Partition kinetic studies of benzoylacetone

Z inorg, aucl. Chem. Vol, 43, No, 10, pp. 2507-2510, 1981 Printed in Great Britain.

0022-19021811102507-4)4502.00t0 © 1981 Pergamon Press Ltd

PARTITION KINETIC STUDIES OF BENZOYLACETONE ACID-CATALYSIS AND MEDIUM EFFECT IN AQUEOUS ACID PHASE KAZUO BABA, HITOSHI WATARAI and NOBUO SUZUKI* Department of Chemistry, Faculty of Science, Tohoku University, Sendal, Japan

(Received 30 October 1980; received [or publication 23 January 1981) Abstract--The partition rate constants and the apparent partition coefficients of benzoylacetone between carbon tetrachloride and water, concentrated aqueous perchloric acid or sulphuric acid phases were determined at 25°C. The enol fractions and the keto-enol tautomerization rate constants in water and aqueous acid solutions were also determined. From the experimental results, the difference of the medium effects of the two acid phases on the partition mechanism was discussed and it was concluded that the partition rate of benzoylacetone is governed by the partition coefficients of keto and enol forms which are remarkably influenced by the medium effects of these acids and the tautomerization rate in aqueous acid phase which increases with increase of the acid concentration.

INTRODUCTION Various/3-diketones have been extensively employed as a useful extractant for a number of metal ions and the study of the partition behaviour of /3-diketones in the two-phase system is important in order to elucidate the solvent extraction mechanisms of metal ions with /3diketones. The extraction of metal ions with/3-diketones from quite high acidic solutions over I mol dm -3 was investigated[I,2], but few studies of the partition behaviour of /3-diketones itself at high acidities have been reported [3]. In previous papers, the partition rate and partition mechanism of acetylacetone [4] and seven n-alkyl substituted /3-diketones[5] between apolar solvents and an aqueous phase ([H+] = 0.001 mol dm -3, I=0.1) were investigated, and it has been clarified that the rate determining step of the partition is the keto-enol tautomerization of /3-diketones in the aqueous phase. The keto-enol tautomerization of/3-diketones, in similar with other general tautomerization reaction, may be catalyzed by the presence of acid or base. The base-catalysis in the tautomerization of acetylacetone was investigated [6] but there is no report concerning with the acid-catalyzed tamomerization of/3-diketones. In the present study, we examined the partition rate of benzoylacetone (1-phenyl1, 3-butanedione) in carbon tetrachloride/concentrated aqueous perchloric acid or sulphuric acid system and discussed the influence of the acid-catalysis and the medium effect of these acids on the partition behaviour of benzoylacetone.

EXPERIMENTAL Reagents. Benzoylacetone was recrystallized three times from absolute ethylether, and it gave the melting point 57.6--58.4°C. Redistilled water was used, Carbon tetrachloride was purified by the ordinary method [8]. Perchloric acid and sulphuric acid were standardized against sodium carbonate solution. Aqueous acid solutions were prepared from weighed quantities of concentrated acids and water. Sodium perchlorate and sodium chloroacetate were recrystallized from water. All of other reagents were of a reagent grade.

*Author to whom correspondence should be addressed.

