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Green Solvents as a promising approach to degradation of organophosphorate Pesticides Paulina Pavez, Guillermo Oliva, and Daniela Millan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01923 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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ACS Sustainable Chemistry & Engineering

Green Solvents as a promising approach to degradation of organophosphorate Pesticides

Paulina Pavez,* Guillermo Oliva and Daniela Millán Facultad de Química. Pontificia Universidad Católica de Chile. Casilla 306, Santiago 6094411, Chile.

Author Information ∗Corresponding authors. Tel.: +56-02-23541743; fax: +56-02-26864744; e-mail: [email protected] Present address: Facultad de Química, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago 6094411, Chile.

ACS Paragon Plus Environment


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Graphical Abstract

Abstract Considering the main characteristics of ionic liquids versus bio-based solvents, which is the most appropriate solvent for degradation of some organophosphorate pesticides in order to obtain a more eco-friendly process? The solvent effect was studied by mean of half-lives (t1/2,h) through a kinetic study, which was followed by 31P NMR and UV/Vis. The results shown that the use of bio-based solvents as media for degradation of Fenitrothion (1) and Paraoxon (2), not only leads to a more efficient degradation but also assures a more sustainable process.

Keywords: ionic liquids, bio-based solvent, organophosphorate pesticides , 31P NMR


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Introduction The growing development of chemical industry has generated an enormous environmental impact over the years.1 As a consequence, severe demands for sustainable processes are motivating chemist to develop cost-effective and environmentally benign reaction systems, such as recycling, reducing solvent use or definitely switch to solvents with better environmental profiles.2, 3 Although, to run reactions without solvents seems to be the best alternative, their use is unavoidable in many cases4 since organic solvents are required in many chemical processes to favor a better contact between reactants or to stabilize transition states and to ensure a convenient separation of products.1 Therefore, if the solvents are essential to carry out a process we should select those solvents safe for the health and the environment.1 The replacement of conventional organic solvents (COS) by a suitable alternative, has become one of the main topics of modern chemistry.5, 6 In the last few years, some work has been done to achieve an innovative solvent, primarily focused on water, supercritical CO2 and more recently ionic liquids (ILs).6 ILs have been considered as an alternative to COS due to their notable properties.3 In addition, one of the most interesting features of these compounds is the large number of cations and anions that can be combined to form new ILs with specific properties.7-9 This has opened the possibility of obtaining a wide range of solvents with very different characteristics and, because of that, they have been labeled as “designer solvents”.10, 11 Nevertheless, although the advantages of using ILs as solvents for various organic reactions, are well accepted,12-17 to denominate a solvent as “green” must meet some of the twelve principles of green chemistry.5 Even though ILs fulfill some of them, this is not enough to classify them as green solvents, because other parameters such as toxicity and price should be taken into consideration.1 Toxicity studies of ILs have been performed to imidazolium-based ILs and the results indicate that toxicity is basically attributed to the anion present in the ILs.18 Bearing this in mind, at this moment there are no real green solvents and these are still required.19



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In this context, very recently bio-based renewable solvents, which are derived from natural ingredients have emerged as a new class of green solvents and they have been used as reaction media in some organic processes.2, 20-23 Some of them such ethyl lactate (EL) and propylene carbonate (PC)

24, 25

p-cymene (CYM), lymene (LYM)26 among others, have

been recognized as safe solvents, non-carcinogenic, non-ozone reducing, and they are completely biodegradable and noncorrosive. They have shown to be effective without compromising the safety and the environmental acceptability of the process.22, 23,

27

The

current literature has reported that also the use of bio-based solvents as reaction medium has improved reaction performance in terms of both selectivity and rate, similarly to that reported for ionic liquids and other solvents.1,

