Cationic Micelles Induced Nitrosation of 1,1,1-Trifluoro-3-(2-thenoyl

Langmuir 2001, 17, 6871-6880

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Cationic Micelles Induced Nitrosation of 1,1,1-Trifluoro-3-(2-thenoyl)acetone in Mild Acid Medium: Reactivity of the Enolate Emilia Iglesias* Departamento de Quı´mica Fı´sica e E. Q. I., Facultad de Ciencias, Universidad de La Corun˜ a, 15071 La Corun˜ a, Spain Received May 4, 2001. In Final Form: July 9, 2001 In a previous paper, it was determined from spectroscopic measurements that 1,1,1-trifluoro-3-(2-thenoyl)acetone (TTeA) was less than 2% enolized in water; the presence of cationic micelles did not increase the enol content, but a strong increase of its acid character was observed; e.g., in 0.02 M tetradecyltrimethylammonium bromide (TTABr), the measured pKa was nearly 3 u lower than that determined in water (Iglesias, E. Langmuir 2000, 16, 8438). In the present work, the nitrosation reaction of TTeA in both water and aqueous micellar solutions of the cationic surfactants TTABr and tetradecyltrimethylammonium chloride is studied. The reaction performed in water at low acidities (e.g., pH > 4) is extremely slow; conversely, in aqueous micellar solutions of cationic surfactants, the reaction is too slow at high acidities (e.g., pH < 2). Cationic micelles induce the nitrosation of TTeA at low acidities because of the rapid formation of enolate anions in the positively charged micellar interface. Rate enhancements higher than 30-fold are observed by the presence of cationic micelles. Kinetic features are mechanistically explained on the basis of a rate-determining step for the attack of the nitrosating agent to the enolate. Depending on the experimental conditions, the kinetically detected nitrosating agents are NO+, N2O3, or XNO (X ) Cl, Br); however, so as to observe nitrosation via dinitrogen trioxide, either in water or in the presence of cationic micelles, it is necessary to work with relatively high [nitrite] (∼10-2 M). When the reaction is performed in water in the presence of Br-, the only nitrosating agent detected is BrNO, and the observed experimental facts are explained by postulating the formation in a reversible step of an intermediate in steady state, which possibly could be the chelate-nitrosyl complex, in analogy with the use of β-diketones as chelating extractants of metal cations.

Introduction The nitrosation of monoketones as well as β-diketones, β-ketoesters, and other related structures to give usually the corresponding oximes is nowadays a well-known process.1-6 The reaction proceeds via the enol tautomer, but either the enolization or the reaction of the enol can be rate-limiting, depending on the relative rates of enolization and nitrosation of the enol and also on the enol content. For the more acidic enols, a reaction pathway via the enolate anion is also possible and in some cases has been detected.7,8 Most β-diketones are in equilibrium with substantial amounts of cis-enols, which are stabilized by strong intramolecular H-bonds. This feature causes an increase in the enol percentage when competition by intermolecular H-bonds with solvent molecules is avoided, as it happens when β-diketones are dissolved in nonpolar aprotic solvents, such as cyclohexane or dioxane.4,9 Electron-withdrawing substituents adjacent to the carbonyl increase the acidity of the enol. In a previous study, it was shown that 1,1,1-trifluoro-3-(2-thenoyl)* Corresponding author e-mail: [email protected]. (1) Williams, D. L. H. Nitrosation; Cambridge University Press: Cambridge, 1988. (2) Leis, J. R.; Penã, M. E.; Williams, D. L. H.; Mawson, S. D. J. Chem. Soc., Perkin Trans. 2 1988, 157. (3) Graham, A.; Williams, D. L. H. J. Chem. Soc., Perkin Trans. 2 1992, 747. (4) Iglesias, E. J. Chem. Soc., Perkin Trans. 2 1997, 431. (5) Iglesias, E. Langmuir 1998, 14 (20), 5764. (6) Iglesias, E. J. Org. Chem. 2000, 65 (20), 6583. (7) Roy, P.; Williams, D. L. H. J. Chem. Res. Synop. 1988, 122. (8) Herve´s-Beloso, P.; Roy, P.; Williams, D. L. H. J. Chem. Soc., Perkin Trans. 2 1991, 17. (9) Iglesias, E. J. Phys. Chem. 1996, 100, 12592.

Scheme 1. Equilibria Involved in TTeA in Water

acetone (TTeA) exists in aqueous strong acid medium, mainly as the keto tautomer; the enol content has been determined as being less than 2%, i.e., the enolization equilibrium constant, KE, in water was measured as 0.018. In aqueous buffered solutions of acetic acid-acetate, the enolate is also present, whose quantities obviously increase with pH; the overall pKa has been determined as 6.1 (Scheme 1). Because of the small enol content, this pKa can be approximated to the acidity constant of the keto tautomer. At high acidity, addition of cationic surfactants forming micelles to aqueous solutions of TTeA does not cause appreciable changes on any of the equilibria stated in Scheme 1. By contrast, the presence of cationic micelles in neutral aqueous solution of TTeA, or even in acetic acid-acetate buffer solutions of pH ∼ 4 (i.e., much lower than the pKa of TTeA), shifts the equilibrium to the enolate formation. Concentrations of the cationic surfactant as low as 0.02 M are enough to observe a complete displacement of the equilibrium toward the enolate formation.10

