Sonication effects on ester hydrolysis in alcohol--water mixtures/Ultraheliefektid estri hudroluusile alkoholi-veesegudes.
Ultrasonic acceleration effects on chemical processes are widely used both in laboratory and industrial practice [1, 2]. Sonication mostly affects reaction rates, yields, and in some cases the ratios of reaction products. Besides bringing about mechanical effects, cavitation induced by sonication can promote many homogeneous and heterogeneous reactions by generating radicals that give rise to chain reactions in solution.
According to current tenets of sonochemistry , an ionic homogeneous reaction that cannot switch to a radical pathway should not be susceptible to ultrasound effects. However, a number of homogeneous polar (heterolytic) reactions have been found that are, in fact, accelerated or even suppressed by ultrasound [3-10]. These effects have been explained by the influence of ultrasound on specific solute-solvent interactions of hydrophobic nature that are not manifested in conventional reaction kinetics [4-11]. Moreover, we were able to show that in these cases the sonochemical effects can be related to the destruction of hydrophobic solute-solvent interactions [8-11].
Recent spectroscopic, X-ray diffraction, and mass spectrometric investigations have shed light on the structure of alcohol-water solutions [12-14]. As to the ethanol-water solutions, these observations suggested formation of associates between ethanol molecules and breakdown of the bulk water structure at [X.sub.E] > 0.1 > ([X.sub.E] is the ethanol molar ratio). In water--ethanol mixtures at [X.sub.E] > 0.15 > a large number of ethanol--water bonds are formed at the expense of water--water bonds, and this situation is described by a cluster model envisaging a stacked ethanol core surrounded by a thin water shell [12-14].
This model has allowed a straightforward interpretation of our results: as a hydrophobic reagent could be hidden inside the clusters it seems to be unavailable for reaction. If ultrasound breaks these hydrophobic clusters, this reagent becomes again available and acceleration of the reaction can be observed. Alkyl acetates were used as probes to study this possible cluster formation effect on the reaction rate. Indeed, sonication affected the acid-catalysed hydrolysis of these esters in the region 0.2 < [X.sub.E] < 0.3 and this effect matched in reverse order with the hydrophobicity of the esters. Butyl acetate should be held most effectively by clusters, and sonication was the least efficient in this case . However, in previous papers [8, 9] we have evasively discussed the observed sonication effects in the region [X.sub.E] < 0.15, as we were unable to suggest any unambiguous mechanism of the solute-solvent interaction.
In this report we have set out a broader investigation into hydrophobic effects of aliphatic alcohols as co-solvents on ester hydrolysis in water, and studied the neutral hydrolysis of 4-nitrophenyl chloroacetate in the presence of small amounts of alcohols. It is important to stress the advantages of this experiment in comparison with the measurement of ester hydrolysis by using the GLC technique (see in ). The spectrophotometric method was used in the former case and this enabled us to apply a very low ester concentration ([10.sup.-5] M) and to exclude the use of an internal standard. The results obtained together with our former data cast light on the nature of sonication effects and the hydrophobic interactions in the region of small to moderate molar ratios of alcohols in water.
Neutral hydrolysis of 4-nitrophenyl chloroacetate
Aqueous solutions were prepared by weighing Milli-[Q.sup.+] water and carefully purified alcohols. The pH of the solutions was maintained at 3.65 [+ or -] 0.05 with HCl. Calculated amounts of the ester or 4-nitrophenol as the reference compound were dissolved in analytical grade acetonitrile. Injection of 10 [micro]L of this solution into 100 mL of the reaction mixture, placed in the reaction cell, provided [10.sup.-5] M initial concentration of the reagents.
The kinetics of the neutral hydrolysis of 4-nitrophenyl chloroacetate was followed spectrophotometrically with a Perkin-Elmer Lambda 2S spectrometer. All hydrolysis reactions were performed without sonication and under ultrasound. Aliquots of 1 mL were withdrawn from the reacting mixtures at appropriate time intervals and the formation of 4-nitrophenol was monitored spectrophotometrically at 317 nm.
Sonication was performed under argon atmosphere with an Elma TI-H-5 MF2 cleaning bath at 25 kHz. The reaction cell, equipped with a mechanical stirrer and a digital thermometer, was reproducibly immersed in the bath thermostatted at 20.0 [+ or -] 0.1[degrees]C. The ultrasonic power of the cleaning bath was adjusted at 50% of the electrical output energy to 8.1 [+ or -] 0.7 W/100 mL in water according to the calorimetric measurements carried out in the reaction cell.
The sonolytic degradation of 4-nitrophenol was monitored identically under the same conditions as the hydrolysis of 4-nitrophenyl chloroacetate.
Acid-catalysed hydrolysis of n-propyl acetate
The acid-catalysed hydrolysis of -propyl n acetate was performed in ethanol--water mixtures in the presence of 1 M HCl. The decrease in the ester concentration was followed by GLC. Ultrasound was generated by a UZDN-2T probe disrupter operating at 22 kHz. Its energy output, estimated calorimetrically, was 55 W/80 mL in water. An immersed sonication horn was used. The reaction temperature was 20.0 [+ or -] 0.3[degrees]C. All runs were carried out at least in duplicates. The standard deviations of mean values of the calculated rate constants did not exceed 10%. For other experimental details see our previous paper .
RESULTS AND DISCUSSION
The hydrolysis of 4-nitrophenyl chloroacetate was carried out at pH = 3.65 in water and in the presence of 1 mol % of ethanol, -propyl n alcohol, and -butyl n alcohol. Experiments were made without sonication and under ultrasound. Results of the kinetic measurements are presented in Table 1.