Kinetic measurement. The partition rate constants of benzoylacetone were obtained by means of the "in-liquid ejection" method at 25 +-0.1°C. The detailed procedure of the partition rate measurement is similar to that of the previous work [9]. The rate constant was obtained as the average value from four measurements, and the reproducibility of the rate constants is within 4%. The keto-enol tautomerization rate constants of benzoylacetone in aqueous phase were obtained by the following procedure. Setting a glass stoppered quartz cell, which contains 3 ml of aqueous solution, in the thermostated (25 _+0.1°C) cell compartment of Hitachi 356 spectrophotometer, a 2.5 x 10 2 mt portion of ethanol solution of benzoylacetone (5 x 10 ~ tool dm 3) was rapidly introduced into the cell. Since benzoylacetone in ethanol contains about 92% enol and 8% keto tautomers[10], a subsequent absorption decreasing at 310am soon after the dissolution was observed as a function of time. Ordinary treatment of first-order kinetics of the absorbance change gave the ketoenol tautomerization rate constant. The rate constant was obtained as the average value from at least seven measurements and reproducibility is within 3%. Equilibrium measurement. The apparent partition coefficients of benzoylacetone were determined by the following procedure. 5 ml of carbon tetrachloride solution of benzoylacetone and the equal volume of aqueous phase were mechanically shaken in 40 ml vial stoppered with glass cap for 30 min-3 hr in the thermostated room at 25 ± 0.5°C. After centrifugation, an aliquot of the aqueous phase was removed into a quartz cell. From the absorbance at 310nm and the predetermined molar absorption coefficient of benzoylacetone, the concentration of benzoytacetone distributed was determined. From the initial concentration and the concentration in the aqueous phase, the apparent partition coefficient (P) expressed as the concentration ratio of benzoylacetone between organic and aqueous phases was calculated. The reproducibility of the four identical measurements for the partition coefficient is within 2%. The enol fraction (f) of benzoylacetone in aqueous acid solution was estimated from the apparent molar absorption coel~cient of benzoylacetone, which is a mixed value for the equilibrium mixture of k'eto and enol forms, and the molar absorption coefficient of the enol form at 310nm which is a characteristic absorption peak for the enol form. The molar absorption coefficient of the enol form in water was determined as follows. An alkaline solution (pH 11) of benzoylacetone (1 10 4 tool dm 3), in which benzoylacetone dissociates completely, was rapidly mixed with diluted perchloric acid (pH 2.5) by means of stopped-flow sample mixing device, and the subsequent decrease in the absorbance at 310am was recorded on a Hitachi RSP-2 rapid-scan speetrophotometer at 25 ± 0.3°C. At the time of

2507

KAZUO BABA et al.

2508

mixing, benzoylacetone is thought to be in the enol form perfectly [11]. Therefore, the molar absorption coefficient of the enol form at 310nm was estimated from the absorbance extrapolated at the time of mixing as 1.62× 104 tool-~ dm3 cm-j. The enol fraction was calculated by f = EapJI.62× 104, where eapp is the apparent molar absorption coefficient of benzoylacetone at 310 nm in a given system.

O

O

'E

RESULTS AND DISCUSSION

Partition mechanism system

in carbon tetrachloride/water

j

0.1

First of all, in order to clarify the partition mechanism of benzoylacetone in carbon tetrachloride/water system, the dependence of the partition rate constant, kp,ob~, on the tautomerization rate constant, kobs, was examined by accelerating the tautomerization rate in the aqueous phase with a base-catalyst such as chloroacetate and

f

• HCO0• catalyst-free

I 0

I I0

5 R o b s / m in -

02

(0)

Fig. 2. Correlation between the partition rate constant in CCIdwater system and the tautomerization rate constant in aqueous phase in the presence of base catalyst.

& E \

0 C H2 CI, CO0-

HCO0-

'

0.04 '

'

formate under the condition that the catalyst does not affect directly the partition and tautomerization equilibria because of its low concentration. The partition rate in the two-phase system and the tautomerization rate were both increased linearly with the concentration of the catalyst as shown in Fig. 1. A linear relationship between kt,.obs and kob,, as is illustrated in Fig. 2, is represented by kp,ob, = Akobs. The slope A's in the two different catalyst systems are in good agreement each other. From the above results, the following scheme (1) can be proposed for the partition of benzoylacetone,

0 ~)8

PK-

ke

I

PE

K(org).----~K(aq).----~E(aq).---~E(org)

bose / tool dm -3

kk

Fig. l(a). Variation of the partition rate constant in CCh/water system with base catalyst concentration in aqueous phase.

(b)

(1)

where K and E denote keto and enol forms, PK and PE the partition coefficients of keto and enol forms and k, and kk are the rate constants of enolization and ketonization, respectively. From the scheme (1), the partition rate constant kp can be represented by the following equation which has been derived for the partition of acetylacetone [4].

ko =Ak

(2)

k = ke +kk

(3)

where E 4~o 5

and l-f + 1 + PE

A=

(4)

d) H C O 0 -

where k is the observed tautomerization rate constant in the aqueous phase. The partition coefficients of keto and enol forms are calculated from the respective equations, 0.08

0 base/moL

drn -3

Fig. I(b). Variation of the tautomerization rate constant in aqueous phase with base catalyst concentration.

