J . Chem. SOC., Furuday Trans. I , 1985,81, 2941-2945 Determination of the Rate of Tautomerization of 1 -Phenylbutane- 1,3-dione (Benzoylacetone) using the Technique of Solubilization into Micelles BY YOSHIKAZU MIYAKA,* MASATO SHICETO~ AND MASAAKI TERAMOTO Department of Industrial Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, Japan Received 3rd January, 1985 The distribution constant and the rate of keto-enol tautomerization of 1 -phenylbutane- 1,3-dione (benzoylacetone) have been measured by a new technique using micellar systems in which the dione exists mainly in its enolic form in the micelles. The tautomerization rate was obtained from the absorbance change observed when an aqueous solution of benzoylacetone was mixed with a surfactant solution above the critical micelle concentration, since the tautomerization rate is much slower than the rate of solubilization of the dione into micelles.The tautomerization rate was accelerated by base catalysts such as hydroxide and acetate ions. P-Diketones such as 4,4,4-trifluoro- 1 -(2-thienyl)butane- 1,3-dione (thenoyl trifluoro- acetone) and 1 -phenylbutane-l,3-dione (benzoylacetone, HB) are widely used as chelating agents in analytical and coordination chemistry. Recently some P-diketones have been developed as commercial extractants of copper. It was reported that p-diketones exist as a mixture of keto and enol t a u t o m e r ~ ~ - ~ and that the rate of complex formation of the enolic form of a given P-diketone with a metal ion is much faster than that of the ketonic f ~ r m .~ - ~ Therefore, in order to explain the kinetics of complex formation of a P-diketone with metal ions, it is necessary to determine the fractions of the keto-enol tautomers and the rate of tautomerization. The rates of tautomerization of P-diketones have been measured by several methods such as the field-impulse methodg and the amperometric method.l0 Watarai et ul. determined the rates of the tautomerization of acetylacetone and benzoylace tone by measuring the absorbance change observed when two aqueous solutions with different dielectric constants were mixed.11-13 However, this method cannot be applied to the study of keto-enol tautomerism in aqueous solutions. In this work the distribution of HB between aqueous and micellar phases was measured in micellar systems on the basis of the fact that the enolic form of benzoylacetone is stable in micelle~.~~g l5 The rate of tautomerization of HB in aqueous solution was determined by following the absorbance changes observed when an aqueous solution of HB was mixed with a surfactant solution above the critical micelle concentration (c.m.c.).The rate of tautomerization was also found to be accelerated by base catalysts such as hydroxide or acetate ion. EXPERIMENTAL Benzoylacetone, sodium dodecyl sulphate (SDS) and dodecyl trimethylammonium chloride (DTACl) were purified by recrystallization. t Present address: Showa Denko Co. Ltd, Omarhi, Nagano, Japan. 394 12942 TAUTOMERIZATION RATE OF BENZOYLACETONE To measure the partition coefficient of HB, equal volumes of aqueous solutions of benzoylacetone and the surfactant were mixed and shaken in a water bath kept at 298 K for ca.1 h. The absorption spectra of the solution were measured by a Shimadzu UV-200s spectrophotometer. In the kinetic study two aqueous solutions were rapidly mixed in a chamber using a four-jet mixer and the time courses for the absorption of the solution at both 310 and 250 nm were measured by a rapid reaction analyser of the stopped-flow type (Union Giken Co. Ltd, RA- 1 100). RESULTS AND DISCUSSION SOLUBILIZATION OF HB INTO MICELLES The effect of SDS concentration on the absorption spectra of the mixed solution of HB and SDS was measured.14 The absorption band at 3 10 nm, assigned to the enolic form, increased with an increase in the concentration of SDS, while that at 250 nm, assigned to the ketonic form, decreased.The absorbance of HB in the presence of micelles is expressed as A = dM1) [HB1t/(l = Eobs[HB]t (1) where E and micellar phases, respectively, and a is defined as are the apparent extinction coefficients of HB in the aqueous and PM = [HB],/[HB] = PuM MM] = @MI (2) where [HB], is the concentration of benzoylacetone in the micellar phase based on the micellar volume, P is the normal distribution constant between the two phases, u, is the volume of the micelle, N is Avogadro's number and [MI is the micelle concentration. To estimate the unknown parameters, a and E,, eqn (1) is modified as follows: Here [S], is the total concentration of the surfactant and n is the aggregation number of the micelle.The values of the c.m.c. were determined as the surfactant concentration at which the absorbance at 310 nm began to increase, and were 3.3 mmol dm-3 for SDS and 9.0 mmol dm-3 for DTACl. The values of (a/n) and E~ were estimated from a linear plot l/([S], - c.m.c.) against 1 /(zObs - E ) and are summarized in table 1. The apparent extinction coefficients of HB solubilized in the micellar phase were similar to those obtained in non-polar solvents. This result suggests that the site of the solubilization of HB is inside the micelle, where the polarity is very low. As the fraction of enolic form is > 98% in non-polar s~lvents,l-~ it is deduced that the benzoylacetone solubilized in the micelle exists almost entirely in the enolic form. The value of P was estimated from eqn (2) using the aggregation numbers of the surfactants, i.e.112 for SDS and 50 for DTAC1.l6 The radius of a spherical micelle was assumed to be 2.0 nm for both surfactant~.