Article pubs.acs.org/IECR
Ultrasound Assisted Synthesis of 4‑Benzyloxy-3methoxybenzaldehyde by Selective O‑Alkylation of Vanillin with Benzyl Chloride in the Presence of Tetrabutylammonium Bromide Sumit M. Dubey and Parag R. Gogate* Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai, 400 019, India ABSTRACT: Cavitational reactors are a novel and promising form of multiphase reactors, based on the principle of the release of a large magnitude of energy due to the violent collapse of the cavities, which can be effectively applied for intensification of chemical synthesis. In the present work, 4-benzyloxy-3-methoxybenzaldehyde has been synthesized by selective O-alkylation of vanillin with benzyl chloride using tetrabutylammonium bromide-based phase transfer catalysis in the presence of ultrasonic irradiations. The reactions were performed in a 150 cm3 capacity glass reactor equipped with reflux condenser, and ultrasonic irradiations were introduced using an ultrasonic horn operating at a frequency of 20 kHz with a power rating of 120 W. The effects of different operating parameters such as quantum of aqueous phase, temperature, catalyst loading, ultrasonic power dissipation, and molar ratio of reactants have been investigated with an aim of maximizing the extent of synthesis of 4-benzyloxy3-methoxybenzaldehyde. Kinetic studies have also been performed, and the activation energy for the reaction has been established. The work has clearly established a superior process for synthesis based on the use of ultrasonic irradiations with higher yields as compared to the conventional approach. reactions. Thompson and Doraiswamy5 have presented an excellent review on the different chemical reactions that can be intensified with the use of ultrasound and also provide detailed discussion on the engineering aspects. Other applications of ultrasound-based cavitational reactors being extensively studied include wastewater treatment, enzymatic reactions, extraction of natural products, nanomaterial synthesis, and food processing applications etc.1,6−9 The application of ultrasound-based reactors has not yet been extensively investigated for the alkylation reactions in the presence of phase transfer catalysts, which are limited by mass transfer effects and/or intrinsic chemical kinetics leading to significantly slower rates of reactions. Thus, there is a need to study alkylation reactions in the presence of ultrasound for better understanding of the mechanism and the engineering aspects of possible intensification. Phase transfer catalysis (PTC) has wide applications in a variety of industries such as intermediates, dyestuffs, agrochemicals, perfumes, flavors, pharmaceuticals, and fine chemical industries.10−12 PTC facilitates the reaction between chemical species situated in different phases, and some of the common examples are the reactions between the salts dissolved in water or present in the solid state and substances dissolved in organic media. Without the presence of catalyst such reactions are usually slow, giving marginal yields, or sometimes do not occur at all. The reacting system typically consists of a heterogeneous two-phase system; one phase is a reservoir of reacting anions or base for the generation of anions, whereas the second organic phase comprises the organic reactants and catalysts (source of
1. INTRODUCTION Process intensification is immensely important in the chemical process industry.1 It plays a very important role especially in organic synthesis to improve processing and thereby enhance profitability. The use of cavitational reactors can create conditions favorable for process intensification of different chemical processing applications including organic synthesis based on different homogeneous and heterogeneous reactions. Cavitation results in the generation of high temperature (in the range of 1000−15000 K) and pressure pulse (in the range of 500−5000 bar) at the center of collapsing bubbles.1 Cavitational events occur at multiple locations in the reactor, creating different chemical and physical effects which can contribute to the process intensification. The physical effects include local turbulence and circulation, which favors the mass transfer and intensifies the reaction which is limited by the mass transfer barrier. Cavitation also results in chemical effects such as the generation of hot spots and reactive species such as free radicals that intensify the chemical process limited by intrinsic chemical kinetics.1 In terms of the nature of the reactions, that is, homogeneous or heterogeneous, the use of ultrasound induced cavitation can intensify the homogeneous chemical reactions by way of generation of local hot spots and reactive free radicals, whereas the intensification of heterogeneous reactions is based mainly on the mechanical effects of cavitation resulting in enhanced mass transfer rates based on the turbulence and better contact between the different phases due to the formation of fine emulsions. In general, the use of ultrasound in chemical synthesis results in increased yield, reduction in processing time, increased conversion, reduction in byproducts formation, and increased selectivity toward product formation.2−4 In the past, ultrasound has been most commonly applied to intensify esterification, transesterification, hydrolysis, polymerization, and oxidation © 2014 American Chemical Society
Received: Revised: Accepted: Published: 7979
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Figure 1. Experimental setup for ultrasound-assisted synthesis.
