COMMUNICATIONS TO THE EDITOR
Solvent Effect on the Dimerization of N-Me thylaniline Publication cost borne completely b y The Journal of Physical Chemistry
Air: Ellison and Meyerl have recently determined the dimerization constant K D of N-methylaniline (NMA) in cyclohexane, at 25", using a dielectric method based upon Onsager's theory. While the value obtained compared well with that found by Lady and Whetsel12 in the same solvent, using the infrared technique, an important difference (by approximately a factor of 2) was noted between their value and the value which we had calculated from thermodynamic activity coefficients as determined by an isothermal liquid-vapor equilibrium study of NMA-carbon tetrachloride mixt u r e ~ . According ~ to Ellison and AIeyer the discrepancy lies either in our theory, or in our data. I n our opinion, however, this discrepancy lies simply in that association constants are solvent dependent and that the comparison has been made with our K Dvalue as measured in carbon tetrachloride. Fortunately, we had performed a similar analysis of vapor-liquid equilibrium data for the KMA-cyclohexane m i ~ t u r e , ~ which seems t o have escaped the attention of Ellison and Meyer. Table I gives K D values in the same solvent, cyclohexane, obtained by three different techniques and it may be seen that they are in reasonable agreement.
enthalpies of mixing tertiary amines with carbon tetrachloride or cyclohexane. In our previous publication4 we had investigated the self-association of aniline and S M A in cyclohexane, carbon tetrachloride, and benzene and had refound the classification of solvents according to their "degree of inertia" with respect to molecular interactions."1° (1) H. R. Ellison and B. W. Meyer, J . Phys. Chem., 74, 3861 (1970). (2) J. H. Lady and K. B. Whetsel, ibid., 71, 1421 (1967). (3) G. Pannetier and L. Abello, Bull. SOC. Chim. Pr., 1645 (1968). (4) L. Abello, B. Servais, M. Kern, and G. Pannetier, ibid., 4360 (1969). (5) K. Sosnkowska-Kehiaian, K. Orzel, and H Kehiaian, BUZZ. Acad. Polon. Sci., Ser. Sci. Chim., 14, 711 (1966). (6) M. L. Josien, J . Chim. Phys., 61,24 (1964). (7) P. V. Huong and J. C. Lassegues, Spectrochim. Acta, Part A , 26, 269 (1970). (8) J. C. Lassegues and P. V. Huong, Method. Phys. Anal., 5 , 69 (1969). (9) H. Buchowski, J. Devaure, P. V. Huong, and J. Lascombe, Bull. SOC.Chim. FT.,2532 (1966). (10) Professor Ellison has informed the Editors that he is in agreement with the authors' comments on ref 1.
LABORATOIRE DE C I N ~ T I Q U CHIMIQUE E FACULTE DES SCIENCES DE PARIS 1, RUE G U YDE LA BROSSE 75-PARIS v" FRANCE
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Micellar Effects on the Hydrolysis Rate of Triethylamine-Sulfur Trioxide
Table I : Dimerization of N-Methylaniline in Cyclohexane at 25" Teohnique Ir Dielectric Thermodynamic
0.434 i 0.025 0.422 i 0.085 0.36 i 0.02
2 1 4
The somewhat smaller dimerization constant, inferred from the thermodynamic data, may be due to the fact that in our treatment the formation of higher NMA associates has also been taken into account. This is in fact necessary when one wishes to explain the behavior of the system at higher concentrations. The smaller dimerization constant of NMA in carbon tetrachloride should be ascribed to the competition between N-€I . . N bond formation and a direct C1. . . N interaction. This explanation is identical with that given by Kehiaian5 for the self-association constant of diethylamine, smaller in carbon tetrachloride than in cyclohexane, and bases on a comparison of the
Publicaton costs assisted bg the University of Maine
Sir: It is well established that the rates of certain organic reactions are markedly altered in the presence of micellar solutions. Electrostatic as well as hydrophobic interactions are thought to be the principal causes of these rate effects.lV2 Generally it is found that reactions catalyzed by cationic surfactants are unaffected or inhibited by anionic surfactants and vice v e r ~ a . ~ We - ~ reasoned that reaction of a zwitterionic (1) For reviews see (a) E. H. Cordes and R. B. Dunlap, Accounts Chem. Res., 2, 329 (1969); (b) E. J. Fendler and J. H. Fendler, Advan. Phys. Org. Chem., 8, 271 (1970). (2) C. Gittler and A . Ochoa-Solano, J . Amer. Chem. SOC.,90, 5004 (1968). (3) C. A. Bunton and M. J. Winch, Tetrahedron Lett., 44, 3881 (1970). (4) E. J. Fendler, R. R. Liechti, and J. €1. Fendler, J . Org. Chem., 35, 1658 (1970). (5) G. J . Buist, C. A. Bunton, L. Robinson, L. Sepulveda, and M. Stam, J . Amer. Chem. Soc., 92, 4072 (1970). (6) C . R. Cramer and J. L. Berg, J . Phys. Chem., 72, 3686 (1968).