28-30

Moreover, due to the fact

their

production cost is less than the synthesis of ILs they can provide alternatives for conventional solvents. Considering the main advantages and disadvantages of the ILs and bio-based solvents, the question is: which is the most appropriate solvent for a particular reaction in order to obtain a more eco-efficient and eco-friendly process? To shed light on this question, we have chosen to study nucleophilic substitution reactions of two organophosphorate pesticides considered toxic such as, Fenitrothion (1) and Paraoxon (2) (see scheme 1) using ILs and bio-based solvents as reaction media (to compare the results obtained we have used some COS). The toxicity of these pesticides is attributed to irreversible inhibition of the acetylcholinesterase enzyme, which is essential for central nervous system.31 Besides, the rate constants values of spontaneous hydrolysis of 1 and 2 are very small (2.4x10-3 M-1s-1 and 1.4x10-7M-1s-1 respectivlely),32, 33 indicating a clear persistence of both pesticides in the environment. The toxicity and ecological problem due to the use of these pesticides, together with their environmental contamination and bioaccumulation in soils and groundwater, motivated us to look for a more efficient and greener way to the degradation of 1 and 2 (scheme 1). To investigate the effects of ILs and bio-based solvents on the degradation of these pesticides, in this work we have studied the reaction of 1 and 2 with piperidine in twelve ILs and six bio-based solvent, which are shown in Scheme 2. We have chosen bio-based solvent commercial available and coming from different natural sources such as: lymene (LYM) and cymene (CYM) from citrus fruit peels,26 ethyl lactate (EL) from the anaerobic


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fermentation of corn starch,25 propylene carbonate (PC) is a derivative of glycerol,20 gluconic acid (GLU) comes from fruit and honey34 and finally 2-methyltetrahydrofurane (2MeTHF) as sugar-derivative.35 The choise of these solvents provide a wide range of solvent parameters.

Scheme 1: Pesticides under study



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Scheme 2: Ionic liquids and bio-based solvent used as reaction media.

Experimental Section Materials. Bio-based solvents, conventional organic solvents (COS), piperidine, Fenitrothion® and Paraxon® were purchased. All ionic liquids were dried before use on a vacuum oven at 70 °C for at least 12h and stored in a dryer under nitrogen and over calcium chloride. Water content in ILs was less than 0.1% by Karl Fisher titration. It is importat to note that EL contains 1% of EtOH from its synthesis. Considering that have


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been reported that EtOH could act as nucleophile in some reactions,36 we have discarded the possible nucleophilic behavior of EtOH present in EL toward these pesticides. Kinetic measurements. The kinetic study of the reaction of 1 with piperidine in all solvents (COS, ILs and bio-based solvents) was performed spectrophotometrically (diode array) in the range 300- 500 nm, by following the appearance of products for at least four half- lives, by means of a Hewlett-Packard 8453 instrument. On the other hand, the reactions of 2 with piperidine in bio-based solvents were studied by 31P NMR obtained on a 400Hz spectrometer, following the disappearing of the signal of 2. In both cases, at least a 10-fold excess of total amine over the substrate was employed and each measurement was made in triplicate ( experimental details in Supporting Information). Product studies for degradation of 1. The single peak in the 31P NMR spectra found in all solvents was assigned to demethylfenitrothion 1B in Scheme 3, by comparison with the products reported by Bujan et al.32 The products formed in degradation of 2. Nuclear magnetic resonance. To confirm the structural assignment of the products O,Odiethyl piperidinephosphate diester (3) and diethyl phosphate (4),

31

P NMR spectra were

recorded for the reactions of O,O-diethyl chlorophosphate with piperidine and NaOH, respectively, in 2-MeTHF (see Figures S1 and S2 in Supporting Information). These reactions involve exclusive attack at the phosphoryl center. O-Ethyl 4-nitrophenyl phosphate diester (5) was prepared previously by us.17 Spectrophotometry. The products 4-nitrophenoxide (7) was identified by comparison of the visible spectrum with an authentic sample. 1-piperidino-4-nitrobenzene (8), (see Scheme 4), was identified by comparison of the final visible spectrum of the of 4-nitro-1chlorobenzene with piperidine in the ionic liquids and in some bio-based solvents. Typical UV-vis bands were found for compounds 7 and 8 at 420 nm and 325 nm, respectively (spectra not shown). Gas chromatography-mass spectrometry (GC/MS). Compound 3B (of Scheme 4) was identified by GC/MS (r.t. =22.37 min, m/z =254). The fragments generated by this structure are also shown in Figure S3 in Supporting Information. (experimental details in Supporting Information)