10.1021/la010662h CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001

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Iglesias

Scheme 2. Possible Equilibria of Nitrous Acid in Aqueous Acid Medium

In the present work, the reactivity of TTeA in the nitrosation reaction, performed in both water and aqueous micellar solutions of the surfactants tetradecyltrimethylammonium bromide (TTABr) and tetradecyltrimethylammonium chloride (TTACl), is studied. The nitrosating agents used were those derived from aqueous mineral acid solutions of sodium nitrite (Scheme 2).11,12 In strong mineral acid, the nitrosation reaction performed in the presence of cationic micelles is extremely slow to follow. The kinetically detected nitrosating agents in water were NO+ (or H2NO2+, KNO ) 3.5 × 10-7 mol-1 dm3)13 and N2O3 (Kn ) 3 × 10-3 mol-1 dm3)14 when no Xwas added to the reaction medium; however, in the presence of Cl- or Br- ions, the only nitrosating agent observed was XNO (KXNO ) 1.14 × 10-3 or 0.051 mol-2 dm6 if X ) Cl or Br, respectively).15,16 All of these equilibrium constants refer to 25 °C. By contrast, in aqueous buffered solutions of acetic acid-acetate, the nitrosation reaction conducted in the absence of cationic micelles is extremely slow. The kinetically detected nitrosating agents in aqueous micellar solutions of cationic surfactants were XNO and N2O3, even in the presence of Cl- or Br- ions. Experimental Section TTeA was an Aldrich product of the highest purity and was used as supplied. The surfactant TTABr, from Sigma and of the highest available purity, was used as received. TTACl was prepared through the ion exchange of a solution of TTABr, using an Amberlite IRA-400(Cl) ion-exchange resin, followed by elution with distilled water. All other reagents were supplied by Merck and were used as received. All solutions were prepared with water doubly distilled from permanganate solution. TTeA was dissolved in dioxane. The percentage of dioxane in the final sample mixture was always less than 0.2% v/v. The pH was controlled using buffer solutions of acetic acid-acetate, which were prepared with acetic acid and NaOH. The pH was measured with a Crison 2001 pH meter equipped with a GK2401B combined glass electrode and calibrated using commercial buffers of pH 4.01 and pH 7.02 (Crison). The reported [buffer] refers to the total buffer concentration. A Kontron-Uvikon (model 941) double-beam spectrophotometer with a thermostated cell holder was used to record the reaction spectra or to monitor the nitrosation reaction. Kinetic measurements were performed under pseudo-first-order conditions, with sodium nitrite concentration (henceforth [nitrite]) greatly exceeding the [TTeA]. In each kinetic experiment, the integrated method was applied, fitting the experimental data (absorbance-time, A-t) to the first-order integrated equation expressed in eq 1 with A0, At, and A∞ being the absorbance readings at zero, t, and infinite times, respectively, and with ko being the observed rate constant (measured in s-1). All measure(10) Iglesias, E. Langmuir 2000, 16, 8438. (11) Ridd, J. H. Adv. Phys. Org. Chem. 1978, 16, 1. (12) Williams, D. L. H. Adv. Phys. Org. Chem. 1983, 19, 381. (13) Baylis, N. S.; Dingle, R.; Watts, D. W.; Wilkie, R. G. Aust. J. Chem. 1963, 16, 933. (14) Markovits, G. Y.; Schwartz, S. E.; Newman, L. Inorg. Chem. 1981, 20, 445. (15) Schmid, H.; Hallaba, E. Monatsh. Chem. 1956, 87, 560. (16) Schmid, H.; Fouad, E. Monatsh. Chem. 1957, 88, 631.

Figure 1. (A) Repeat scans every 8 min showing the absorbance variation due to the nitrosation of TTeA (7.5 × 10-5 M) at [nitrite] ) 1.67 × 10-3 M and [H+] ) 0.033 M (HBr); (B) repeat scans every 2 min showing the reaction spectrum due to the nitrosation of TTeA in aqueous micellar solution of [TTABr] ) 0.027 M at [nitrite] ) 1.67 × 10-3 M and 0.067 M acetic acid-acetate buffer of pH 4.11. ments were performed at 25.0 ( 0.1 °C:

At ) A∞ + (A0 - A∞)e-kot

(1)

Results Reaction Spectrum. The spectrum of TTeA recorded in aqueous medium (either in strong mineral acid, [HCl] ) 0.033 M, or in 0.067 M acetic acid-acetate buffer of pH 4.15) is that of the keto form and shows a broad absorption band between 230 and 330 nm, approximately, which is composed of two overlapping bands whose maximum absorptions are centered at 266 and 300 nm. When sodium nitrite (1.67 × 10-3 M) is added to the reaction mixture, the nitrosation reaction spectrum consists essentially of a displacement of the band absorption to the red (higher wavelengths): the first maximum decreases with time while the second one increases, but the effect is small. Isosbestic points around 240 and 272 nm can be clearly observed (Figure 1A). These facts might be interpreted a priori as due to the transformation of the keto form into the oxime (Scheme 3) through the reaction of the nitrosating agents (NO+, N2O3, or XNO; X ) Cl, Br, ...) with the enol. In aqueous buffered solutions of acetic acidacetate of pH 4.15, the spectrum resembles that obtained in aqueous hydrochloric acid, but addition of sodium nitrite causes a very small variation with time similar to that shown in Figure 1 (i.e., the nitrosation reaction is much

Nitrosation of 1,1,1-Trifluoro-3-(2-thenoyl)acetone

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Table 1. Experimental Conditions and Results Obtained in the Nitrosation Reaction of TTeA in Aqueous Acid (HBr) Medium at [Nitrite] ) 1.67 × 10-3 M and [H+] ) 0.033 M [TTeA] (10-5 M)

A0 (λ ) 313 nm)

A∞ (λ ) 313 nm)

∆ (mol-1 dm3 cm-1)

ko (10-4 s-1)

3.2 4.8 6.4 12.8

0.193 0.245 0.325 0.593

0.298 0.419 0.528 0.994

3263 3521 3250 3173

9.5 9.6 9.4 9.1

slower because the nitrosating agent concentration decreases with [H+]). The reaction spectrum recorded in 0.027 M cationic surfactant TTABr at [H+] ) 0.033 M (HCl) shows only an appreciable decomposition of HNO2, i.e., the structured band absorption between 330 and 400 nm, due to HNO2, decreases with time. Conversely, if acetic acid-acetate buffer of pH 4.15 is used instead of HCl, the spectrum of TTeA shows a new and much stronger absorption band centered at 341 nm and attributed to the enolate anion.10 Under these experimental conditions, a rapid decrease of the absorption due to enolate with time (t1/2 ≈ 6 min) is observed (Figure 1B), which means that the nitrosation reaction goes through the enolate anion, a species much more reactive than the enol. Well-defined isosbestic points are not observed, which may account for the rapid formation of an intermediate followed by its evolution to the more stable oxime. Reaction in Water. The nitrosation reaction of TTeA in water was studied by monitoring the increasing absorbance at λ ) 313 nm, presumably due to the oxime formation. Equation 1 fit perfectly experimental absorbance-time (A-t) data (r > 0.999). Some of the representative experimental conditions and results are gathered in Table 1. Influence of [Nitrite]. The influence of nitrite concentration was analyzed under the experimental conditions of [H+] ) 0.032 M (by using HClO4), [TTeA] ) 6.4 × 10-5 M, and ionic strength, I ) 0.53 M, controlled with NaClO4. Figure 2A shows the variation of the overall rate constant, ko, as a function of nitrite concentration. The results are consistent with eq 2 where β ) 2.1 ((0.1) × 10-2 mol-1 dm3 s-1 and γ ) 3.8 ((0.2) mol-2 dm6 s-1 (r ) 0.9993). This rate equation suggests the occurrence of two parallel reaction pathways that go through two other different activated complexes. Following previous nitrosation studies, a rate equation of type (eq 2) is attributed to nitrosation by both NO+ and N2O3. Both reagents are generated in aqueous acid solutions of sodium nitrite; nevertheless, the formation of dinitrogen trioxide requires relatively high nitrite concentrations. A good linear relationship between ko/[nitrite] and [nitrite] is obtained, from whose intercept and slope values, quite similar results of β and γ, respectively, to those obtained by nonlinear regression analysis are determined. When the influence of [nitrite] was studied at [H+] ) 0.065 M, ko vs [nitrite] also follows eq 2, but with β ) 2.5 ((0.2) × 10-2 mol-1 dm3 s-1 and γ ) 1.7 ((0.15) mol-2 dm6 s-1 (i.e., when doubling [H+], the value of γ is approximately half, whereas β remains unchanged within the experimental error):