Figure 2. Schematic representation of synthesis of 4-benzyloxy-3-methoxybenzaldehyde.
benzyl chloride (L.R.) were obtained from Thomas Baker Chem. Pvt. Ltd., Mumbai, India. Vanillin (A.R.) was obtained from HiMedia Laboratories Pvt. Ltd., Mumbai, India. Sodium hydroxide pellets (G.R.) were obtained from Merck India Ltd., Mumbai, India, whereas sodium chloride (A.R.) was obtained from S.D. Fine-Chem. Ltd., Mumbai, India. All the chemicals were used as received from the supplier. 2.2. Reactor Details. Ultrasonic irradiations were introduced into the liquid solution using an ultrasonic horn with a tip diameter of 1 cm and operating at a frequency of 20 kHz with a rated power dissipation of 120 W. The ultrasonic horn was procured from M/s Dakshin Pvt. Ltd., Mumbai, and the initial calorimetric studies revealed that the energy efficiency was about 12%. The reactions were studied in a 5.0 cm (i.d.), 150 cm3 capacity glass reactor with a three-neck lid, which was equipped with reflux condenser. The reactor was kept in a water bath to maintain the desired temperature. A water bath used for maintaining the operating temperature was procured from M/s Ganesh Scientific Industries, Mumbai, India. A schematic representation of the experimental setup has been shown in Figure 1. 2.3. Experimental Procedure. Reactions were carried out in a three phase L−L−L system. An aqueous phase consists of vanillin, NaOH, and NaCl, whereas the organic phase consists of benzyl chloride in toluene. TBAB catalyst was added in the required amount to this system which forms a third middle layer to facilitate the reaction. In a typical reaction scheme, benzyl chloride is taken as the limiting reactant with excess of vanillin and NaOH, and the reaction proceeds toward complete conversion of benzyl chloride. The progress of the reaction was monitored on the basis of the disappearance of benzyl chloride in the organic phase. The reaction scheme for the selective Oalkylation of vanillin with benzyl chloride using a phase transfer catalyst (TBAB) has been schematically depicted in Figure 2.
lipophilic cations). The reacting anions are continuously introduced into the organic phase in the form of lipophilic ion-pairs with lipophilic cations supplied by the catalyst.10 The principle of PTC is based on the ability of certain phase-transfer agents to facilitate the transport of reagent from one phase into another (immiscible) phase wherein the other reagent exists. Thus, reaction is made possible by bringing together the reagents which are originally in different phases. However, it is also necessary that the transferred species is in the active state for effective phase transfer catalytic action, and that it is regenerated during the organic reaction.11 Ultrasound has been reported to increase the effectiveness of the phase transfer catalysts,12−14 though not much work has been reported in the area of alkylation reactions. Considering these facts, the present work deals with investigating the effect of ultrasound and important operating parameters on the synthesis of 4-benzyloxy-3-methoxybenzaldehyde based on the selective O-alkylation of vanillin with benzyl chloride in the presence of a phase transfer catalyst, tetrabutylammonium bromide (TBAB). 4-Benzyloxy-3-methoxybenzaldehyde is synthesized mostly via the L−L two-phase systems, which limit the conversion and also requires significantly longer reaction times. The synthesis approach via the L−L−L system using a phase transfer catalyst gives advantages over the twophase system such as ease in the separation and increased conversion with less processing time.15 4-Benzyloxy-3-methoxybenzaldehyde (4-benzyloxy-vanillin) is widely used as a perfume and as a starting material for the synthesis of thalifoline and ephedrine or in the synthesis of flavonoid compounds.
