A NEW TECHNIQUE FOR THE DIRECT STUDY OF REACTANT

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May20, 1961

COMMUNICATIONS TO THE EDITOR

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(Nujol) of II.HC1, III.HCI and the crude mixture of hydrochlorides were identical. We believe that the racemization proceeds through the intermediacy of the symmetrical ion IV HO.-@ CH3

-

ACO.-@

-

c-

I

II.HC1 [e] III*HCl [e]

+ 39.5"

@ C H ~ 4- ACOIV

0.0"

The reverse reaction, the attack on I V by acetate ion, occurs with inversion to give 111. L( -)-2P-Tropanol afforded ~(+)-2@-tropanyl [O!]~D' acetate (hydrochloride2 m.p. 219-222 +3.5" (2.5% HzO)) in high yield when refluxed for three hours with acetic anhydride. The "wrong" stereochemistry of this isomeric alcohol precludes the formation of the ion IV.

TIME

Fig. l.-Alternate cathodic and anodic chronopotentiograms for Fe(II1) adsorbed 011 the platinurn electrode; current density was 100 microamperes per c m 2 .

solution and immersed in solutions free of dissolved reactant. This slow rate of desorption may be taken advantage of to study the adsorbed reactants under much to be preferred conditions where the (3) The rates of racemization and ionization of 2-tropanol derivaonly reaction going on a t the electrode is due to the tives are being studied in another laboratory; private communication adsorbed reactant. from Prof. H. L. Goering. Chronopotentiometry6 is ideally suited to the S. ARCHER STERLING-WINTHROP RESEARCH INSTITUTET. R. LEWIS study of adsorbed reactants because experiments RENSSELAER, NEW YORK M. R. BELL can be carried out rapidly compared to the rate J. W. SCHULENBERG of desorption in reactant free solutions. Figure 1 RECEIVED APRIL11, 1961 shows a set of chronopotentiograms for Fe(II1) and Fe(I1) adsorbed on a 0.1-cm.2 platinum electrode, in which the direction of the current was A NEW TECHNIQUE FOR THE DIRECT reversed a t each successive transition time. These STUDY OF REACTANT ADSORPTION AT PLATINUM ELECTRODES chronopotentiograms were obtained by immersing Sir: the electrode in a solution 0.85 F in Fe(C1O4l3 Adsorption of reactants on mercury and platinum in 1 F HC104 for 50 seconds, removing the elecelectrodes frequently has been proposed as a likely trode, washing it thoroughly with distilled water, step in the kinetics of electrode r e a ~ t i o n s . ' ~ 2 ~placing 3 ~ ~ i t in the chronopotentiometric cell conUsually the presence of adsorption has been in- taining oxygen-free 1 F HC104, and recording the ferred from experiments that also involved the chronopotentiograms. As would be expected for adsorbed reactants, reaction of the unadsorbed reactant dissolved in the body of the solution. Commonly, when the the chronopotentiograms in Fig. 1 are completely bulk concentration of the reactant is large enough unaffected by whether or not the solution is stirred to produce appreciable adsorption on the electrode, during their recording. Furthermore the ratios almost all of the current in electrochemical ex- of the cathodic to anodic transition times are periments results from the reaction of the unad- approximately unity rather than one-third, the sorbed reactant a t the electrode, so that the inter- value obtained in the case of diffusion-controlled pretation of the experimental results is difficult and chronopotentiograms.6 This observation shows no direct information on the nature of the adsorbed that the Fe(I1) produced by the reduction of Fe(II1) remains adsorbed on the electrode. species is obtained. Conclusive proof that these chronopotentiograms Lorenz5 has resorted to the use of platinized platinum electrodes to increase the proportion of result from adsorbed Fe(II1) and Fe(I1) is obthe current corresponding to adsorbed reactants. tained by comparing the observed chronopotentioHowever, platinization of the electrode often leads grams with the theoretical equation for a chronoto larger and undesirable contributions to the cur- potentiogram from adsorbed reactants. In the rent from the charging of the double layer and this case of a reversible diffusion-controlled chronopotentiogram for the reduction of Fe(lI1) the equaleads to experimental difficulties. We have found recently that a number of react- tion of the wave isfi ants of electrochemical interest remain adsorbed on platinum electrodes for 10 to 20 minutes even when the electrode is removed from the reactant where E is the electrode potential, El/, is E when (1) H. Matsuda and P. Delahay, Collection Csechodm, Chcm. t = r/4, r is the transition time, and t is time. Coinmun., 19. 2977 (1960). . . The corresponding equation in the case of a n ad(2) H:A. Laitinen and J. E. B. Randles, Trans. Faraday SOL.,61, 54 (1955). sorption chronopotentiogram is (3) P. Delahay and I. Trachtenberg, J . A m . Chem. (1958). ( 4 ) A. Frumkin, Abstract No. 172, Electrochem.

SOL, 80, 2094

SOC.Meeting, Philadelphia, 1959. ( 5 ) W. Lorenz and H . Miihlberg, 2. Elekfvochrm., 69, 730, 736 (lV56).

( 6 ) P. Delshay, "New Instrumental Methods in Electrochemistry," lnteracience Publiwhers, IDC.,New Uork, N. Y..1064, Ch. 8.