ko ) β[nitrite] + γ[nitrite]2

(2)

The study of the influence of [nitrite] analyzed at [Br-] ) 0.017 M yields the results depicted in Figure 2B. A perfect linear relationship between ko and [nitrite] with a negligible intercept at the origin may be observed. The results account for eq 3 with δ ) 0.227 ((0.005) mol-1 dm3

Figure 2. Variation of the overall rate constant as a function of [nitrite] for the nitrosation reaction of TTeA under the experimental conditions: (A) [H+] ) 0.032 M (HClO4), I ) 0.53 M (NaClO4); (B) [H+] ) 0.032 M (HClO4), [Br-] ) 0.017 M, and I ) 0.25 M. Table 2. Overall Rate Constant Obtained in the Nitrosation of TTeA at [Nitrite] ) 1.67 × 10-3 M and [Br-] ) 0.032 M as a Function of [H+] Controlled with Perchloric Acid [H+] (M)

ko (10-3 s-1)

[H+] (M)

ko (10-3 s-1)

0.017 0.025 0.033 0.050 0.067

0.92 0.94 0.94 0.96 1.01

0.10 0.13 0.17 0.22

1.04 1.11 1.03 1.015

s-1 (r ) 0.999), which suggests nitrosation solely by nitrosyl bromide, BrNO.

ko ) δ[nitrite]

(3)

The same behavior is observed between ko and [nitrite] (for [nitrite] varying between 5 × 10-3 and 0.05 M) when the values of the overall rate constant were determined at [Cl-] ) 0.040 M, but the slope of the straight line is lower: δ ) 0.097 ((0.001) mol-1 dm3 s-1 (r ) 0.9995). Bromide ion catalysis in nitrosation is always greater than chloride ion catalysis because the difference in the KXNO values (0.051 mol-2 dm6 for BrNO and 1.14 × 10-3 mol-2 dm6 for ClNO at 25 °C) outweighs the difference in the bimolecular rate constants corresponding to both nitrosating agents: nitrosyl chloride is more reactive than nitrosyl bromide. Influence of Ionic Strength. The influence of ionic strength (controlled with NaClO4) was studied at [nitrite] ) 1.67 × 10-3 M and [H+] ) 0.032 M (by using HBr). The overall rate constant varies between ko ) 0.92 × 10-3 s-1 at I ) 0.032 M and ko ) 1.03 × 10-3 s-1 at I ) 0.55 M. The absence of an ionic strength effect indicates that, in the rate-limiting step, at least one neutral species is involved.

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Figure 3. Influence of (A) [Cl-] and (B) [Br-] on the pseudofirst-order rate constant for the nitrosation of TTeA in aqueous acid medium at [nitrite] ) 1.67 × 10-3 M, [H+] ) 0.032 M; ionic strength is 0.55 M in the case of Cl- and 0.45 M in the case of Br-.

Influence of Acidity. The influence of [H+] (controlled with HClO4) was investigated at [nitrite] ) 1.67 × 10-3 M and [Br-] ) 0.032 M. The obtained results are reported in Table 2, from which one may conclude that there is no effect from the acidity within the range investigated. Influence of Halide Ion Concentration. The influence of [Cl-] was studied at [H+] ) 0.032 M (HClO4), [nitrite] ) 1.67 × 10-3 M, and I ) 0.55 M (NaClO4). The variation of the observed rate constant, ko, as a function of [Cl-] is shown in Figure 3A. The results are summarized by eq 4, with η ) 6.8 ((0.2) × 10-5 s-1 and F ) 8.37 ((0.08) × 10-4 mol-1 dm3 s-1 (r ) 0.9995):

ko ) η + F[Cl-]

(4)

This is the normal observed behavior for the influence of Cl- concentration on nitrosation reactions; the intercept at the origin is interpreted as being due to the nitrosation pathway via NO+, whereas the slope accounts for the nitrosation by ClNO, whose concentration increases with that of Cl-. However, a non-negligible intercept at the origin is also observed in reversible reactions. The oxime hydrolysis in acid medium is not completely negligible. In fact, when the reaction is slow, such as in the experiments performed in the absence of Br-, the overall change in absorbance, i.e., A∞ - A0, varies with the [Cl-] (e.g., from ∆A313 nm ) 0.124 at [Cl-] ) 0.03 M to ∆A313 nm ) 0.193 at [Cl-] ) 0.55 M), which reveals the small reversible character of the reaction. The influence of bromide ion concentration was studied under the experimental conditions of [H+] ) 0.032 M

Figure 4. (A) Influence of [nitrite] on the pseudo-first-order rate constant of the nitrosation of TTeA performed in 0.083 M acetic acid-acetate buffer of pH 4.11 in the presence of 0.019 M indicated cationic surfactant. The insert shows the absorbance readings at 341 nm as a function of [nitrite]. (B) Plot of ko/[nitrite] against [nitrite].