2. MATERIALS AND METHODS 2.1. Materials. Tetrabutylammonium bromide (TBAB) (extrapure 98%) was obtained from SISCO Research Laboratories Pvt. Ltd. Mumbai, India. Toluene (A.R.) and 7980
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Initially, an aqueous phase was prepared by taking 0.02 mol of vanillin, 0.025 mol of NaOH and 0.12 mol of NaCl in water. Sodium chloride was used to saturate the aqueous phase. To this mixture, 9.3 × 10−3 mol of TBAB catalyst was added. The organic phase which is composed of 0.02 mol of benzyl chloride was then added, and finally the total volume was made up to 25 mL with toluene and then added in the reactor. In this reaction mixture, the ultrasound probe was immersed at a depth of 1 cm. The operating ultrasound parameters used for the investigations were as follows: duration of the pulse, 5 s on and 5 s off; power, 90 W; fixed frequency at 20 kHz. At the end of the reaction, the organic phase, which contains 4-benzyloxy-3-methoxybenzaldehyde and toluene, was washed with an excess amount of water to remove any traces of catalyst, and the solvent was distilled under vacuum. The solid material was recrystallized using ether to get a pure 4-benzyloxy-3methoxybenzaldehyde. The formation of the product using this strategy was confirmed based on the earlier reported work of Yadav and Lande.15 The focus of the present work was more on investigating the intensification of the reaction based on the kinetic aspects which was solely decided on the basis of conversion of benzyl chloride. It is important to check the reproducibility of the obtained data which indeed was attempted in the present work. The experiments were repeated in duplicate, and the experimental errors were observed to be within ±2% of the reported data. 2.4. Analysis Techniques. The samples were withdrawn at regular intervals and analyzed by a gas chromatograph (GC) (Chemito model 8610) equipped with flame ionization detector (FID). A stainless steel column (3.25 mm × 4 m) packed with a liquid stationary phase of 10% OV-17 was used in the analysis. The samples were directly injected by taking a 3 μL sample volume in a SGE syringe (10 μL capacity). The temperature program was set as initial 100 °C with 5 min hold followed by gradual 10 °C per min rise until 300 °C followed by 5 min hold. The conversion of reaction was quantified based on the disappearance of benzyl chloride present in the organic phase.
Figure 3. Comparison of ultrasound-based and three-phase synthesis in absence of ultrasound: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; TBAB, 3 g; benzyl chloride, 2.3 mL; toluene, 22.7 mL; water, 20 mL; temp = 90 °C.
the degree of intensification is very significant when compared with the conventional approach based on the use of the normal two phase L−L system (as evident from existing literature). Schrittwieser et al.16 has reported that 20 h of processing time yielded 87% at room temperature in the conventional stirringbased two-phase approach, whereas Xia et al.17 reported 95% yield with overnight stirring. It has been reported that the presence of a strong aldehyde group in vanillin restricts the extent of conversion and hence the use of modifications in terms of the L−L−L three-phase system and the ultrasonic irradiations play a significant role. 3.2. Effect of Quantum of an Aqueous Phase. Phase transfer catalyst (TBAB) has tendency to dissolve in water and hence can prevent the formation of third phase which thereby decreases the conversion. To avoid this, NaCl was added to saturate the aqueous phase, so that the TBAB catalyst is not dissolved in the aqueous phase and the third catalyst (middle) phase remains intact. The effect of quantity of water on the yields of 4-benzyloxy-3-methoxybenzaldehyde was studied in the presence of ultrasound. The quantity of water was varied as 18 mL, 20 mL, and 23 mL. It was observed as per the results reported in Figure 4, that 20 mL of water gives the maximum conversion. Beyond this optimum quantity, as the amount of water increases, more and more TBAB dissolves in water, lowering the effective quantum available for the reaction resulting in a decreased conversion. It is essential that the optimum quantity of water needs to be used for better conversion so as to maintain the maximum quantum of TBAB present in the middle phase. Yadav and Lande15 also reported the requirement of optimum quantum of NaCl to saturate the aqueous phase which also confirms the requirement of optimum water. 