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Electrospray ionization mass spectrometry (ESI-MS): The detection of phosphorylated species 4 and 5 (of Scheme 4), formed in the reaction of 2 with piperidine in EL, was undertaken on an ABSciex Triple Quad 4500 (UHPLC-MS/MS) mass spectrometer equipped with a Turbo Ion Spray (AB Sciex) ion source.

A microsyringe pump delivered the mixed reaction of 2 with piperidine in EL at infinite time dissolved in 10% (vol/vol) acetonitrile into the ESI source at a flow rate of 10 µL/min. (experimental details in Supporting Information)

Results and discussion First-order rate constants (kobsd) for the reaction of 1 with piperidine using some COS, ILs and bio-based solvents and the rate constants for the reaction of 2 with piperidine in biobased solvents were determined by UV-vis and NMR techniques. The first-order rate constants (kobsd) for the reaction of 2 with piperidine in ILs were obtained from literature.17 The rate law for all the reactions studied is given by eqn (1), where P and S represent one of the products and pesticides 1 and 2, respectively. The rate constants (kobsd) were obtained in the presence of total piperidine excess and are shown in Tables S1 and S2 in Supporting Information. d [P] = kobsd [ S ] dt

(1)

From rate constants values (kobsd), the half-lives (t1/2) of degradation of both pesticides were obtained in different reaction media. Figure 1 shows the t1/2 values obtained for degradation of 1.


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Figure 1: Half-life (t1/2,h) for degratadion of 1 with piperidine (0.2 M) in several solvents. Each measurement was made in triplicate.

As we can see in Figure 1, a t1/2 value of 6.4 hours was found in aqueous solution for degradation of 1 with piperidine. Nevertheless, the t1/2 values calculated in ILs are at least one order of magnitude lower than those found in aqueous solution. In addition, we use some bio-based solvents for the degradation of 1. As can be seen in Figure 1, the t1/2 values obtained in CYM, LYM, 2-MeTHF and EL are somewhat lower that those in aqueous solution. Interestly, when propylene carbonate (PC) is the reaction media, it is observed that after 10 min piperidine has degraded half of the initial concentration of the pesticide, with a t1/2 value similar to those found in ILs where degradation of 1 shows half-life values in the range of 4-11 min. (See the insert of the Figure 1). It is important to mention that we also tried to perform the study using glycerol as solvent. The kobsd value for the reaction was impossible to measure under the same experimental conditions. This result is not surprising considering the extremely high viscosity of glycerol (η25ºC 934 mPa s). Therefore, we decided to use a glycerol derivative, PC, with η value (η25ºC 2.5 mPa s) similar to that of water (η25ºC 0.89 mPa s).20 As observed in the Figure 1, the degradation of 1 in PC presents the lowest t1/2 value in comparison with other bio-based solvents. Therefore, considering the high cost and questioned toxicity of ILs,37-38 we are prone to accept that PC would be the most appropriate solvent to obtain a more eco-



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efficient destruction of 1 through a nucleophilic substitution reaction. In addition, PC is a carbonate ester derived from propylene glicol and is prepared by the carbonation of the epoxides. The process is particularly attractive since the production of these epoxides consumes carbon dioxide. Thus this reaction is a good example of a green process.2, 39 On the other hand, it is noteworthy that the t1/2 value obtained in PC is very similar to those obtain in DMSO and MeCN. Therefore, for this particular reaction, we could replace toxic aprotic polar solvents by one (PC) with a similar kinetic behaviour, but without environmental problems, cheaper and coming from a green process.

