Herbicidal ionic liquids derived from renewable sources

Abstract

A novel family of sugar-based herbicidal ionic liquids has been synthesized and the physical properties of the obtained salts were characterized. The herbicidal function was introduced to the new ionic liquids by the anion (4-chloro-2-methylphenoxyacetate or 2,4-dichlorophenoxyacetate) while the potential biodegradability and non-toxicity originated from the cation (based on D-glucose).

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Introduction

The design of environmentally benign ionic liquids (ILs) has been one of the active areas of green chemistry over the past 10 years. Chemical and physical properties of ILs can be tailored by modifying the structure of the cation and anion. In order to obtain ‘fully green’ ILs, the initial materials must be non-toxic and should be renewable.¹ The synthesis of these compounds from environmentally friendly and renewable raw materials, e.g. amino alcohols (choline² and ephedrine³), hydroxy acids,⁴ amino acids,⁵ and terpenes⁶ is becoming more beneficial compared to the use of compounds derived from fossil feed stocks.

Sugars are ideal raw materials for the synthesis of ILs from the viewpoints of both environmental and economic concerns. Research on sugar-based ILs has been intensively promoted in the last few years,⁷ e.g. ILs derived from methyl D-glucopyranose,⁸ glucose-tagged triazolium ILs,⁹ ILs obtained from isosorbide,¹⁰ mono- and bis-ammonium ILs from isomannide¹¹ and ILs with a pentofuranose unit.^12,13 The example of tetraalkylammonium ILs with a sugar moiety in the anion structure derived from D-galacturonic and D-glucuronic acids has already been reported.¹⁴ Certain sugar-derived quaternary ammonium salts containing iodides and bromides, which are not classified as ILs, have also been described in the literature.¹⁵ Recently, we have demonstrated the possibility of using a renewable and relatively inexpensive initial material, such as D-glucose, as the target precursor for an IL cation connected with a weakly coordinating bistriflamide anion. The resulting hydrogen-bond-rich ILs showed high activity in Diels–Alder reaction.¹⁶

Herbicidal ionic liquids (HILs) are a novel group of third generation ILs, which was first described in 2011. HILs are organic salts with a melting point below 100 °C, which comprise at least one compound with herbicidal activity (usually the anion).¹⁷ HILs combine specific biological properties with selected physicochemical properties and may be used as efficient weed control agents as well as phytopharmaceuticals.¹⁸ To date, there are numerous scientific reports describing HILs, which were synthesized using commonly applied herbicides, e.g. 2,4-D,^19,20 MCPA,^21–23 MCPP,^17,23 MCPB,²⁴ dicamba,²⁵ fomesafen,²⁶ glyphosate,²⁷ clopyralid,²⁸ metsulfuron-methyl²⁹ and bentazone.³⁰ Furthermore, it is also possible to use two compounds with different biological properties as substrates, which results in dual-function HILs.³¹ Recent findings confirm that compounds with fungicidal properties (e.g. tebuconazole or propiconazole) may be successfully utilized for the synthesis of ILs, which will protect the crops against pathogenic fungi.³²

Currently compounds from the phenoxy acid group, such as 4-chloro-2-methylphenoxyacetic acid (MCPA) and 2,4-dichlorophenoxyacetic acid (2,4-D), are among the most commonly used herbicides. These compounds are auxins, substances which stimulate plant grow at lower concentrations but reduce plant growth and cause weed death at high concentrations. The presently applied herbicidal formulations contain phenoxy acids in the form of salts (either sodium, potassium or ammonium) or esters. However, this corresponds to certain disadvantages in terms environmental contamination due to mobility and unintended transport, since salts exhibit high water solubility, whereas esters are characterized by high vapor pressure. HILs are a potential solution to this problem. They are characterized by negligible vapor pressure and their water solubility may be notably limited by the use of appropriate cations. Additionally, HILs exhibit several other advantages, such as: reduced required dose per hectare, lower toxicity (toxic phenoxy herbicides may become nontoxic as HILs¹⁷) and enhanced properties (HILs may improve the immunity of plants to pathogenic diseases caused by viruses^33,34 or fungi³⁵). As a result, HILs may be considered as efficient and environmentally friendly alternatives to the presently used commercial herbicidal formulations. In this work, a novel family of sugar-based HILs has been synthesized. These salts were designed by the combination of dedicated properties, such as herbicidal function (which was introduced by the anion) and potential biodegradability and non-toxicity (originating from the cation).