(HClO4) and [nitrite] ) 1.67 × 10-3 M. The obtained results are displayed in Figure 3B. The plot of ko vs [Br-] profiles was adapted to eq 5 with σ ) 3.40 ((0.07) × 10-2 mol-1 dm3 s-1 and φ ) 7.5 ((0.3) mol-1 dm3 (r ) 0.9992):

ko )

σ[Br-] 1 + φ[Br-]

(5)

This unusual ko vs [Br-] profile in nitrosation reactions indicates that Br- should intervene in a reversible step of the reaction mechanism upon an intermediate that is in the steady state. Reaction in Cationic Micelles. The kinetic features of the nitrosation reaction of TTeA in aqueous micellar solutions of the cationic surfactants TTACl and TTABr were investigated by noting the decrease in absorbance at 341 nm due to the enolate of TTeA. The acid medium was generated from buffer solutions of acetic acid-acetate. Influence of [Nitrite]. The pseudo-first-order rate constant (ko) measured as a function of [nitrite] in aqueous micellar solutions of 0.019 M cationic surfactants TTACl or TTABr at 0.083 M acetic acid-acetate buffer of pH 4.11 is shown in Figure 4A. A comparison with the data in Figure 2 indicates that, for example, at [nitrite] ) 0.01 M, the rate enhancement in micelles is higher than 30fold but that the real beneficial effect is even more


Langmuir, Vol. 17, No. 22, 2001 6875

Table 3. Overall Rate Constant Values Obtained in the Nitrosation of TTeA in Aqueous Micellar Solutions of 0.019 M TTACl at 0.075 M Acetic Acid-Acetate Buffer of pH 4.11 and [Nitrite] ) 1.67 × 10-3 M as a Function of Chloride Ion Concentration [Cl-] (M)

ko (10-4 s-1)

A0 (λ ) 341 nm)

∆A ) A0 - A∞

0.0 0.033 0.050 0.067 0.10 0.13 0.18 0.25 0.33 0.40

8.89 9.42 9.87 9.49 9.83 9.01 9.04 8.73 8.48 8.35

0.944 0.831 0.768 0.741 0.671 0.614 0.541 0.484 0.434 0.405

0.883 0.743 0.677 0.639 0.559 0.512 0.460 0.403 0.353 0.322

important if one also takes into account the difference in acidity in both experimental situations. The ko vs [nitrite] profiles follow eq 2 (the corresponding parameters are now called βm and γm), a fact readily interpreted as due to competitive nitrosation reactions involving XNO and dinitrogen trioxide, respectively, as it has been pointed out for the reaction in water. The modified plots of ko/ [nitrite] vs [nitrite] yield straight lines with no negligible intercept (βm) at the origin (Figure 4B), which undoutedly means that the nitrosation via XNO is significant. At the same X- concentration, the [BrNO] is higher than the [ClNO], and the same trend is observed in βm values, which are higher with TTABr (nitrosation by BrNO) than with TTACl (nitrosation by ClNO). By contrast, the slope of the lines is approximately the same; in other words, γm values are independent of the nature of micelle counterions. This finding should be evident if one takes into account that γm values are due to nitrosation by N2O3, whose formation is independent of the presence of Br- or Cl- ions. On the other hand, by increasing the [nitrite], one can also observe a clear decrease on the net absorbance change. The decrease in absorbance is due to the reduction of the enolate ions concentration at the micellar interface. Because at this pH the predominant form of nitrite is NO2- (the pKan of HNO2 is 3.1),17 the competition between these ions and enolate ions for the micellar interface is important at high [nitrite]. The inset of Figure 4A shows the effect of [nitrite] on the absorbance readings. Influence of Halide Ion Concentration. The influence of [Cl-] on the nitrosation of TTeA in aqueous micellar solutions was investigated at [nitrite] ) 1.67 × 10-3 M, [TTACl] ) 0.019 M, and 0.075 M acetic acid-acetate buffer of pH 4.11. The obtained data listed in Table 3 show that there is practically no influence of [Cl-] on ko values, even though the enolate concentration at the micellar interface decreases notably, as either the initial absorbance readings (A0) or the ∆A341 nm (A0 - A∞) evidence. Again, the ionic exchange between Cl- and enolate anions is responsible for the absorbance decrease. These results allow us to assert that, under these experimental conditions, the nitrosation reaction that takes place in the micellar interface only is studied. Contrary to the case of Cl-, the nitrosation of TTeA conducted in aqueous micellar solutions of TTABr is strongly catalyzed by Br- addition. The influence of [Br-] was analyzed at [TTABr] ) 0.022 M, [nitrite] ) 1.67 × 10-3 M, and 0.067 M acetic acid-acetate of pH 4.11. The measured pseudo-first-order rate constant as a function of [Br-], added to the reaction medium as NaBr, is plotted in Figure 5. The ko vs [Br-] profile describes an ascending

curve that levels off at high [Br-]. (The total bromide ion concentration present in the bulk aqueous phase also includes the critical micelle concentration (cmc) of TTABr and the ionized Br- ions of the surfactant, which are equal to R[TTABr]m, with R being the micellar ionization degree (R ) 0.24)5). Data in Table 4 also show the sharp decrease in the absorbance readings at 341 nm as the amount of Br- ions increases. As already mentioned, this experimental observation reflects the competition process between enolate and Br- ions for binding to the cationic micellar interface.10 Influence of [Buffer]. The influence of buffer concentration at [nitrite] ) 1.67 × 10-3 M and [TTABr] ) 0.10 M or [TTACl] ) 0.019 M was also investigated. The obtained results are listed in Table 5. As one can observe, the overall rate constant enhances slightly with [buffer]. This small variation may be attributed to the increase of [H+] as acetate ions bind to the cationic micelles. The ionic exchange equilibrium constant (AcO-w + Br-m h AcO-m + Br-w) is unfavorable to AcO- ions, that is, KBrAcO ) 0.1 and KClAcO ) 0.50.18 In the binding process of AcO- ions to the micellar interface, not only Br- or Cl- are displaced

(17) Tummavouri, J.; Lumme, P. Acta Chem. Scand. 1965, 19, 617; 1968, 22, 2003.

(18) Bartet, D.; Gamboa, C.; Sepu´lveda, L. J. Phys. Chem. 1980, 84, 272.