~~ The values obtained are shown in table 1 with the partition constants of HB between several non-polar solvents and the aqueous phase. The values of P in micellar solutions are similar to those between organic and aqueous phases. /([$h - c*m*c*) = (&M -E)/(Eobs (a/n) - (a/n)- (3) RATE OF TAUTOMERIZATION OF HB When the two solutions of HB and surfactant (above twice the c.m.c.) were mixed, the absorbance at 310 nm increased exponentially with time and that at 250 nm decreased. The relaxation times determined at both wavelengths were the same. The dependences of the observed first-order rate constant (kobs = 7-l) on the concentrations of SDS and DTACl are shown in fig.1 (a) and (b), respectively. The observed rate constant decreased with the increase surfactant concentration.Y. MIYAKE, M. SHIGETO AND M. TERAMOTO 2943 0-06 0-05 0.04 - 'm 0.03- --- 9 Yo 0.02- Table 1. Extinction coefficients of HB in micellar solution and in organic solvents ( E ~ ) at 3 10 nm, a/n, distribution constants between the micellar and aqueous phases and distribution constants between the organic and aqueous phases I I I I I (a 1 - - - - - 0 - Q A surfactant or solvent E~ at 310 nm a/n P (b 1 0.05 - - 0.04 - - 0.03 - - 0.02 - - n SDS 1.26 x 104 120 670 DTACl 1.32 x 104 93 101 DTABr 1.32 x 104 90 CTACl 1.26 x 104 425 101 n-heptane 1.49 x lo4 benzene 1.47 x 104 - 1090 - - - - - water 5.44x 103 f l 0.01 1 0 ' I 1 I I 1 I 0 0-01 0-02 0.03 0.04 0.05 0.06 [ SDS] ,/mol dm -3 I 1 P n v) *O 0-01 t 01 I I 1 I I I I 0 0.01 0.02 0.03 0.04 0.05 0-06 0.07 (DTACl] o/mol dm-3 Fig.1. Effect of micellar concentration on the relaxation rate in micellar solutions of (a) SDS and (b) DTACl. Results were calculated from absorbance at 0, 310 and A, 250 nm, respectively. The pH and the ionic strength of the solutions were adjusted to 4.0 and 0.1 mol dm-3, respectively, with HCl and NaC1.2944 0.05 0.04- Tm 0.03- Ai0 --. YJ n TAUTOMERIZATION RATE OF BENZOYLACETONE! I I I - - n - O 0 - V 0.01 0*02* 2 4 6 0 10 PH Fig. 2. Effect of pH on the relaxation rate. [S], = 15 mmol dmP3 and [HB] = 0.05 mmol dm-3. As the rates of solubilization of organic substances into micelles are very rapid and the rate constants are 100 s-I,17 the process of solubilization relaxation could not be observed in this study.It is considered that the observed relaxation process is attributed to the tautomerization of benzoylacetone in the aqueous phase. The following mechanism is thus proposed : k EM k' H K e H E (HE),. (4) where HK and HE are the ketonic and enolic forms of benzoylacetone, respectively. It is assumed that only the enolic form of HB exists in the micelles, and also that the solubilization process is maintained at equilibrium { P,, = [(HE),]/[HE]). The observed first-order rate constant of relaxation can be expressed as follows : where KT defined by eqn ( 5 ) is the tautomerization equilibrium constant.The solid lines in fig. l ( a ) and (h) are the results calculated from eqn ( 5 ) using the values of a/n in table 1 and a rate constant k = 0.020 s-l, which is very close to the value reported by Harada et ~ 1 . ~ (0.018 s-l). It is concluded that the rate of tautomerization of HB in the aqueous phase can be reasonably explained by chemical relaxation caused by the very fast solubilization of HB into the micelles. The effect of the pH of the aqueous phase on kobs is shown in fig. 2. If the pH of the aqueous solution is comparable to or above the pK, of benzoylacetone, i.e. 8. I, dissociation of HB occurs and the rate of dissociation of the ketonic form is accelerated by hydroxide i ~ n . ~ . ~ ~ The observed rate constant, /cobs, can then be derived as follows : Here Kak is the dissociation constant of the ketonic form and has the value 2.3 x lop9 mol The value of k,, was determined as 2000 dm3 mol-1 s-l from the data shown in fig.2. Eigen et ul. obtained a value of ko, of 4 x lo4 dm3 mol-1 s-l for acetylacetone (pK, = 8.9) by the field-pulse m e t h ~ d . ~ The value for 2- thenoyltrifluoroacetone (pK, = 6.4) was estimated as 130 dm3 rno1-I s-l from the rate of 1 : 1 complex formation between 2-thenoyltrifluoroacetone and NiII or CuT1.18 It is thus deduced that the rate constant decreases with a decrease in pK,.Y. MIYAKE, M. SHIGETO AND M. 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