Results and discussion

D-Glucose was selected as the raw material for the cation synthesis and has been transformed to the final material in 4 steps. The procedures used for the modification were described elsewhere^8,36,37 (Scheme 1).

Scheme 1 The synthesis of quaternary ammonium salts using D-glucose.

The first step included the acetylation of β-D-glucose (I) which yielded a single β-anomer (II). In the next step, the glycosylation of penta-O-acetyl-β-D-glucopyranose with 2-bromoethanol, using BF₃·Et₂O as the catalyst, yielded 2-bromoethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (III). Trace amounts of disaccharide and trisaccharide bromoalkyl derivatives, which were detected by MS analysis, as well as unreacted starting material were isolated using column chromatography to obtain pure products with a yield of 50%. The key step consisted of the quaternization reaction of amines R¹N(CH₃)₂ (where R¹ = CH₃, C₄H₉, C₈H₁₇ C₁₂H₂₅, C₁₆H₃₃) with 2-bromoethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside which allowed to obtain the ammonium bromides (IV) with high yields (Table 1). The purification included extraction of N-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-trimethylammonium bromide with water. Purification using column chromatography was applied in case of other products.

Table 1 The synthesized salts based on herbicidal anions

Salts	R^1	R^2	Anion	Yield (%)	State at 20 °C
a Melting point Tm = 121 °C.b Tm = 134 °C (Table S1, ESI).
Va	CH3	CH3	MCPA	98	Wax
Vb	C4H9	CH3	MCPA	95	Liquid
Vc	C8H17	CH3	MCPA	96	Wax
Vd	C12H25	CH3	MCPA	98	Liquid
Ve	C16H33	CH3	MCPA	96	Wax
VIa	CH3	Cl	2,4-D	96	Solid^a
VIb	C4H9	Cl	2,4-D	92	Solid^b
VIc	C8H17	Cl	2,4-D	95	Wax
VId	C12H25	Cl	2,4-D	93	Wax
VIe	C16H33	Cl	2,4-D	94	Wax

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The last step of the synthesis process was focused on the anion exchange in the reaction of (IV) with potassium 2-methyl-4-chlorophenoxyacetate MCPA or potassium 2,4-dichlorophenoxyacetate 2,4-D, which resulted in ten salts with yields above 90% (Table 1). The acid–base reaction was not used due to the possibility of hydrolysis of the acetyl groups combined with the D-glucose molecule. Six of the prepared salts (Va, Vc, Ve, VIc, VId, VIe) were waxes, two (Vb, Vd) were liquids, however they were characterized by a very high viscosity. Only two of the obtained compounds (VIa, VIb) were solids (T_m > 100 °C). Eight of the synthesized salts can be classified as ionic liquids. The structure of the prepared new salts was confirmed by NMR analysis. In addition, all products were completely miscible with water and the colour of their applied water solutions were stable for the period exceeding one month. Thermal stability of the synthesized salts was determined by thermogravimetric analysis (TGA) and the obtained data were presented in Table 2. The analysis showed high thermal stability with decomposition temperatures (T_onset50) in the range of 235–340 °C (Fig. S1, ESI†). According to literature,³⁸ the thermal stability of ILs depends on the structure of the cation and increasing the length of the alkyl chain leads to less stable compounds. However, our results confirmed a different thermal behavior of the long-chain functionalized salts with herbicidal anions, since the elongation of the alkyl substituent increased their thermal stability.²⁵ It is important to note that the values obtained for the 2,4-D ILs derived from D-glucose exhibited slightly better properties.

Table 2 Thermophysical properties of obtained salts

Salts	Tm^a (°C)	Tg^b (°C)	Tonset5%^c (°C)	Tonset50%^d (°C)
a Melting point.b Glass transition.c Decomposition of 5% of the sample.d Decomposition of 50% of the sample.
Va	—	12	164	235
Vb	—	20	208	285
Vc	—	10	191	259
Vd	—	13	224	327
Ve	—	8	191	287
VIa	121	21	178	251
VIb	134	—	202	247
VIc	—	22	201	279
VId	—	12	221	340
VIe	—	15	204	296

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The synthesized salts mostly exhibited two thermal decomposition steps (Fig. S2 and S3, ESI†). Only the salt VIe with 2,4-D anion showed a three-step decomposition.