Figure 5. Influence of [Br-] on the pseudo-first-order rate constant for the nitrosation of TTeA performed in 0.067 M acetic acid-acetate buffer of pH 4.11 in the presence of cationic micelles of TTABr (0.022 M) at [nitrite] ) 1.67 × 10-3 M. Table 4. Overall Rate Constants, ko, and Absorbance Readings at 341 nm Obtained in the Nitrosation of TTeA in Aqueous Micellar Solutions of 0.022 M TTABr at 0.067 M Acetic Acid-Acetate Buffer of pH 4.11 and [Nitrite] ) 1.67 × 10-3 M as a Function of Bromide Ion Concentration [Br-] (M)

ko (10-3 s-1)

A0 (341 nm)

A 0 - A∞

0.0 3.5 × 10-3 7.0 × 10-3 1.0 × 10-2 1.4 × 10-2 2.1 × 10-2 2.8 × 10-2 3.5 × 10-2 0.042 0.050 0.067 0.10 0.13 0.20 0.27

2.40 2.68 3.25 3.56 4.08 4.59 4.87 5.23 5.56 4.95 5.85 5.84 5.95 6.65 5.79

0.900 0.853 0.837 0.815 0.775 0.714 0.623 0.617 0.570 0.582 0.469 0.435 0.390 0.310 0.285

0.777 0.721 0.680 0.650 0.600 0.520 0.476 0.434 0.391 0.406 0.348 0.262 0.221 0.148 0.129

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Table 5. Rate Constants and Absorbance Readings at 341 nm Measured as a Function of [Buffer] for the Nitrosation Reaction of TTeA ([TTeA] ) 6.4 × 10-5 M) at pH 4.11 and 25 °C [TTACl] ) 0.019 M; [nitrite] ) 3.3 × 10-3 M [buffer] (M) 0.050 0.067 0.083 0.10 0.13 0.17

ko

(10-3 2.45 2.54 2.64 2.67 2.84 2.96

s-1)

[TTABr] ) 0.10 M; [nitrite] ) 1.7 × 10-3 M

A0 (341 nm)

A 0 - A∞

[buffer] (M)

ko (10-3 s-1)

A0 (341 nm)

A 0 - A∞

0.998 0.976 0.959 0.942 0.901 0.851

0.854 0.832 0.827 0.792 0.749 0.701

0.050 0.067 0.10 0.13 0.20 0.25 0.30

2.81 3.01 3.35 3.50 3.71 3.77 3.83

0.857 0.833 0.783 0.780 0.710 0.681 0.628

0.691 0.670 0.618 0.605 0.546 0.510 0.486

Table 6. Rate Constants, ko, and Total Absorbance Decrease, ∆A, at 341 nm Measured in the Nitrosation of TTeA in Aqueous Micellar Solutions of 0.019 M TTACl at [Nitrite] ) 1.67 × 10-3 M as a Function of Both Acetic Acid-Acetate Buffer Concentration and pH [buffer] (M)

pH

ko (10-4 s-1)

∆A341 nm

0.150 0.153 0.153 0.153 0.153 0.153 0.153 0.153 0.075 0.076 0.076 0.076 0.076 0.076 0.076 0.076

2.92 3.36 3.84 4.10 4.36 4.63 4.83 5.24 3.10 3.44 3.87 4.11 4.37 4.64 4.84 5.25

65.2 48.0 18.6 8.87 3.85 1.46 0.588 0.104 64.0 42.4 16.7 7.92 3.29 1.18 0.492 0.0856

0.177 0.386 0.675 0.834 0.956 0.996 1.001 1.110 0.265 0.481 0.776 0.950 1.061 1.072 1.049 1.137

from the micellar interface but also enolate ions are displaced according to the absorbance variation. Influence of pH. The effect of the acidity on the nitrosation of TTeA in the presence of TTACl micelles ([TTACl] ) 0.019 M) at two buffer concentrations and [nitrite]) 1.67 × 10-3 M was also analyzed. The results are gathered in Table 6. The total observed change in absorbance at 341 nm increases with pH; however, at approximately pH > 4, all TTeA molecules are bound to TTACl micelles in the enolate form, which is the only chromophore responsible for the absorption at this wavelength. The overall rate constant varies with pH in the contrary trend, i.e., the faster rates are measured at low pH values. On the other hand, the effect of buffer concentration is very small. Influence of [Surfactant]. Finally, rates of nitrosation of TTeA were measured as a function of [surfactant] in aqueous micellar solutions of either TTACl or TTABr surfactants in buffered solutions of 0.067 M acetic acidacetate of pH 4.11 at [nitrite] ) 3.3 × 10-3 M (in the case of TTACl) or [nitrite] ) 1.7 × 10-3 M (in the case of TTABr). As Figure 6A shows, the observed rate constant decreases with TTACl concentration when the [TTACl] is higher than the cmc (5.2 × 10-3 M)5, but ko increases sharply at [TTACl] ≈ cmc, and no reaction is observed in the absence of micelles. The contrary applies to the case of TTABr (Figure 6B): again, there is no reaction in the absence of micelles, but ko increases continuously with the increase in micelle concentration; however, a sharp increase is observed at low TTABr concentration, but at approximately [TTABr] > 0.02 M, the overall rate constant increases linearly with the surfactant concentration. The insets in Figure 6 show the absorbance variation as a function of [surfactant]. Whereas with TTACl there is no change of ∆A341 nm at [TTACl] > cmc, with TTABr the ∆A341 nm increases sharply at [TTABr] ≈ cmc (2.7 × 10-3

Figure 6. Variation of the overall rate constant obtained in the nitrosation of TTeA in 0.067 M acetic acid-acetate of pH 4.11 as a function of (A) [TTACl] at [nitrite] ) 3.3 × 10-3 M and (B) [TTABr] at [nitrite] ) 1.67 × 10-3 M. (The inserts show the absorbance change (∆A ) A0 - A∞) measured at 341 nm as a function of [surfactant].)

M), passes through a maximum, and decreases smoothly at high [TTABr]. In the first situation, no ionic exchange between the counterions of TTACl and enolate ions is observed, while with TTABr the Br- counterions displace the enolate ions from the micellar interface. Discussion Reaction Mechanism in Water. In strong acid medium, the prevailing form of TTeA is the keto tautomer.10 Nevertheless, the reactive forms toward electrophiles, such as the possible nitrosating agents NO+, N2O3, or XNO (X ) Cl, Br, ...), are either the enol or the enolate ion, which exist only in minor proportions under the experimental conditions previously reported. The lack of acidity effect is clear evidence that the reactive form must be solely the enolate ion. In addition, at low [nitrite]


