (2) W. S. Anderson, J . Phys. Chem., 63, 765 (1959). (3) P. C. Chang. N. C. Yang, and C. D. Wagner, J. Am. Chsm. Soc., 81, 2060 (1959).
(4) C. D. Wagner, Tetrahedron, 14, 164 (1961). ( 5 ) C. D. Wagner, J . Phys. Chem., 6 6 , 1158 (1962). (6) E. Collinson, F. S. Dainton, and D. C . Walker, Trans. Faraday Soe., 67, 1732 (1961). (7) C. D. Wagner and V. A. Campanile, Nucleonics, 17, 99 (1969).
The gas-liquid chromatogram of products from the low-temperature irradiation was nearly identical with that of the irradiation a t 25’. Thus. the products must arise from “hot” processes or from reactions involving no activation energy, an observation like that already made with 1-hexene. Mass spectrometric examination of the CS proclucts established that they consisted mostly of diolefins and triolefins, with mostly vinyl (terminal) but some vinylene (RCH = CHR) double bonds. The Clz products were di-, tri-, and some tetraolefins, always with vinyl unsaturation but, freqricntly inrluding vinylene double bonds. The low yield of cyclic compounds (G = 0.12) inay be attributed to the high collision eficirncy of the ion-molwulc reaction. The active positive center of the molecule readily golarims a neighboring molecule and reacts with it, and only seldom finds its own opposite double bond in a position to react. From these data the mechanism of formation of Cg compounds is not clear. The similarity in structure between Cs and Clz suggests condensation of the molecule ion with a molecule, followed in a fraction of the events by splitting at the relatively weak third bond to give CSand Ca. ( 8 ) C. D. Wagner,
J. Phys. Chew+, 64, 231 (1960).