The thermal behaviors of the synthesized HILs were evaluated by differential scanning calorimetry (DSC). The results were presented in Table S2.† No crystallization temperatures were measured upon heating to 150 and cooling to −100 °C. All the synthesized compounds based on the MCPA anion were amorphous ILs and exhibited high glass transition temperatures ranging between 8.3 and 19.5 °C. It is likely that the observed thermal properties were also influenced by the structure of the anion. The replacement of the MCPA anion with 2,4-D caused a slight increase in glass transition temperature values. Moreover, two salts containing short alkyl substituents and the 2,4-D anion (VIa, VIb) exhibited melting points above 100 °C. The glass transition temperature was not observed only in the case of salt VIb. Overall, the thermal properties of the prepared salts were dependent on the length of the N-alkyl substituent, however no simple relationship can be established. Generally, the elongation the alkyl substituent on the nitrogen atom decreased T_g of the compounds.²⁹ The lowest glass transition temperatures (T_g = 8.3 and 9.7 °C) were measured for the MCPA-based ILs, which comprised the hexadecyl and octyl group, respectively.

In the case of salts derived from 2,4-D, the lowest glass transition value (T_g = 12.4 °C) was observed for VId, which contained the dodecyl substituent.

The surface tension of aqueous solutions of bromides IV and salts with 2,4-D VI as a function of the log of concentration has been plotted in Fig. 1 and in S4 (ESI†) for HILs V with MCPA anion. As can be seen, the surface tension values of the aqueous solutions showed a progressive decrease with increasing concentration and remained constant above the CMC.³⁹ The synthesized bromides, which were used for the synthesis of salts comprising the herbicidal anions, exhibited surface activity. Our aim was to ensure that the surface active properties in the obtained salts originated from the cation. From these plots, we calculated the surface active parameters for the obtained salts and the corresponding bromides. The surface tension values at the CMC (γ_CMC), maximum surface excess concentration (Γ_max), minimum area per molecule (A_min), the negative logarithm of the concentration of salts in the bulk phase required to reduce the surface tension of the water by 20 mN m⁻¹, which represents efficiency of surface adsorption on an air–water interface (pC₂₀), and contact angle (CA) were obtained from surface tension measurements and given in Table 3. It can be observed that the CMC value gradually decreased with the elongation of alkyl chain in the cation from 106.41 to 0.09 mmol dm⁻³ for bromides IVb–e. The CMC values obtained by surface tension measurement were compared with the results presented by Quagliotto et al.³⁷ Similar values of CMC were obtained for bromides by conductivity and surface tension measurements.

Fig. 1 Surface tension (γ) as a function of the logarithm of concentration of bromides IV and salts VI.

Table 3 Surface active parameters – critical micelle concentration, surface tension at CMC (γ_CMC), maximum surface excess (Γ_max), minimum area per molecule (A_min), adsorption efficiency (pC₂₀) and contact angle (CA) of obtained bromides IV as well as salts V and VI

Salt	CMC (mmol L^−1)	γCMC (mN m^−1)	Γmax (μmol m^−2)	Amin (10^−19 m^2)	pC20	CA (°)
IVb	106.41	37.80	1.77	9.36	1.69	81.2
IVc	27.47	37.35	1.16	14.35	2.69	74.8
IVd	2.01	35.46	1.24	13.41	3.61	73.7
IVe	0.09	38.96	1.82	9.11	4.69	80.9
Vb	20.80	34.43	2.75	6.04	2.25	82.7
Vc	12.39	34.34	1.30	12.79	3.13	69.8
Vd	1.38	32.60	2.01	8.26	3.73	53.8
Ve	0.11	33.79	3.07	5.40	4.48	56.3
VIb	20.70	31.09	3.08	5.38	2.30	73.2
VIc	17.78	32.59	1.29	12.88	3.08	60.7
VId	1.56	31.36	1.62	10.24	3.94	59.9
VIe	0.10	35.52	3.16	5.25	4.45	66.4