In order to understand what is the mechanism of degradation of 1 through a nucleophilic substitution reaction, a product analysis of this reaction was carried out by 31 P-NMR. (See Figures S4-S23 in Suporting Information). The single peak found in all solvents was attributed to demethylfenitrothion (1B) which is produced by the nucleophilic attack of piperidine exclusively at the aliphatic C atom by a SN2(C) pathway, as shown in Scheme 3. The absence of attack to the P=S group in 1 by nucleophiles may be due to the small electronegativity difference between P and S in the thiophosphoryl group of pesticide 1, resulting in a little polarized P= S bond and, therefore, a poorly electrophilic P atom in 1. In contrast, the large electronegativity difference between P and O in the phosphoryl group of 2 makes this group more polarized, and therefore, a more electrophilic P atom. This could be the reason why attack by both nucleophiles (piperidine and EL) to thionophosphate group in 1 was not observed.

Scheme 3. Reaction pathway for degradation of 1 by piperidine in all solvents used in this study.


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To rationalize how the t1/2 values found in this reaction depend on the nature of the solvent, we used a multiparameter linear solvation energy relationship (LSER) to perform a regression analysis and to know the statistical weight of each solvent parameter. Among the most used relationships is one based on Kamlet–Taft descriptors, which divides solvent polarity into hydrogen bond donating ability (α), hydrogen bond accepting ability (β), and a combination of dipolarity and polarisability (π∗).40 Each parameter is empirically obtained and has been measured in a wide range of solvents, including ionic liquids and bio-based solvents.21, 41-55 (see Table S3, in Supporting Information) A generalized LSER is given in eqn (2), where XYZ is a variable term proportional to free energy; in this work is the logarithm of the half-life (t1/2) of degradation of both pesticides. Solvent parameters and t1/2 values obtained for degradation of 1 and 2 are shown in Table S3, in Supporting Information. Thus, a statiscally relevant result was obtained when we correlated log t1/2 for the degradation of 1 with each solvent parameter by iterative step of excluding the variables; these results are shown in Table 1.

XYZ = XYZο + aα + bβ + sπ ∗

(2)

A treatment based on a multiparametric regression with Kamlet–Taft’s solvent parameters failed when the data from COS were analyzed together with those from ILs and bio-based solvents, LSER 1, Table 1. Therefore, to improve the correlation we decided to include another parameter to account for all the experimental observations. Thus, the square of the Hildebrand solubility parameter ( δH2 ) was considered in the multiparameter regression. Interestingly, when Hildebrand parameter is introduced in the multiparametric regression, *

2

( XYZ = XYZ 0 + aα + bβ + sπ + hδH ) the correlation improves considerably, LSER 2 (See Table 1).



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Table 1. Statistical data from the multiparametric regression procedure including α, β, π* Kamlet–Taft’s parameters and Hildebrand solubility parameter ( δH2 ), for degradation of 1.

a

LSER

XYZ

XYZ0

ac

bc

sc

1

log(t1/2)

1.23(0.41)

2.01(0.63)

1.14(0.83)

-3.35(0.82)

2

log(t1/2)

0.91(0.10)

0.75(0.17)

-4x10-4(0.25)

3

log(t1/2)

0.91(0.09)

0.76(0.12)

4

log(t1/2)

0.66(0.17)

-

b

h/10-3 c

Fa

Rb

-

7

0.54

-2.96(0.22)

1.27(0.09)

125

0.97

-

-2.96(0.13)

1.26(0.08)

184

0.98

-

-2.47(0.23)

1.44(0.15)

63

0.90

c

Statistical F. Correlation coefficient. Standart deviations are given in parentheses.

As we can see in Table 1, the poor statistical significance of both, hydrogen bond donating ability (α) and hydrogen bond accepting ability (β) parameters, and the high statistical heigh of polarizability (π), and Hildebrand parameters ( δH2 ) see LSER 2 and LSER 3, suggest that changes in the t1/2 values for 1 degradation would not be governed by hydrogen bonding interactions, but rather by weaker interactions such as coulombic (π parameter) and Van der Waals interactions ( δH2 parameter). This analysis is reasonable considering that the the transition state (TS1) formed in the nucleophilic attack of piperidine to aliphatic carbon of 1, have a low charge separation than those TS2 formed if the piperidine would attack to phosphoryl center of 1. Therefore TS1 would be lower polarizable and solvents with high π value would have a negative effect on the rate constants of 1 degradation.