Langmuir, Vol. 17, No. 22, 2001 6877

Scheme 3. Possible Nitrosation Products of TTeA

in the absence of X-, the only nitrosating agent is the nitrosonium ion, NO+. In the presence of Cl- or Br-, even at high [nitrite], the observed kinetic features indicate nitrosation via only XNO (X ) Cl, Br); that is, the ratedetermining step involves the reagents XNO (neutral) and the enolate anion, a fact that explains the lack of ionic strength influence. On the other hand, the influence of [Br-], stated in eq 5, is indicative of a reaction mechanism that requires the presence of an intermediate formed in a reversible step with the intervention of Br- and that must be in the steady state. All of these observations lead one to propose the reaction mechanism outlined in Scheme 4. As a first consideration, two possible intermediates are proposed as follows: the C-nitroso compound (I1) or the chelate-nitrosyl complex (I2). In the former case, the slow step should be the isomerization of the C-nitroso compound to the oxime (Scheme 3), of which there are no references of being slow. The latter proposal assumes the rapid formation of the chelate-nitrosyl complex, which would rearrange slowly to the C-nitroso compound. The chelatenitrosyl complex formation is supported by the use of these trifluorodiketones as chelating extractants of metal ions, such as Fe3+, In3+, Al3+, .... The formation of chelating complexes between β-diketones and hydrated metal cations is well-documented in the literature.19-21 From Scheme 4 and taking into account that [XNO] ) KXNO[nitrite][H+][X-] and [E-] ) Ka[TTeA]o/[H+], one arrives easily at eq 6, where Ka ) ([E-][H+])/([KH] + [EH]).

ko )

k1KXNOKa[X-][nitrite] k-1 1+ [X ] k2

(6)

The comparison between eq 5 and eq 6 gives φ ≡ k-1/k2 ) 7.5 mol-1 dm3 and σ ≡ k1KBrNOKa[nitrite] ) 3.4 × 10-2 mol-1 dm3 s-1, which yields k1 ) kBrNO ) 8.0 × 108 mol-1 dm3 s-1 for the bimolecular rate constant between the BrNO and the TTeA enolate (Ka ) 7.0 × 10-7 M).10 As one can see from data in Figure 3B, at low [Br-], such as 0.017 M, ko varies linearly with [Br-]; in other words, [Br-]k-1/ k2 , 1, and then, the δ parameter in eq 3 (0.227 mol-1 dm3 s-1) is equal to k1KBrNOKa[Br-], which yields k1 ≡ kBrNO ) 5.2 × 108 mol-1 dm3 s-1, in good agreement with the value of the same rate constant obtained from the study of the influence of bromide ion concentration. Because of hydration, Cl- is a poorer nucleophile than Br-, so it is necessary for high [Cl-] to make [Cl-]k-1 comparable to k2, i.e., so as to kinetically observe the accumulation of the intermediate. As a consequence, due to [Cl-]k-1/k2 , 1, the plot of ko vs [Cl-] results in a straight line (eq 4). The small but significant intercept at the origin is attributed to the oxime hydrolysis in acid medium, and F ≡ k1KClNOKa[nitrite] ) 8.4 × 10-4 mol-1 dm3 s-1. This experimental value, together with those of KClNO and Ka, gives k1 ) 8.8 × 108 mol-1 dm3 s-1 for the bimolecular rate (19) Komatsu, Y.; Honda, H.; Sekine, T. J. Inorg. Nucl. Chem. 1976, 38, 1861. (20) Sekine, T.; Yumikura, J.; Komatsu, Y. Bull. Chem. Soc. Jpn. 1973, 46, 2356. (21) Sekine, T.; Yomatsu, Y. J. Inorg. Nucl. Chem. 1975, 37, 185.

Scheme 4. Nitrosation Mechanism of TTeA in Aqueous Acid Medium

constant between ClNO and TTeA enolate, i.e., higher than the value of kBrNO, in agreement with the higher reactivity of ClNO than BrNO. The less polarizability and the higher electronegativity of Cl than Br make the charge separation of the bond X-NO (δ values in Scheme 4) higher when X ) Cl than when X ) Br; in other words, the electrophilic character of the N-atom in the nitrosyl compound is higher in ClNO than in BrNO. In the absence of X-, the slow step is k1. On increasing the [nitrite], the nitrosation pathway via N2O3 is more noticeable. Then, the expression of the overall rate constant is that of eq 7. In this equation, k3 represents the bimolecular rate constant of the reaction between dinitrogen trioxide and the enolate (E-); one can also notice that this reaction pathway is less important at high [H+]. Comparison of this equation with eq 2 yields γ ≡ k3KnKa/ [H+]; i.e., it decreases on increasing the [H+], and from the two γ values determined at [H+] ) 0.032 or 0.065 M, one gets k3 ) 8.1 × 107 mol-1 dm3 s-1. This value that corresponds to the reactivity of N2O3 toward the enolate of TTeA is lower than the reactivity found with ClNO or BrNO; this is a common observation in nitrosation studies:1,12

ko ) k1KNOKa[nitrite] +

k3KnKa [nitrite]2 [H+]

(7)

On the other hand, comparison of eqs 2 and 7 gives β ) k1KNOKa. From the experimental β value, it results that k1 ) kNO ) 7.5 × 1010 mol-1 dm3 s-1. Even though this rate constant corresponds to the reaction between a cation (NO+) and an anion (E-), the value obtained is too high. The estimated bimolecular rate constant from the Smoluchowski’s equation for a diffusion-controlled process occuring in water at 25 °C is kD ) 7 × 109 mol-1 dm3 s-1. This value should therefore be an approximate upper limit for reactions between neutral molecules, but when electrostatic forces of attraction between reacting univalent ions, such as in our case, are taken into account, the latter value of kD appears multiplied by ∼3.5.22 Then, two reasons could explain the high value of k1: the first is the uncertainty in β values (Figure 2) because of the extrapolation process, and the second is that the oxime hydrolysis, (22) Ridd, J. H. Adv. Phys. Org. Chem. 1978, 16, 1. (b) Laidler, K. J. Chemical Kinetics; Harper Collins Publishers: New York, 1987; Chapter 6.

6878


Iglesias

Scheme 5. Nitrosation Mechanism of TTeA in Aqueous Solutions of Cationic Micelles

which causes the lowest values of ko, would include the rate constant of oxime hydrolysis, an acid-catalyzed process. Reaction Mechanism in Cationic Micelles. The absorption band at λ ) 341 nm is only due to the enolate form. Working in the absence of cationic micelles at pH 4.1 (or lower), the absorption at this wavelength is negligible but not in the presence of micelles. Thus, it is assumed that following the reaction at 341 nm one is monitoring the variation of [E-], which resides at the micellar interface; in other words, in the presence of micelles, the nitrosation occurs mainly in the micelles since in the absence of them, the reaction is too slow. Therefore, Scheme 5 could be proposed. Experiments at Fixed [Surfactant]. Working at constant [surfactant], it is possible to define the apparent acidity constant, Kaap, or TTeA reported in eq 8. From spectroscopic measurements,10 this equilibrium constant has been measured at [surfactant] ) 0.019 M as pKaap ) 3.33 or 3.20 when the surfactant is TTABr or TTACl, respectively. The pKa in water is 6.16.

