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COMMUNICATIONS TO THE EDITOR stants for these reactions as a function of surfactant concentration reveals that catalysis is maximal a t 0.01 M, or less, surfactant and that rate constants either remain constant or decrease slightly at higher surfactant concentrations. At all surfactant concentrations investigated, 0.005-0.03 M , that order of reactivity as a function of substrate structure noted in Table I is observed. Since the concentration of surfactant is generally about 200 times greater than that of substrate, it is likely that the substrates themselves do not alter the micellar structure significantly. Thus, the data provide evidence for the suggestion of Gitler and OchoaSolanolg that hydrophobic interactions may contribute to activation energies for reactions in micellar systems. Such contributions might result from movement of the ionic head group of the substrates into the environment occupied by the nonionic dihydropyridine moiety of the products as the hydrophobic interactions between substrate and micelle are accentuated. This suggestion is consistent with the observation that the absorption maxima of the substrates in the presence of 0.02 M hexadecyltrimethylammonium bromide change uniformly from 271.3 to 272.3 mp as the N substituent is changed from decyl to h e ~ a d e c y l . ~ Third, the surfactant-dependent reactions are made less favorable by salts. For example, association constants are 2300,860, and 300 M-' for 0.0,0.1, and 0.5 M added sodium chloride for the reaction of the N-tetradecyl substrate in the presence of 0.02 2M tetradecyltrimethylammonium bromide a t 30". Second-order rate constants for this reaction decrease from 7.1 to 3.1 to 0.8 M-I sec-' for the same concentrations of sodium chloride. The effectiveness of anions as inhibitors increases in the order F- < NO3- < C1- < Br-, which is related to but not identical with the relative inhibitory capacity of these anions for the surfactant-dependent basic hydrolysis of esters.lf Fourth, the affinity and reactivity of cyanide toward pyridinium ions are increased by zwitterionic surfactants. For example, rate and association constants for the addition of cyanide to N-dodecyl-3-carbamoylpyridinium bromide in the presence of 0.02 M dodecyldimethylammoniopropanesulfonate at 25" are, respectively, 1.0 M-I see-l and 1100 M-I. These figures indicate increases of 71- and 5700-fold in reactivity and affinity of cyanide for this pyridinium €onelicited by the zwitterionic surfactant. This is the only case known to us in which an organic reaction is subject to marked promotion by a zwitterionic surfactant. The source of the rate and affinity increases is not evident. Finally, these reactions are subject to promotion by sonicated aqueous dispersions of ovolecithin. Rate and association constants for the addition of cyanide to Vdodecyl-3-carbamoylpyridinium bromide are inAreased 13- and 350-fold over control values in the presence of 4 X M lecithin. As above, this reaction appears to be the only case identified in which a non-
enzymatic reaction is promoted by biological surfactants (with the exception, of course, of those reactions in which substrate solubilization is the important factor). Promotion by sonicated dispersions of lysolecithin and sphingomyelin has also been observed. All of the above reactions have been followed spectrophotometrically a t 340 mp in aqueous solution. The temperature was maintained at 25" unless noted otherwise. Values of pH were maintained in the vicinity of pH 10 through use of dilute triethylamine buffers.
Acknowledgment. E. H. C. expresses his appreciation to the Escuela de Quimica, Universidad Central, for hospitality during a visit during which a portion of this research was conducted. We are indebted to Procter and Gamble, Inc., for a gift of the dodecyldimethylammoniopropanesulfonate. (4) Related experiments have been described by P. Mukerjee and A. Ray, J.Phys. Chem., 70,2144(1966). ( 5 ) Career Development Awardee of the National Institutes of Health. Research Fellow of the Alfred P. Sloan Foundation. This work was supported by Grant AM 08232 from the National Institutes of Health and by Grant GB 8162 from the National ScienceFoundation.
ESCUELA DE QUIMICA, FACULTAD DE CIENCIAS J. BAUMRUCKER M. CALZADILLA UN~VERSIDAD CENTRAL CARACAS, VENEZUELA M. CENTENO G. LEHRMANN DEPARTMENT OF CHEMISTRY
P. LINDQUIST
INDIANA UNIVERSITY BLOOMINGTON, INDIANA 47401
D. DUNHAM
M. PRICE B. SEARS
E. H. COR DES^ RECEIVED DECEMBER 2, 1969
Solvent Polarity in Electrochemical and Other Salt Solution Studies
Sir: Electrochemical techniques are very useful for the generation of reactive species in organic solvents. The subsequent behavior of these species may be followed by a number of techniques, including that of cyclic vo1tammetry.l Other methods, e.g., potential step methods, can be and have been used to study the subsequent chemical reaction.2 The rate constant k k for the process 0 ne ;=tr R; R -t D can thus be determined. R can be a neutral or charged species and may react through a transition state with a charge separation quite different from that of the initial state. I n such cases, k will be very sensitive to the polarity of solvent.