Degradation of nitrophenol under sonication can lead to defacement of ester hydrolysis kinetics and this issue has been discussed in detail in our previous paper . Therefore the rate of sonolytic degradation of 4-nitrophenol was separately studied under the same conditions as used for the hydrolysis of 4-nitrophenol chloroacetate. The first-order rate constant of this process was found to be [k.sub.I] = 7.5x[10.sup.-5] [s.sup.-1]. This value is more than 50 times smaller than that for the ester hydrolysis (data in Table 1). Thus, the degradation of 4-nitrophenol can be ignored in the kinetics of ester hydrolysis.
Formation of encounter complexes between ester and alcohol, rendering the ester uncreative, was shown by Buurma et al. . Indeed, it is seen in Table 1 that the rate of hydrolysis decreased in parallel with the increase in the hydrophobicity of the co-solvent. However, the sonication effect, characterized by son ([k.sub.son]/k), increased almost linearly with the number of carbon atoms in the alcohol molecule (Fig. 1), resp. with the hydrophobicity of the co-solvent.
Such unexpected at first glance dependence of sonication effects can be related to the weak solvation of esters in this region. While greater hydrophobicity leads to a stronger ground-state stabilization and hence to a greater decrease in the reactivity, ultrasound breaks down the hydrophobic interactions almost entirely, thus giving rise to larger sonication effects for more hydrophobic esters.
The same results can be presented in a different way, as shown in Fig. 2. It can be seen that ultrasound of the applied acoustic power appears to destroy the ester-co-solvent encounter complexes regardless of the hydrophobicity of these compounds.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
These results encouraged us to revisit the sonication effects obtained for the acid-catalysed hydrolysis of aliphatic esters in the region [X.sub.E] 0.15 < , and certain inconsistency of these data forced us to recapitulate kinetic measurements with -propyl n acetate in ethanol-water binary solutions. Rate constants for the acid-catalysed hydrolysis of this ester under ultrasound and without sonication, measured in this study, are presented in Table 2. Unfortunately kinetic measurements were not feasible in pure water because of low solubility of the ester.
The plots of sonication effects son ([k.sub.son]/k) vs solvent composition are presented in Fig. 3. The curves show minima with almost no sonication effect for ethyl and propyl acetates while the more hydrophobic ester, n-butyl n acetate, may show a minimum at a higher ethanol content. We tend to attribute this phenomenon to the preferential solvation of esters in the solution.
[FIGURE 3 OMITTED]
Ultrasound seems to be responsible for the perturbation of the ground-state stabilization of the reactants [10, 11]. Therefore the sonication effect should reflect the solvation mode of ester molecules. In the region of clusters ([X.sub.E] > 0.15) the sonication effects are large for ethyl and propyl acetates, indicating a considerable interaction of esters with the solvent system. On the other hand, moderate or negligible effects at [X.sub.E] < 0.15 manifest relatively weak solvation, thus not dramatically affecting the reactivity when changing under sonication. The minima point at solvent compositions that provide solvation situations for ester molecules with the same reactivity as whether present or destroyed by ultrasound. Propyl acetate, more hydrophobic in comparison with ethyl acetate, apparently requires more alcohol molecules in an equilibrium solvation to show the minimum in sonication effects. As to -butyl n acetate, this highly hydrophobic compound seems to interact with the solvent system to such a power that at a higher content of the alcohol and under our experimental conditions ultrasound is not able to break down entirely the solvation equilibrium of the ester.
While in the cluster region ([X.sub.E] > 0.2) > the sonication effect correlates with the reverse order of the hydrophobicity of the esters in accordance with the supposed strength of the inclusion clusters, at a lower ethanol content in the solution ([X.sub.E] < 0.1) the sonication effect increases with an increase in the hydrophobicity of esters similarly to what was observed for the alcoholic co-solvents.
As a more general conclusion concerning ester hydrolysis, it can be inferred from the results above that in an aqueous alcohol binary mixture the solvent effects are limited almost entirely to the hydrophobic ground-state stabilization by the organic co-solvent.
This work was carried out under the auspices of the European Union COST Action D 32.
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Sander Piiskop, Hannes Hagu, Jaak Jarv, Siim Salmar, and Ants Tuulmets *
Institute of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia
Received 24 October 2007
* Corresponding author, firstname.lastname@example.org
Table 1. Results of kinetic measurements for the neutral hydrolysis of 4-nitrophenyl chloroacetate in water at 20[degrees]C Rate constant k x [10.sup.4] Ultrasonic acceleration Co-solvent, ([s.sup.-1]) ([k.sub.son]/k) 1 mol % Nonsonic Sonic -- 2.89 3.10 1.07 EtOH 3.27 4.17 1.28 n-PrOH 2.66 4.11 1.55 n-BuOH 2.42 4.07 1.69 Table 2. Results of kinetic measurements for the acid-catalysed hydrolysis of propyl acetate at 20[degrees]C Content of ethanol, Rate constant k x [10.sup.4] ([s.sup.-1]) wt % ([X.sub.EtOH]) Nonsonic Sonic 10 (0.042) 0.89 [+ or -] 0.03 1.42 [+ or -] 0.04 13 (0.055) 0.97 [+ or -] 0.07 1.41 [+ or -] 0.1 20 (0.089) 0.99 [+ or -] 0.06 1.20 [+ or -] 0.06 30 (0.144) 0.75 [+ or -] 0.02 0.74 [+ or -] 0.07 40 (0.207) 0.66 [+ or -] 0.04 0.93 [+ or -] 0.08 50 (0.281) 0.64 [+ or -] 0.06 1.11 [+ or -] 0.07 Content of ethanol, Ultrasonic acceleration wt % ([X.sub.EtOH]) ([k.sub.son]/k) 10 (0.042) 1.59 13 (0.055) 1.44 20 (0.089) 1.21 30 (0.144) 0.99 40 (0.207) 1.41 50 (0.281) 1.75