Figure 2 shows a comparison between the plots of t1/2 experimental against t1/2 calculated for the degradation of 1, without regard of the Hildebrand parameter ( δH2 ), Figure 2(A), and when δH2 is included in the plot, Figure 2(B).


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Figure 2. Double logarithmic plot of t1/2 experimental against t1/2 calculated, obtained for the degradation of 1 by piperidine. (A) From LSER 1 (table 1), without considering δH2 . (B) From LSER 3 (table 1), considering α, π, δH2 and neglecting the β parameter.

Fig. 2(B) shows the strong predictable element when π and δH2 parameters are used in combination to account for the t1/2 values found for degradation of 1. Also in Figure 2(B) we can observe the great effect on the t1/2 value found in water when the Hildebrand parameter is considered in the multiparameter regression. These results are in accordance with the dynamic three-dimensional hydrogen-bonded network of liquid water.56 In addition, it is noteworthy that when the t1/2 obtained in EL was considered in the multiparametric regression, the correlation failed considerably. The latter suggests that the degradation of 1 in this solvent may occur by a different mechanism, as explained below. On the other hand, Figure 3 shows the half-life values (t1/2, h) found for the degradation of 2 with piperidine in different reaction media. It is important to note that data for the reaction in ILs and COS were obtained from literature.17 As can be seen, in the experimental conditions of this study, very high values of t1/2 are found in COS, approximately 10 and 30 hrs in MeCN and dioxane, respectively. It is worth mentioning that we have not included the t1/2 value obtained in water, because in the experimental conditions of this study we



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observed only the hydrolysis of 2. In addition, t1/2 values in the range of 2-7 hours were obtained in ILs, and interestingly, very small t1/2 values when bio-based solvents are the media for the degradation of 2. As observed in the insert of Figure 3, in CYM, LYM, 2MeTHF, EL and GLU the degradation of 2 shows t1/2 values of 27, 18, 15, 2 and < 1 min, respectively.

Figure 3: Half-life times (t1/2, h) for the degradation of 2 with piperidine (2.8 M) in various bio-based solvent. Each measurement was made in triplicate. Data for the reaction in ILs and COS were obtained from literature.17

To understand the mechanism of the degradation of 2 in bio-based solvents, we performed a product analysis by NMR technique. Progressive

31

P NMR spectra obtained for

degradation of 2 in 2-MeTHF and GLU are shown in Supporting Information, Figures S24S25. The results show that the decrease of the signal of 2 (-7.3 ppm) is simultaneous to the increase of three other phosphorous signals at 9.2; 0.1 and -6.2 ppm. They are attribuited to: (i) O,O-diethyl piperidinophosphate diester (3), (ii) diethyl phosphate (4), and (iii) Oethyl 4-nitrophenylphosphate diester (5), shown in scheme 4. In addition, at long reaction times a new signal approximately at 8 ppm appears in the 31P NMR spectra, atribbuited to


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the formation of a new phosphorylated product (6), formed by a new piperidine attack to the phosphorus atom of compound 5, as shown in Scheme 4. These results indicate that the degradation of 2 in these bio-based solvents is similar to those found in ILs.17

Scheme 4: Nucleophilic attack of piperidine (PI) to 2 in ILs,17 GLU and 2-MeTHF, by three reaction paths: (A) at phosphorus, (B) at the C-1 aromatic carbon, and (C) at the aliphatic carbon. In addition, the progressive 31P NMR spectra obtained for degradation of 2 with piperidine in solvents derived from citrus fruit peel, such as CYM and LYM (see Figures S26 and S27; in Supporting Information, respectively), show only two signals that appear at aproximately 0.1 and -6.2 ppm, which can be assigned to the formation of the phosphorylated species 4 and 5, shown in scheme 4. From integration of the

31

P NMR

signals of the products formed from piperidine attack to the different electrophilic centers



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of 2 in solvents used in this study (see Figures S24−S27), and considering the results obtained from ref. 17, the relative products distribution was calculated in the different reaction media. Figure 4 shows this behavior schematically.