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The CMC values for IVd–e were at 2.01 and 0.09 mmol L⁻¹ respectively, however the values reported in the study of Quagliotto et al. were at 2.63 and 0.21 mmol L⁻¹ (obtained by Du Noüy ring method³⁷), and 5.48 and 1.25 mmol L⁻¹ (by conductivity measurement). However, the CMC values could not be directly compared due to different measurement methods. The CMC values for HILs with MCPA anion (Vb–e) decreased from 20.80 to 0.11 mmol L⁻¹. This demonstrates that the CMC values of these HILs were slightly similar compared to those with the 2,4-D anion and the same alkyl chain length. Therefore, the following order may be established based on the comparison of CMC values of HILs and their precursors: IV > V ≈ VI. This order results from the fact, that herbicidal anions are structurally bigger and more hydrophobic compared to halides, which decreases the electrostatic repulsion and facilitates the formation of micelles.⁴⁰ The relationship between log CMC value and alkyl substituent chain length for bromides IV as well as ILs V and VI has been shown in Fig. 2. Elongation of the alkyl chain in surface active HILs led to a decrease of the CMC value, which results from higher hydrophobic interactions between the longer chains.⁴¹ The obtained salts exhibited a linear dependence (R² > 0.999) between the CMC value and the number of carbon atoms in the alkyl substituent in the cation. The values of pC₂₀ obtained for the investigated salts were listed in Table 3. It can be seen that the values of pC₂₀ increased linearly with the elongation of alkyl chain in the cation (Fig. S5, ESI†). Some literature reports suggest that the pC₂₀ directly depends on the amount of carbon atoms in the alkyl chain⁴² and allows to describe the relation between surface activity and herbicidal activity.²¹ Surface activity allowed for the reduction of the contact angle of a drop of alkyl-based bromides and solution of the obtained salts. The best wettability values of paraffin surface were observed for HIL comprising MCPA and 2,4-D with alkyl chain containing 12 atoms of carbon, which amounted to 53.8° and 59.9°, respectively. All salts (V and VI) exhibited herbicidal activity. Their effectiveness was dependent both on the plant species and the structure of the cation. The influence of HILs V on weed control has been shown in Table S1, ESI.† The new HILs exhibited lower efficacy against cornflower compared to the commercial herbicide used as reference. The differences were statistically significant except HIL Vb. The efficacy of the tested HILs has been shown in Fig. 3. HILs exhibited a similar effect as the commercial herbicide in case of white mustard HIL Vd, was characterized by the highest efficacy, while Va demonstrated the lowest biological activity.

Fig. 2 Relation between log CMC and the number of atoms in alkyl substituent for bromides IV as well as salts V and VI.

Fig. 3 Weed control by different forms of MCPA (A) and 2,4-D (B) against white mustard (Sinapis alba L.) and cornflower (Centaurea cyanus L.).

The herbicidal efficacy of salts VI against cornflower and white mustard has been shown in Table S2, ESI.† The efficacy increased with the increase of the alkyl chain length in the cation. This phenomenon can also be observed on the photograph presented in the ESI (Fig. S6 and S7, ESI†). The lowest efficacy was exhibited by salts VIa and VIb, which exhibited lower effect than the reference. VIe was an effective HIL with a similar efficacy compared to the standard. The activity of the reference compound did not differ significantly from salts with alkyl chain groups containing 4, 8, 12, 18 atoms of carbon (for cornflower) and 8, 12, 18 atoms of carbon (in the case of white mustard). The influence of surface activity on the efficacy of HILs depended on the tested plant species. The relation between herbicidal efficacy against white mustard (expressed as reduction of fresh plant weight) and surface active parameter pC₂₀ for the obtained HILs comprising the 2,4-D anion have been shown in Fig. 4. A linear increase of the herbicidal activity with the increase of pC₂₀ was observed.

Fig. 4 Relation between efficacy and the value of pC₂₀ for white mustard and 2,4-D based salts.

Experimental

Materials

4-Chloro-2-methylphenoxyacetic acid (purity 95%) and 2,4-dichlorophenoxyacetic acid (purity 99%) were acquired from Organika–Sarzyna S.A. Poland and PESTINOVA Poland. α-D-Glucose, 2-bromoethanol, BF₃·OEt₂, trialkylamines were purchased from Sigma-Aldrich. All solvents (methanol, acetone, chloroform) were supplied by AlfaChem (Poznan, Poland) and used without further purification. Deionized water (DI) was obtained from demineralizer HLP Smart 1000 (Hydrolab, Straszyn, Poland). Other chemicals were received from Sigma-Aldrich (Poznan, Poland).

General procedures

Penta-O-acetyl-β-D-glucopyranose (II). Compound II was prepared according to the method described by Michihata et al. (2013).³⁶ Powdered α-D-glucose (46.84 g, 0.26 mol) was slowly added over a period of 15 min into a gently refluxed Ac₂O (244 mL, 2.58 mol) containing NaOAc (10.66 g, 0.13 mol). Next, the mixture was refluxed for 5 min and cooled down to RT. The reaction was quenched by the addition of cold water and left in the fridge overnight. The precipitate (white powder) was filtered off and washed with H₂O until the odor of the acetic acid disappeared. The crude product was purified by recrystallization (EtOH) to obtain the final product (94.29 g, 93% yield) as a white powder.