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COMMUNICATIONS TO THE EDITOR

Here El,, is E when t = r/2. A plot of E us. log t / ( r - t ) for a chronopotentiogram such as those in Fig. 1 gave a straight line with a slope of 0.063 volt. This compares favorably with the theoretical value of 0.059 volt. The analysis of chronopotentiograms for adsorbed reactants by means of plots of E vs. log t / ( ~- t ) is of particular value in cases where more than one form of the adsorbed reactant is possible. For example, cathodic chronopotentiograms for iodine adsorbed on a platinum electrode would obey the equation (3) if the iodine were present on the electrode as iodine molecules (3)

But the chronopotentiogranis would obey equation 2 if the adsorbed iodine were present as iodine atoms. Experimentally, chronopotentiograms for adsorbed iodine give straight lines with slopes near the theoretical value for plots according to equation 2 but not with equation 3. Thus i t may be concluded tentatively that iodine adsorbed on platinum electrodes is present as atomic iodine. The extension of this technique should provide much useful information on the nature of reactants adsorbed on electrodes. It may even be possible to determine directly the contribution of adsorbed reactants to exchange currents measured in solutions of the reactants by performing rapid galvanostatic experiments with electrodes containing adsorbed reactants in separate, reactant-free solutions. Further studies of this technique and its application to the examination of adsorbed Fe(II), Fe(111), 1%and I- are in progress and will be reported later. This work was supported by the U. S. Army Research Office under Grant No. DA-ORD-31124431491. Helpful preliminary experiments were performed by Karl Pool. CONTRIBUTION No. 2696 GATESAND CRELLINLABORATORIES OF CHEMISTRY CALIFORNIA INSTITUTE OF TECHNOLOGY FREDC. ANSON PASADENA, CALIFORNIA RECEIVED MARCH 27, 1961 BICOORDINATE PHOSPHORUS : BASE-PCFs ADDUCTS'

Sir: The cyclopolyphosphines (PCF3)d and (PCF3)52 react easily and reversibly with trimethylphosphine or trimethylamine to form (CH3)3PPCF3 or (CH)3NPCF3. These evidently employ the PCFa unit in the same role as the =O, =NH, and =CH2 units in the system of analogous R3P and R3N compounds discussed by Wittig and Rieber, a with the interesting difference that the PCF3 complexes a t mom temperature easily dissociate to the tertiary base and (PCF3)4and 6, thus offering a way to improve the chemical availability of the PCF, unit. (1) This research was supported by the United Stales Air Force under successive subcontracts of Prime Contracts 9 F 33(616)-5135 and 6913, monitored by the Materials Laboratory, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio. (2) W.Mahler and A. B. Burg, J . Am. Chem. Soc.. 80, 8161 (1958). (3) G . Wittig and M. Rieber, A n n . , 662, 181 (1949).

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It is also interesting that the R3PPCF3compounds are isomers of the diphosphines R2PPR2,and represent the first authentic examples of bicoordinate phosphorus in neutral molecules. Formation of the Trimethylphosphine Complex. --When 0.990 mmole of pure (7>CF3)4and 4.22 mmoles of (CH3)3Pin a closed tube were warmed slowly from - 196", combination occurred with transient appearance of a yellow liquid but cnding with a white solid. Removal of the excess (CHJ),3P in high vacuum a t -45" left a residue containing 0.9S(CH3)3Pper PCF3 unit. 'CVith the absorption of small measured portions of (CHa)SPby (PCF3)d [weighed sample) a t constant temperature, constant pressure was observed except for a distinct rise as the combining ratio approached 1:l. With 114 mm. pressure of ( C H S ) ~ P a t 21" (1 h r ) the ratio in the solid phase reached 0 985(CK3)J3 per PCF3 unit, but decreased as a small side reaction fouled the mercury manometer. The quickly reproducible dissociation pressures in the mid-range of composition were 12.0 mm. a t 23' and 53 mm. a t 43', determining the equation IogP,, = 11.276 - 3020/T (calcd. a t O", 1.69 mm.; obsd., 1.65). Defining Ke, as pressure in atm., this gives AFO = 13.52 - 0.0384 T for the dissociation of the solid complex to gaseous (CHJ)3P and the slightly volatile mixture of (PCFI)4and (PCFJb. Complete dissociation could be accomplished by high-vacuum fractional condensation, with all of the (CH3)3P passing a trap a t -78'. The trappedout (PCF3), compounds showed an average vaporphase mol. m-t. of 414, indicating six ( P C F Y ) ~ (solid) to one (PCF3)B(liquid) in the equilibriunl mixture. Catalytic Reorganization of the Polyphosphines. -The reversible formation of the PCF, complex implied that (CH&P would catalyze the interconversion of (PCF3)4 and (PCFI)5. In f x t , a one mole-per cent. addition of (CH3)3P t o pure (PCF3)6 caused a 50% conversion to (PCF3)d during 45 hr. a t 25". Such a conversion previously had required heating to 260°.2 In solution in ether or hexane, the catalyst converts the tetramer mostly to the pentamer, but without solvent, the tetramer (m.p. 66") is strongly favored by its solid-state energy. Higher (PCF3) polymers2 are not found in the equilibrium mixtures. A possibly similar catalysis occurred during a very fast 90% conversion of the triphosphine H2(PCF3)31to (HPCF3)sand (PCF3),,by a trace of (CH3)3Pa t 25'. Solution Behavior of the Complex.-The nearly colorless solutions of (PCF3)4 in liquid (CH3)3P show average mol. ivt. values (by vapor-tension lowering a t 0")a t least 25y0 higher than expected for pure (CH3)yPPCF3.even a t PCF3 concentrations as low as 0.7 mole per cent. Also, when the solutions are chilled suddenly to -78" and the free (CH3)3Pis distilled off, the residues never contain more than 0.5s (CHa)3Pper PCF3 unit. Thus in solution with much excess (CI3J)J' the complex (CH3)BPPCFs is less completely formed than in the solid with little excess (CH3)3P. Apparently the conversion of solid tetramer (or liquid pentamer) to solid (CH3):PPCF3 is favored by the greater