PK = ~_~P, -

PE--~P _

fo

(5)

where fo is the enol fraction of benzoylacetone in the

04

2509

Partition kinetic studies of benzoylacetone organic phase. In carbon tetrachloride/water system, the calculated equilibrium factor, A=0.0161, which was obtained by eqns (4) and (5) using the independently measured values (kob,=3.17min -~, f=0.310 and P = 540) and a enol fraction in carbon tetrachloride reported previously [12] (fo = 0.976), was good agreement with the slope in Fig. 2, 0.0177. In the same but catalyst-free system, the partition rate constant was observed as 5.25 x 10-~ r a i n ' and this was also good agreement with the calculated one, 5.10 x 10 : min-~. Therefore it can be stated that the partition mechanism of benzoylacetone in carbon tetrachloride/water system is represented by scheme (I) and the dominant process governing the partition rate is the keto-enol tautomerization reaction in the aqueous phase.

'_.q

902

0

Acid-catalysis in carbon tetrachloride]aqueous acid system In carbon tetrachloride/aqueous sulphuric acid or perchloric acid systems, k~,ob~and kobs were determined and shown in Fig. 3 as a function of the acid concen(a) ©4

E

05

Z o

O:

./

0

HzS04 HCI.04

/

i 5

t

I~? k'obs/mln

Fig. 4. Correlation between the partition rate constant in CC14/aqueous acid system and the tautomerization rate constant in aqueous acid phase. Broken line denotes the correlation observed in base catalyst system (see Fig. 2). tration. Both the rate constants were distinctly enhanced by the addition of these acids, and this suggests a catalytic effect of these acids for both the rate processes. The results show perchloric acid has a larger catalytic effect for the partition rate than sulphuric acid does, while sulphuric acid is more effective for the tautomerization rate. Furthermore, as exemplified in Fig. 4, a correlation between k~,,ob, and kob~ does not show a linear relationship through origin (see Fig. 2). From the above results, it is expected that the addition of sulphuric acid and perchloric acid into aqueous phase not only causes the acid-catalyzed tautomerization in aqueous phase but also affects the equilibrium factor. Consequently, the effect of the acids on the partition rate should be treated by considering the medium effect of acids on the partition and tautomerization equilibria.

oc~d / mo~, d m -3

Fig. 3(a). Variation of the partition rate constant in CCh/aqueous acid system with the acid concentration.

Medium effect of sulphuric and perchloric acids A preliminary experiment showed that the UV absorption maxima (250 and 310 nm) of benzoylacetone in 55

(b

H2S04 o, 5 0

_9o

'c

25

15

11

H2S04

O5 0

II ocid/tool

I

I 2 dm -~

Fig. 3(b). Variation of the tautomerization rate constant in aqueous acid with the acid concentration. INC Vol. 43, No. 10--U

0

I

2 acid/tool,

5 drn -~

4

Fig. 5. Dependence of the partition coefficients of keto and enol tautomers of benzoylacetone on the acid concentration.

KAZUO BABA et al.

2510

Table 1. The rate and equilibrium data of benzoylacetone in CCh/aqueous acid systems at 25°C acid mol dm"3 HCIO4 O.OOl 0.21 0.59 0.98 1.58 2.08 4.18

f

p

kobs min"l

kp~obs min"l

k

*

min-

0.325 0.321 0.304 0.298 0.290 0.277 0.261

547 475 366 302 226 180 77.1

3.29 3.28 3.70 4.77 6.91 ll.O

0.054 0.058 0.083 0.12 0.22 0.40

0.054 0.06l 0.085 0.13 0.24 0.45

0.325 0.318 0.307 0.298 0.287 0.276

557 609 657 713 763 675

4.60 5.72 7.82 12.6

0.073 0.086 0.097 0.17 0.26 0.42

0.074 0.083 O.lO 0.15

H2SO4 0.31 0.63 0.92 1.57 2.54 4.15

* Calculated by equations (2) and (4), where fo = 0.976 [12].