+
(1) P. Delahay, "New Instrumental Methods in Electrochemistry," John Wiley and Sons, New York, N.Y.,1954. Also see R.9. NicholsonandLShain, Anal. Chem., 36,706 (1964). (2) W. M. Schware and I. Shain, J . Phys. Chem., 69,30 (1966);70, 854 (1966); J. T.Lindquist and R. S. Nicholson, J . Electroanal. Chem.,
16,445 (1968).
Volume 74,Number 6 March 6, 1970
COMMUNICATIONS TO THE EDITOR
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-I
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1
i
50 I -60
i
....!,..
i
MTHF
-5 0
.4
0 -30 -20 LOG S A L T CONCENTRATION, M
-10
I
00
Figure 1. A plot of 2 value vs. the negative of the logarithm of salt concentration for a number of nonpolar solvents. Open circles ( 0 )represent measurements made with pure l-ethyl-4carbomethoxypyridinium iodide; closed circles (a) are based on solutions of tetra-n-butylammonium perchlorate containing sufficient (7% or less) l-ethyl-4-carbomethoxypyridinium iodide to permit facile observation of the charge-transfer maximum. Circles with diagonal lines ($) are derived from measurements on solutions of tetra-n-butylammonium perchlorate containing 15% l-ethyl-4-carbomethoxypyridinium iodide (DME solutions). Data for acetonitrile and chloroform solutions (containing 0.13 M ethanol) are taken from ref 6 and are shown as 8 (CHaCN) or X (CHCb). All other solvents were highly purified and degassed.
We wish to point out that the electrolyte required in the solutions for adequate conductivity (e.g., 0.1 M tetra-n-butylammonium perchlorate (TBAP)) markedly raises the polarity of the solvent over that of the pure material. We have utilized 2 values as a measure of the polarity of such solutions. Z values are empirical measures of solvent polarity based on the position of the charge-transfer band of l-ethyl-4-carbomethoxypyridinium i ~ d i d e . Our ~ results show that, the lower the polarity of the pure solvent, the greater the increase in polarity brought about by a particular concentration of salt. Some solvents will exhibit a somewhat greater (or lesser) change than this rough generalization implies (e.g., chloroform). Data on the 2 values of 1,Zdimethoxyethane (DME), acetonitrile, and other solvents relevant to electrochemistry are presented in Figure 1 as a log (salt concentration) vs. 2-value plot. The Z values of the pure solvents are derived by extrapolating Z-value measurements to infinitely dilute solutions. The Z-value increment for the change from zero
The. Journal of Physical Chemistry
salt to 0.1 M TBAP is quite large for DME (-6.8 kcal/ mol) and significant for acetonitrile (1.5 kcal/mol). It is remarkable that the limiting Z value for very high concentrations of TBAP in DME is close (ea. 66.4 kcal/ mol) to the 2 value found for molten tri-n-hexylammonium perchlorate (66.9 kcal/mol) .4 A Z-value increment of 2.8 kcal/mol can increase the rate of an electron-transfer reaction by a factor of 10, as shown for the case of 1-ethyl-4-carbomethoxypyridinyl radical and 4-nitrobenzyl chloride.6 Electrontransfer reactions are clearly among the elementary processes to be considered for the reactions of species generated electrochemically, and a proper consideration of solvent polarity is necessary for comparison with kinetic results obtained by other means. Salt effects in low polarity media can be studied by means of Z-value measurements. The charge-transfer bands observed for l-ethyl-4-carbomethoxypyridinium iodide (Le., the 2 values) exhibit much greater sensitivity to increases in salt concentration above 3 X 10-4 M in DME and methylene chloride than below that concentration (Figure 1). A similar effect is observed in chloroform solutions.6 Electrostatic interactions may be responsible for these salt effects since dielectric relaxation measurements on tetraalkylammonium picrate salts in benzene do not support the notion that ionic aggregates more complex than ion pairs are present in solutions below 0.01 11/1.7-9 We suggest that 2-value measurements will provide intrinsically useful information on organic salt solutions and that they will be useful for the interpretation of kinetic data derived through electrochemical experiments. (3) Cf.E.M.Kosower, "An Introduction t o Physical Organic Chemistry," John Wiley and Sons, New York, N. Y.,1968,Section 2.6. (4) J. E.Gordon, J . Amer. Chem. Soc., 87, 4347 (1966). (5) E.M.KosowerandM.Mohammad, ibid.,90,3271 (1968). (6) E.M.Kosower, ibid., 80,3253 (1968). (7) Cf.Table 2.13 in ref 3. (8) G. Williams, J. Phys. Chem., 63, 534 (1960). (9) M.Davies and G. Williams, Trans.Faraday Soc., 56,1619 (1960). (10) To whom correspondence should be addressed. (11) Support from the Army Research Office (Durham) and the National Institutes of Health is gratefully acknowledged.
M. MOHAMMAD DEPARTMENT OF CHEMISTRY E. M. KOSOWER*O,~~ STATEUNIVERSITY OF NEWYORK STONY BROOK, NEWYORK 11790 RECEIVED DECEMBER 19, 1969