Kaap )

([E-]w + [E-]m)[H+] [KH] + [EH]

) Ka(1 + Ks[Dn]) (8)

On the other hand, the nitrosating agents must be XNO (X ) Cl with TTACl or X ) Br with TTABr) and N2O3, the latter being observed at high [nitrite]. Moreover, at pH 4.1, equilibrium 1 in Scheme 2 must be taken into account as well as the binding of the nitrosating agents to micelles reported in Scheme 5. These considerations, together with the appropriate mass balance equations, lead us to eq 9 to relate ko with both [nitrite] or [H+] at fixed [surfactant]. Herein, V (0.14 dm3 mol-1) is the molar volume of the micellar interface, [X-]w ) cmc + R[Dn]; Dn is the micellized surfactant; and k1m and k3m are the bimolecular rate constants in the micellar phase for the nitrosation of the enolate by XNO or N2O3, respectively; that is, ko ) k1m[XNO]m[E-]m + k3m[N2O3]m[E-]m.

ko )

Ka[H+]2[nitrite]

[

k1m

(Kan + [H+])(Kaap + [H+]) V2

Ks′′KXNOKs[X-]w +

]

(k3m/V2)Ks′KnKs [nitrite] (9) Kan + [H+] In analogy with eq 2 obtained in water, it is easy to identify the expressions for both βm and γm from eq 9. The experimental values determined for both parameters, by adjusting eq 9 to the experimental data of ko- [nitrite], are reported in Table 7 along with the values of k1mKs′′ and k3mKs′, calculated from βm and γm, respectively. The values of both Ks′′ and Ks′ are unknown; however, taking into account the simplicity of XNO or N2O3 structures

with that of HNO2, one could assume that Ks′′ or Ks′ will approximate the binding constant of HNO2 to micelles (estimated as 0.48 mol-1 dm3).23 Then, the reactivities of ClNO, BrNO, or N2O3 in the micellar phase may be estimated. The resulting values are lower than those determined in water by factors higher than 100, a finding that reveals the lower polarity of the microenvironment within the micellar interface where the reaction takes place. However, this important difference found in reactivity cannot be solely attributed to polarity effects. It was believed that the stabilility of enolate anions in a region of high positive potential (such as the cationic micellar interface) would also play a decisive role in the reduction of the reactivity. As a consequence, the negative charge of enolate anion should be mainly located at the O-atoms with the concomitant reduction in the nucleophilic character of the reactive center, the C3-atom. When working at [nitrite] ) 1.67 × 10-3 M and [TTACl] ) 0.019 M at variable pH, the expression of ko is that of eq 9 reduced in only the first term of the sum because of the low [nitrite] used. Then, by remembering that pKan ) 3.1 and pKaap ) 3.2 at pH > 4, approximately, a linear relationship should be observed between log(ko) and pH with a negative slope equal to 2 (i.e., ko ) A[H+]2/(KanKaap) or log(ko) ) log C - 2 × pH). Figure 7 shows the corresponding plot, which at pH > 4 results in a straight line of slope -1.87 ( 0.04. At constant [TTACl] and variable [Cl-], the overall rate constant remains unchanged. The expected effect should be an increase of ko with [Cl-] due to the increase of both [ClNO], mainly in the water phase, and [ketone], also in water. However, the observed lack of effect is justified due to both the slow rate in water, where the enolate concentration is quite low, and the very small effect of increasing [ClNO] at the micellar interface. Nevertheless, in the case of fixed [TTABr] and varying [Br-], addition of more Br- enhances ko values. As a consequence of the higher [BrNO] than that of [ClNO], the nitrosation rate of the enolate in the aqueous phase is faster in the presence of Br- than Cl-; then, reaction in water is observed, and due to higher polarity of the water phase than that of the micellar interface, a catalysis is observed. Experiments at Variable [Surfactant]. When varying the [surfactant], it is necessary to work with Ks instead of Kaap, since the latter is not constant. In addition, low [nitrite] was used so as not to observe reaction via dinitrogen trioxide, and the NO+ cannot be the nitrosating agent at the interface of cationic micelles. So then, the only possible nitrosating agent should be XNO. Starting with the results obtained in the presence of TTABr micelles, under the experimental conditions of data in Figure 6B, the nitrosation reaction occurs between BrNO and E- (enolate ions). As no Br- was added to the system, TTABr micelles should behave as functionalized micelles (or as reactive counterion-type micelles)24 since Br- ions intervene in the formation of the nitrosating agent, BrNO. In this case, the pseudo-first-order rate constant should increase with increasing TTABr and leveloff at high surfactant concentration. Thus, once the substrate (the enolate ion) is completely bound, the pseudofirst-order rate constant for reactive counterion surfactants should remain constant with increasing [surfactant]. Moreover, addition of Br- to this system should cause no changes in the observed rates, but the results in Figure 6B are the contrary. (23) Garcıá-Rıó, L.; Iglesias, E.; Leis, J. R.; Penã, M. E. Langmuir 1993, 9, 1263. (24) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213.


Langmuir, Vol. 17, No. 22, 2001 6879

Table 7. Experimental Conditions and Parameters Obtained in the Nitrosation of TTeA in 0.083 M Acetic Acid-Acetate Buffer of pH 4.11 Performed at Fixed [Surfactant]a r

km1Ks′′ d

km3Ks′ d

153 ( 158 ( 2c

0.9999 0.9992

106

1.2 × 1.1 × 106

2.4 × 2.5 × 103

1.14 × 1.14 × 10-3

3.2 3.2

0.84 ( 0.12b 1.07 ( 0.03c

180 ( 13b 160 ( 6c

0.999 0.9994

2.2 × 105 2.8 × 105

2.8 × 103 2.5 × 103

0.051 0.051

3.2 3.2

1.69 ( 0.02b

151 ( 20b

0.9993

4.0 × 105

2.4 × 103

0.051

3.2

surfactant

βm (mol-1 dm3 s-1)

TTACl (0.019 M)

0.255 ( 0.226 ( 0.013c

TTABr (0.019 M) TTABrf

0.016b

γm (mol-2 dm6 s-1) 2b

a

See Figure 4. b Nonlinear regression analysis. c Linear regression analysis. M and [buffer] ) 0.15 M.