aqueous sulphuric and perchloric acid solutions undergo progressive red shifts with increase of the acid concentration on account of the protonation of benzoylacetone. This red shift was not observed under the acid concentration of 4.5 mol dm -3, and the protonated species is negligible. Hence, a variation of the enol fraction of benzoylacetone with the acid concentration under 4.5 mol dm -3, where an additional species other than the keto and the enol tautomers is not present, was examined spectrometrically. The observed enol fraction (f) in aqueous acid phase and the apparent partition coefficient (P) in carbon tetrachloride/aqueous acid system are listed in Table 1. The enol fractions decrease with increase of the acid concentration, and this seems to be that the keto form is relatively more stabilized than the enol form by the addition of acids. The apparent partition coefficients of benzoylacetone varied depending on the concentration of acids. Namely, in perchloric acid system the partition coefficients are monotonously decreasing with increase of the acid concentration. On the other hand, the partition coefficients in sulphuric acid system gradually increases, pass through a maximum at around 2.5 mol dm -3, and finally decreases. From the observed values of the enol fraction and the apparent partition coefficient, the partition coefficients of keto and enol forms of benzoylacetone can be calculated separately be eqn (5). In Fig. 5, the partition coefficients of the keto and enol forms are logarithmically plotted against the acid concentr~tion. As the plots for both of the tautomers in perchloric acid system is linearly decreasing, the keto and enol forms are strongly salted-in with perchloric acid. In surphuric acid system, however, keto and enol forms are slightly salted-out up to 2.5 mol dm -3 and then salted-in. The salt effects of these acids are directly correlated with the equilibrium factor shown as eqn (4). For example, the equilibrium factor in 1.58 mol dm -3 perchloric acid system is about three times as large as the one in the same concentration of sulphuric acid system. This predicts that the partition rate in perchloric acid system is larger than that in sulphuric acid systems

if the tautomerization rates in these acid systems are not influenced by the acids. From the detailed examination of the acid catalysis and the medium effect, the general validity of eqns (2) and (4) are demonstrated, and the partition rate constant can be calculated at different conditions. The calculated partition rate constants, listed in the last column in Table 1, are good agreement with the observed ones. Therefore, it is concluded that the partition mechanism of benzoylacetone in carbon tetrachloride/aqueous acid system is represented by scheme (1) as well as in carbon tetrachloride/water system, and the partition rate depends on the nature of acid and its concentration, that is, the acid-catalyzed tautomerization and the medium or "salt" effect on the partition coefficients of each of the tautomers. In other words, the strongly salting-in effect of perchloric acid makes up for the lower catalytic activity of this acid for the tautomerization rate and, after all, perchloric acid is apparently more effective catalyst for the partition rate than sulphuric acid. REFERENCES

I. J. P. Mckaveney and H. Freiser, Anal, Chem. 29, 292 (1957). 2. S. Suzuki and Y. Inoue, Bull. Chem. Soc. Japan 39, 1705 (1%6). 3. K F. Fouch6, J. lnorg. Nucl. Chem. 32, 3369 (1970), 4. H. Watarai and N. Suzuki, J. lnorg. Nucl. Chem. 38, 310 (1976). 5. H. Watarai and N. Suzuki, J. lnorg. Nucl. Chem. 40, 1909 (1978). 6. M.-L. Ahrens, M. Eigen, K. Kruse and G. Maass, Ber. Bunsenges. Physik. Chem. 74, 380 (1970). 7. M. K. Eidinoff, J. Am. Chem. Soc. 67, 2072 (1945). 8. A. Weissberger, E. S. Proskauer, J. A. Riddick and E. E. Toops, Jr., Technique of Organic Chemistry, Vol. 7. Interscience, New York (1955). 9. H. Watarai and N. Suzuki, lnorg. Nucl. Chem. Lett. 10, 431 (1974). 10. R. A. Morton, A. Hassan and T. C. Calloway, J. Chem. Soc. 883 (1934). 11, M. Eigen, Pure Appl. Chem. 6, 97 (1963). 12. Y. Yoshimura and N. Suzuki, Anal. Chim. Acta 85, 383 (1976).