Figure 4: Relative nucleophilic attack of piperidine to 2 in bio-based solvents and in ILs from literature.17 Attack of piperidine at: (gray columns) the phosphorus atom by SN2(P) pathway, (white columns) the C-1 aromatic carbon by SNAr pathway, and (blue columns) the aliphatic carbon by SN2(C) pathway. These results support the idea that bio-based solvents can also change the reactivity of a particular compound, similarly to that found in ILs.28-30 As we can see in Figure 4, the product distribution obtained in degradation of 2 is strongly dependent on the nature of the solvent; therefore the selectivity of this reaction is an important factor in order to choose the most suitable solvent. We can observe that in most of the ILs and in 2-MeTHF, the SN2(C) pathway (blue columns in Figure 4) is the most important route for degradation of 2 (46− 62%) and when [Bmpy]DCA is the reaction media, the most important route is the SN2(P) pathway (54%). Interestingly, the results in Figure 4 show that the degradation of 2 in CYM and LYM proceeds only through two reaction pathways: the nucleophilic attack of piperidine at the aromatic center (path B) and at the aliphatic carbon (path C) of Scheme 1. The absence of nucleophilic attack of piperidine at the phosphoryl center of 2, could be related with the low polarisability of LYM and CYM (π*, see Table S3, in Supporting Information), which means that these solvents would stabilize less the transition state (TS)


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of the SN(P) pathway in comparison with those TS derived from the SN(C) and SNAr pathways. In addition, Figure 5 shows the progressive 31P NMR spectra obtained for degradation of 2 in EL. It can be observed that the decrease of the signal of 2 (-7.3 ppm) is simultaneous to the increase of three other signals at aproximately 0.1, -2.1 and -6.2 ppm. The results suggest that at least one of the pathways for degradation of 2 in EL proceeds by a different mechanism compared to that previously described in the other solvents used in this study.

Figure 5. Progressive 31P NMR spectra obtained for degradation of 2 with piperidine (2.8 M) at 25 ºC in EL.

On the other hand, Figure S28 in Supporting Information, shows the aromatic region of the 1

H NMR spectra for the reaction of 2 in EL. The results show the presence of 4-nitrophenol

as product of the reaction, indicating a nucleophilic attack of EL to the phosphorus atom of pesticide 2, in accordance with Scheme 5.



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Scheme 5: Nucleophilic attack of ethyl lactate (EL) to 2 by three reaction paths: (A) at phosphorus, (B) at the C-1 aromatic carbon, and (C) at the aliphatic carbon.

The presence of 4-nitrophenol was also observed by the increase of a band at 400 nm in the UV-vis spectra (Figure S29 in Supporting Information). On the other hand, compound 3B of Scheme 5, was identified by GC/MS (r.t. =22.37 min, m/z = 254.09), and the principal fragments corresponding to m/z =181.06 (100%); m/z =125.09 (80%); m/z =153.09 and m/z =80.97 are shown in Figure S3 in Suporting Information. In addition, the nucleophilic attack by EL to the C-1 carbon of the aromatic ring of 2 (path (B) in Scheme 5), leads to compounds 4 and ethyl 2-(4-nitrophenoxy)propanoate (8B). On the other hand, compounds 5 and ethyl 2-ethoxy propanoate (9B) are observed when EL attack occurs at the aliphatic carbon of pesticide 2, by the SN2(C) pathway (path (C)), see Scheme 5. Compouds 4 and 5, were confirmed by ESI-MS in the negative mode (see Figures S30-S31 in Supporting Information) It is noteworthy that we carried out the same reaction of 2 with piperidine in ethyl propionate (EP) as reaction medium. This solvent is similar to EL except that it does not present the hydroxyl group (see structures in Figure S32, in Supporting Information). Progressive