3.3. Effect of Temperature. To determine the effect of reaction temperature on 4-benzyloxy-3-methoxybenzaldehyde formation, the reaction was carried out at different temperatures of 60, 70, 80, 90, and 100 °C. Reaction was carried out for 1 h and sampling was done at regular intervals of time. The obtained results have been shown in Figure 5. It has been observed that the conversion increases significantly with an increase in the temperature, which confirms that reaction was intrinsically kinetically controlled. Quantitatively speaking, with an increase in the temperature from 60 to 90 °C, the extent of
3. RESULTS AND DISCUSSION 3.1. Comparison of Ultrasound-Assisted Approach with the Conventional Synthesis. Synthesis of 4-benzyloxy3-methoxybenzaldehyde using an ultrasound-based approach has not been studied in any of the earlier investigations, and hence before proceeding to detailed kinetic study and optimization of different operating parameters, it was necessary to check the extent of intensification that can be obtained due to the use of ultrasound. For checking the effect of use of ultrasound, additional experiments were performed using only mechanical stirring at a speed of rotation of 600 rpm, and other operating conditions were kept constant (three-phase L−L−L system with the use of PTC). The obtained results have been shown in Figure 3. It can be seen that the use of ultrasound increases the extent of conversion from 77.9% in the case of the approach without the use of ultrasound to 97.1% in the case of the ultrasound-based approach. Further, the selectivity toward product has been observed to be 100%, and no other byproduct was observed in the case of ultrasound-assisted synthesis. It is also important to note here that the observed intensification is due to the use of ultrasound and not due to the possible increase in the temperature; it has been reported that an increase in temperature above 90 °C has negligible effect on the progress of the reaction.15 It is also interesting to note here that 7981
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Figure 4. Effect of quantum of an aqueous phase on extent of conversion: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; TBAB, 3 g; benzyl chloride, 2.3 mL; toluene, 22.7 mL; temp = 90 °C.
Figure 6. Effect of TBAB catalyst loading on the extent of conversion: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; benzyl chloride, 2.3 mL; toluene, 22.7 mL; water, 20 mL; temp = 90 °C.
phase occurred at a TBAB amount of 3 g. At any quantity of catalyst below 3 g, the middle phase disappeares as all TBAB catalyst dissolves in the water and the system converts itself into an L−L system and thereby the conversion becomes lower. In the L−L two-phase system, conversion shows linear variation for 1 and 2 g. It was found that formation of a third phase creates nonlinearity in conversion and there is a significant increase in the conversion from 44.53% for 1 g to 96.83% for 3 g catalyst loading in 1 h of reaction time. A further increase in catalyst concentration above 3 g resulted in settling of the TBAB catalyst at the bottom of the reactor leading to detrimental results. Similar results were reported by Vivekanand and Wang19 for synthesis of 2-phenylvaleronitrile using TBAB catalyst, where rate of reaction increased with an increase in the amount of quaternary ammonium salt until an optimum loading. 3.5. Effect of Molar Ratio (Benzyl Chloride/Vanillin). The effect of molar ratio of benzyl chloride to vanillin on the synthesis of 4-benzyloxy-3-methoxybenzaldehyde was studied by varying the molar ratio as 0.75:1, 1:1, and 1.5:1. The obtained results have been shown in Figure 7. It was found that the middle phase disappeared as the quantity of benzyl chloride
Figure 5. Effect of temperature on the extent of conversion: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; TBAB, 3 g; benzyl chloride, 2.3 mL; toluene, 22.7 mL; water, 20 mL.