Figure 7. Log(ko) against pH obtained in the nitrosation of TTeA at [nitrite] ) 1.67 × 10-3 M in aqueous micellar solutions of TTACl (0.019 M) in acetic acid-acetate buffer: (b) 0.15 and (2) 0.075 M.

Similar discrepancies have also been observed by Bunton25 and Nome.26 These deviations from the theoretical behavior expected by the pseudo-phase ionexchange model27 have been explained on the basis of a proposal of an additional reaction pathway across the interfacial boundary (km/w). A dynamic micelle structure with monomers protruding out of the hydrophobic core should facilitate the solubilization and transfer of hydrophobic substrates, such as the enolate of TTeA, across the Stern layer to the interfacial boundary where the phase-transfer catalysis takes place. Therefore, the inclusion of a phase-transfer catalysis as an additional reaction pathway introduces a new kinetic term, as eq 10 states, which describes a reaction pathway across the micelle-water interface.

rate ) k1m[E-]m[BrNO]m + km/w[E-]t[BrNO]t

(10)

From eq 10, one can easily derive eq 11, which describes the observed pseudo-first-order rate contant, ko, as a function of [TTABr], where a ) (k1m/V)Ks′′KsKaKBrNO[H+][nitrite]R/(Kan + [H+]), b ) KaKs/(Ka + [H+]), and d ) km/wKBrNO[H+]2[nitrite]/(Kan + [H+]).

ko )

a[TTABr]m 1 + b[TTABr]m

+ d[TTABr]t

(11)

The solid line in Figure 6B corresponds to the fit of eq 11 (25) Bunton, C. A.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979, 101, 1253. (b) Bunton, C. A.; Gan, L. H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981, 85, 4118. (26) Nome, F.; Rubira, A. F.; Franco, C.; Ionescu, L. G. J. Phys. Chem. 1982, 86, 1881. (b) Neves, M. F. S.; Zanette, D.; Quina, F.; Moretti, M. T.; Nome, F. J. Phys. Chem. 1989, 93, 1502. (27) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 509.

d

103

KXNOe 10-3

pKan

In mol-2 dm6 s-1. e In mol-2 dm-6. f At [TTABr] ) 0.10

to the experimental points with a ) (1.135 ( 0.095) mol-1 dm3 s-1; b ) (560 ( 55) mol-1 dm3; d ) (1.08 ( 0.03) × 10-2 mol-1 dm3 s-1; and cmc ) 2.8 × 10-3 M (r ) 0.9993). The value of b has also been determined from spectroscopic measurements10 as b ) 465 mol-1 dm3, which is in good agreement with the kinetically determined value. From the results obtained for a and d, one determines that k1mKs′′ ) 2.8 × 105 mol-2 dm6 s-1 (if V ) 0.14 dm3 mol-1) and km/w ) 1.8 × 107 mol-1 dm3 s-1, respectively. The former value is in good agreement with that determined from the influence of [nitrite] by working at fixed [surfactant] and reported in Table 7. Likewise, the reactivity across the boundary interface (km/w) is between that determined in pure water (kXNO ) 8 × 108 mol-1 dm3 s-1) and that determined in the micellar phase (k1m ∼ 6 × 105 mol-1 dm3 s-1 if Ks′′ ≈ 0.0.48 mol-1 dm3, vide supra). This finding is in agreement with the anisotropic nature of the micellar interface, whose properties vary from those of pure water to those of the internal core of the micelle (a hydrocarbonlike solvent). The reaction pathway across the boundary interface is further supported by the absorbance variation with [TTABr]. The inset of Figure 6B shows this variation; note that at [TTABr] > 0.03 M, approximately, the absorbance decreases with the increase of [TTABr]; by contrast, at more or less the same surfactant concentration, ko increases linearly with [TTABr]. The decrease in absorbance is evidence of the ionic exchange between Brand enolate ions, where the equilibrium ion-exchange constant (i.e., Br-m + E-w h Br-w + E-m) is KI ) 450, but the same process with Cl- gave KI ) 1750.10 This means that, in the presence of TTABr micelles, the possibility of finding enolate ions across the boundary interface can be considered. Contrarily, the inset of Figure 6A shows the variation of the net absorbance change at 341 nm due to the nitrosation reaction as a function of [TTACl]. The measured cmc, by electrical conductance measurements,5 of TTACl was 5.2 × 10-3 M. One can see that at [TTACl] > cmc the absorbance change remains constant, i.e., the ionexchange process is not detected at the [Cl-] introduced in the system with the surfactant, which is understandable by looking at the higher value of KI in the case of Cl-. Then, the total ketone concentration is in the form of the enolate anion at the micellar phase; consequently, the reaction must occur only in the micelles. The observed inhibition is caused by the dilution effect of the reactants in the micellar phase on increasing the surfactant concentration. This reaction in other experimental conditions is currently being investigated by working with other cationic surfactants of different counterions and chain length and also by extending the experiments to other substrates, such as 1,1,1-trifluoracetylacetone (which is less hydrophobic than TTeA), to arrive at a more general treatment for micellar catalysis, mainly, one that concerns the reaction pathway across the boundary interface.

6880


Conclusions Kinetic features attained in the present study corroborate the findings observed in the UV-vis spectroscopic study of the behavior of TTeA analyzed both in the absence and in the presence of cationic micelles: low enol content of TTeA ( ClNO > BrNO > N2O3, and the measured values of the bimolecular rate constants are close to those accepted for an encounter-controlled process. In mild acid media (pH > 4), cationic micelles induced the nitrosation reaction due to the enolate ion generation

Iglesias

because of its stabilization in a region of high positive potential, such as the interface of a cationic micelle. In addition, the reaction through the enolate is faster than with the enol, which moreover is present in minor proportions at quantities too small to observe reaction at convenient rates. Reactivities in cationic micelles follow the expected trend, but they are lower than the corresponding values determined in water, in agreement with the low polarity and water content of the micellar interface. In the case of TTABr micelles, the reaction across the boundary interface is kinetically detected, and as expected, the reactivity determined in this region is between the reactivities found in pure water and in the micelle, which reveals the anisotropic nature of the micellar interface. Acknowledgment. Financial support from the Direccioń General de Investigacioń (Ministerio de Ciencia y Tecnologıá) of Spain (Project BQU2000-0239-C02) is gratefully acknowledged. LA010662H

Cationic Micelles Induced Nitrosation of 1,1,1-Trifluoro-3-(2-thenoyl

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