31

P NMR spectra obtained for the reaction of 2 with piperidine in EP (see

Figure S33, in Supporting Information) shown the absence of the signal at -2ppm. This result would confirm that the signal at -2ppm corresponds to product 3B formed by a nucleophilic attack by hydroxyl group of EL to the phosphoryl center of 2, as shown in


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Scheme 5. In addition, we observed that the nucleophilic attack by the hydroxyl group of EL does not occur in the absence of piperidine (not shown). These results indicate that the reaction proceeds by a mechanism of basic catalysis by piperidine. Having demonstrated a different mechanism for degradation of 2 in EL, we could think that the degradation of 1 in the same solvent occurs by a nucleophilic attack of EL instead of piperidine to C-aliphatic of 1. The latter can explain the different kinetic behaviour of degradation of 1 in EL. The t1/2 value obtained for degradation of 1 in EL, never follow the trend of the multiparametric regression (see above, Figure 2). On the other hand, we obtained pseudo-first-order rate coefficients (kobsd) for each of the reaction routes (SN2(P), SNAr, and SN 2(C)), using the logarithmic plots of the

31

P NMR

area, for degradation of 2 at different times, and the relative products distribution. The kobsd values found in the different media used in this study are shown in Table S3 in Supporting Information. With the kobsd values for the SN2(P) reaction routes in the degradation of 2, we have drawn Figure 6. The same way as discussed above, Fig. 6(B) shows the strong predictable element when π∗ and δH2 parameters are used in combination to account for the t1/2 values found in the SN2(P) reaction routes in the degradation of 2.

Figure 6. Double logarithmic plot of t1/2 experimental against t1/2 calculated for the SN2(P) reaction routes for degradation of 2. (A) Without considering δH2 . (B) considering α, π∗, δH2



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and neglecting the β parameter. Data for the reaction in ILs and COS were obtained from literature.17

Finally, we have demostrated that degradation of 2 in the bio-based solvents yields phosphate products less lipophilic (compouns 4 and 5) and therefore less toxic to the human being, in the same way as reported when ILs are the reaction solvents.17 In addition, very small t1/2 values were found when bio-based solvents are the media for the degradation of 2. Therefore, the use of bio-solvents as media for the reaction of 2 not only leads to a more efficient degradadtion but also assures a more sustainable process.

Conclusions In this work we have investigated the effect of solvents on the degradation of two pesticides, Fenitrothion (1) and Paraoxon (2), through a nucleophilic substitution reaction, using solvents of different nature, such as ILs, bio-based solvents and some COS. The kinetic results obtained in this study show that when ILs and PC (a bio-based solvent) are used as reaction media for the degradation of 1, the process is more efficient, showing half-life values in the range of 4-11 min. In addition, very small t1/2 values are observed when bio-based solvents are the media for degradation of 2. Therefore, considering the high cost and questioned toxicity of ionic liquids, we are prone to accept that bio-based solvents would be the most appropriate ones to obtain a more eco-efficient destruction of both pesticides through a nucleophilic substitution reaction. Acknowledgments This work was supported by project ICM-MINECON, RC-130006-CILIS Chile, and FONDECYT grant 1130065 and FONDECYT Postdoctoral Grant 3150122. Supporting Information Available. Stacked 31P NMR plot for the reaction of 1 and 2 with piperidine in all solvents, GC/MS chromatogram and mass spectrum of compound 7. Product analysis by Mass spectrum of compound 4 and 5, Experimental procedure and kinetics results. This material is available free of charge via the Internet at http://pubs.acs.org/.


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Green Solvents as a promising approach to degradation of organophosphorate Pesticides

Paulina Pavez,* Guillermo Oliva and Daniela Millán Facultad de Química. Pontificia Universidad Católica de Chile. Casilla 306, Santiago 6094411, Chile

Green and sustainable solvents were used as reaction medium to degradate organophosphorate pesticides in order to obtain a more eco-friendly process.


Green Solvents as a Promising Approach to ... - ACS Publications

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