conversion increased from 45.7% to 97.1% in constant reaction time as 1 h. It was also observed that at 90 and 100 °C, the conversion was almost similar. Also, it has been observed that an increase in temperature from 90 to 100 °C resulted in degradation of the product and hence the effective yield decreased. Yadav and Lande15 also reported the optimum temperature requirement for the maximum conversion of the alkylation reaction. Similar results were also reported by Escudero et al.18 for the ultrasound assisted N-alkylation of imidazole, and it was established that a liquid sonicated at a temperature nearer to the boiling point of the medium diminishes the enhanced effect that can be obtained by the use of ultrasound. 3.4. Effect of TBAB Catalyst Loading. The rate of a catalytic reaction is always proportional to the number of active sites, especially when the mass transfer resistance is completely eliminated. Thus, the rate of reaction should be dependent on the catalyst quantity. The catalyst quantity was varied over a range of 1 to 4 g in the presence of ultrasound and the obtained results have been shown in Figure 6. It was observed that the formation of a middle phase (L−L−L system) takes place only after the addition of a certain critical amount of catalyst to the reaction mixture. In the present case, the formation of a middle
Figure 7. Effect of molar ratio (benzyl chloride/vanillin) on the extent of conversion: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; TBAB, 3 g; toluene, 22.7 mL; water, 20 mL; temp = 90 °C. 7982
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was carried using an ultrasonic horn operating at 20 kHz frequency at different power dissipation values of 70, 90, and 100 W. The obtained results are shown in Figure 9. It was
was increased, shifting the reaction from a three-phase system to a L−L system, which leads to a reduction in the reaction rate. The changes in the phase formation were attributed to the phase equilibrium of the system. The polarity of organic phase changes, and it completely dissolves the middle phase.15 It was found that the conversion increases with a decrease in benzyl chloride concentration. The conversion for molar ratio (1.5:1) was only 55.8% and that for a molar ratio (1:1) was 97.1%. Vivekanand and Wang19 reported similar results of increased conversions only until an optimum concentration of reactants (molar ratio). It was also confirmed that the increased concentration beyond the optimum value disturbs the phase equilibrium and shifts the reaction equilibrium to the backward direction, and hence the conversion at equilibrium decreases. Yadav and Lande15 also reported that at much higher molar ratios in the L−L−L system, the system converts itself to the L−L system, and hence the conversion decreases. Thus, it can be concluded that it is important to establish the optimum reactant ratio for maximum benefits as it is generally assumed fact that the rate of reaction would increase with an increase in the molar ratio. 3.6. Effect of NaOH Quantity. The effect of sodium hydroxide amount on the synthesis of 4-benzyloxy-3-methoxybenzaldehyde was also studied by varying the quantity of NaOH over the range of 0.4−1.2 g under otherwise similar reaction conditions. The obtained results have been given in Figure 8. It can be seen that, for low concentrations of NaOH,
Figure 9. Effect of ultrasound power on the extent of conversion: vanillin, 7 g; NaCl, 7 g; TBAB, 3 g; NaOH, 1 g; toluene, 22.7 mL; benzyl chloride, 2.3 mL; water, 20 mL; temp = 90 °C.
found that the conversion after 40 min was almost the same for all powers. At 90 W, the conversion was relatively high in the initial period. This possibly indicates that the extent of cavitational effects are similar at these power dissipation levels and that 90 W is possibly above the critical level required for intensification due to the ultrasound effects. 3.8. Comparison of L−L−L System and L−L System. At a catalyst concentration just below the critical concentration (3 g), the three-phase system is converted into an L−L two-phase system. If a little amount of TBAB is added above the critical concentration (3 g), the L−L−L system is obtained. The comparison between the two types of operation has been given in Figure 10 to clearly confirm the fact that the formation of the
Figure 8. Effect of NaOH quantity on the extent of conversion: vanillin, 7 g; NaCl, 7 g; TBAB, 3 g; toluene, 22.7 mL; benzyl chloride, 2.3 mL; water, 20 mL; temp = 90 °C.