The Journal of Physical Chemistry, Vol. 76, N o . 11, 1071
COMMUNICATIONS TO THE EDITOR
substance, where the possibility of binding with either type of charged micelle exists, might prove to be an exception to this general rule. I n the present communication, we wish to report the preliminary results of our studies of such a system. Hydrolysis of triethylamine-sulfur trioxide (I) is
believed' to occur via a nucleophilic attack on sulfur by water and thus results in the ejection of triethylamine with concomitant formation of the bisulfate ion. Rate data for this reaction were obtained by pH titration of the liberated acid contained in aliquots of the reaction mixture using dilute sodium hydroxide solution. I n Table I are listed pseudo-first-order rate
0.020 (ETAB) moles
Figure 1. The effect of added ETAB on the hydrolysis rate of triethylamine-sulfur trioxide a t 77.9". Table I : Pseudo-First-Order Rate Constants for Hydrolysis Triethylamine-Sulfur Trioxide at 77.9" in the of 0.01 Presence of Surfactants at Varying Concentrations Surfactant
None ETAB ETBB DTAB
' See ref 7 , value 1.10
1 x 10-2 2 x 10-2 2 x 10-2 9 . 7 x 10-3 2 x 10-2
1 .0oa 5.04 9.88 3.17 2.30 3.36
X lo-* sec-'.
constants for the hydrolysis of I alone and in the presence of the cationic surfactants, eicosanyltrimethylammonium bromide (ETAB) and dodecyltrimethylammonium bromide (DTAB), and the anionic surfactant, sodium hexadecylsulfonate (SHS). There clearly exists in this reaction a rate acceleration effect for both anionic and cationic surfactants. The smooth increase in rate upon addition of the cationic surfactant ETAB is shown in Figure 1. At higher concentrations of ETAB, the typical dampening of the effect occurs, signalling the approaching saturatioh of the micellar pseudophase. It is likely that hydrophobic interactions are superimposed on the electrostatic effects which bring the
T h e Journal of Phpical Chemistry, Vol. 75, N o . 11- 1971
substrate into the micellar region for more facile hydrolysis. Some interactions could well aid in departure of the triethylamine moiety with its lessening positive charge. A manifestation of the hydrophobic assistance may be the smaller catalytic effect observed with shorter surfactant chain length as illustrated in Table I with ETAB and DTAB. This idea is further supported by some kinetic measurements on the methyl analog, trimethylamine-sulfur trioxide. The rate of hydrolysis of trimethylamine-sulfur trioxide in the presence of 0,02 M ETAB at 77.9" relative to that in water at the same temperature is 1.72; the corresponding ratio for triethylamine-sulfur trioxide is 9.33. Our investigation of this reaction is continuing with emphasis on experiments designed to test the importance of hydrophobic interactions.
Acknozoledgrnent. The authors gratefully acknowledge support in part by a Frederick Gardner Cottrell Grant-In-Aid from The Research Corporation. (7) B. E. Fleischfresser and I. Lauder, Aust. J . Chem., 15, 251 (1962).
DEPARTMEKT OF CHEMISTRY OF MAINE UNIVERSITY ORONO, MAINE 04473
MICHAEL D. BENTLEY* SUS.4N E. BOWIE ROBERT D. LIMOGES
RECEIVED FEBRUARY 1, 1971