the conversion was also low, which can be attributed to the fact that the formation of an ion-pair between the vanillin and catalyst does not proceed unless the HCl is neutralized. NaOH is added in the system to neutralize HCl in the system. The maximum conversion was obtained with 1 g of NaOH, and a further increase in the NaOH concentration increased the side reaction (Cannizzaro reaction) due to active aldehyde group in vanillin. Similar results were reported by Yadav and Lande15 for conventional synthesis of 4-benzyloxy-3-methoxybenzaldehyde. Thus, it is important to select an optimum concentration of NaOH just sufficient to neutralize the acid content present in the system. This needs to be established using laboratory scale investigations. 3.7. Effect of Ultrasound Power. For investigations related to the effect of ultrasonic power dissipation, the reaction
Figure 10. Comparison of the L−L−L system and the L−L system.
third phase in the system gives significant process intensification. It was also established that the two-liquid phases (the normal L−L PTC) reaches a maximum of 93% conversion, with 85% selectivity to 4-benzyloxy-3-methoxybenzaldehyde in 8 h at 90 °C (data not shown in the figure). The limiting conversion may be attributed to the fact that TBAB is completely dissolved in the aqueous phase which restricts the reaction. On the contrary, with the creation of the middle 7983
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ultrasound was 13.17 kcal/mol which is lower than the reported literature value of 21.5 kcal/mol for the conventional approach.15
catalyst-rich phase, either by the addition of salt or a little catalyst beyond the critical value, the conversion increases to 97.1% within only 1 h. This intensification occurs because the catalyst being in the middle phase increases the transfer of required ions from both the organic and aqueous phase into the middle phase and reaction occurs at much faster rate increasing the overall conversion. 3.9. Kinetics of Reaction. For synthesis of 4-benzyloxy-3methoxybenzaldehyde via the L−L−L phase system, first-order kinetics has been considered.15 A plot of −ln(1 − XA) against time has been shown in Figure 11 at various temperatures to
4. CONCLUSIONS The current study revealed that cavitation is an efficient technology for synthesis of organic chemicals with significant process intensification in terms of increase in the conversion. Liquid−liquid reaction system can be elegantly modified to convert it into a liquid−liquid−liquid PTC system, and improved conversion can be observed using the new approach. Use of phase transfer catalyst with ultrasound in the synthesis of 4-benzyloxy-3-methoxybenzaldehyde has revealed an improved approach for synthesis giving process intensification in terms of higher yield. An increase in conversion of benzyl chloride and 100% selectivity toward product was observed in the presence of ultrasound as compared to the conventional approach for the synthesis of 4-benzyloxy-3-methoxybenzaldehyde. The kinetic studies have clearly established that the use of ultrasound has enabled to decrease the activation energy for the process.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 22 33612024. Fax: +91 22 33611020. E-mail: pr.
[email protected]. Notes
Figure 11. Validity of kinetic model at different temperature: vanillin, 7 g; NaOH, 1g; NaCl, 7 g; TBAB, 3 g; benzyl chloride, 2.3 mL; toluene, 22.7 mL; water, 20 mL.
The authors declare no competing financial interest.
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validate the first order mechanism. The rate constants were obtained and used to find the activation energy by the Arrhenius plot which gives a relationship between the reaction rate constant (k), absolute temperature (T), and the energy of activation (E) as follows:
REFERENCES
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⎛ E ⎞⎛ 1 ⎞ ln(k) = ⎜ − ⎟⎜ ⎟ + ln(A) ⎝ R ⎠⎝ T ⎠
The Arrhenius plot ln(k) against (1/T) to determine the activation energy is shown in Figure 12 where the slope of the straight line gives the value of activation energy and the intercept on the y-axis gives the value of the frequency factor. The activation energy for the L−L−L system in the presence of
Figure 12. Arrhenius plot for L−L−L system. 7984
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