¹H NMR (600 MHz, CDCl₃) δ ppm = 2.02 (s, 3H), 2.04 (s, 6H), 2.09 (s, 3H), 2.12 (s, 3H), 3.85 (ddd, 1H, J = 2.2, 4.6, 10.0 Hz), 4.12 (dd, 1H, J = 2.2, 12.5 Hz), 4.29 (dd, 1H, J = 4.6, 12.5 Hz), 5.12–5.115 (m, 2H), 5.26 (t, 1H, J = 9.4 Hz), 5.72 (d, 1H, J = 8.3 Hz). ¹³C NMR (150 MHz, CDCl₃) δ ppm = 20.6, 20.8, 20.9, 61.6, 67.9, 70.3, 72.8, 72.9, 91.8, 169.0, 169.3, 169.4, 170.1, 170.6. ESI-MS [M + Na]⁺ calcd: 413.1060, found: 413.1056.

2-Bromoethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (III). Compound III was prepared according to the method described by Quagliotto et al. (2005).³⁷ A solution of penta-O-acetyl-β-D-glucopyranose (II) (62.45 g, 0.16 mol) and 2-bromoethanol (13.9 mL, 0.19 mol) in dry dichloromethane (250 mL) was placed in the dark and cooled down to 0 °C. Next, BF₃·Et₂O (100 mL, 0.81 mol) was slowly added into the solution. The reaction was stirred at 0 °C for 3 h and for 20 h at RT. At the end, the reaction mixture was diluted with dichloromethane (50 mL) and then poured into cold water (250 mL) with vigorous stirring. The organic layer was separated and washed repeatedly with water and saturated sodium bicarbonate. The organic phase was dried over anhydrous sodium sulfate and concentrated on a rotary evaporator, and the resulting residue was purified by chromatography with silica gel using 2% methanol in dichloromethane as solvent. A white crystalline solid was obtained, yield 37.01 g (50%).

¹H NMR (600 MHz, CDCl₃) δ ppm = 2.01 (s, 3H), 2.03 (s, 3H), 2.07 (s, 3H), 2.09 (s, 3H), 3.43–3.50 (m, 2H), 3.72 (ddd, 1H, J = 2.4, 4.8, 10.0 Hz), 3.80–3.84 (m, 1H), 4.11–4.18 (m, 2H), 4.25–4.28 (m, 1H), 4.58 (d, 1H, J = 7.9 Hz), 5.00–5.03 (m, 1H), 5.09 (t, 1H, J = 9.4 Hz), 5.23 (t, 1H, J = 9.5 Hz). ¹³C NMR (150 MHz, CDCl₃) δ ppm = 20.5, 20.6, 29.8, 61.8, 68.3, 69.7, 71.0, 71.9, 72.5, 100.9, 169.3, 170.1, 170.5. ESI-MS [M + Na]⁺ calcd: 477.0372, found: 477.0375.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-trimethylammonium bromide (IVa). Bromide IVa was prepared according to the method described by Dmochowska et al. (2006).¹⁵ 2-Bromoethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (III) (5.00 g, 10.98 mmol) was placed in a two-necked round-bottom flask, dissolved in anhydrous ethanol (20 mL), mixed with 33% ethanolic solution of trimethylamine (15 mL) and refluxed under nitrogen atmosphere. After 8 h the solution was evaporated to dense oil, dissolved in water and extracted with dichloromethane. The aqueous layer was concentrated under reduced pressure at temperature below 40 °C and yielded compound IVa (5.18 g, 92%).

¹H NMR (600 MHz, CDCl₃) δ ppm = 2.00 (s, 3H), 2.04 (s, 3H), 2.05 (s, 3H), 2.08 (s, 3H), 3.19 (s, 9H), 3.59–3.66 (m, 2H), 3.77–3.79 (m, 1H), 4.04–4.07 (m, 1H), 4.23–4.38 (m, 3H), 4.61 (d, 1H, J = 8.0 Hz), 4.96 (dd, 1H, J = 9.7 Hz), 5.06 (t, 1H, J = 9.7 Hz), 5.21 (t, 1H, J = 9.6 Hz). ¹³C NMR (150 MHz, CDCl₃) δ ppm = 20.7, 20.8, 54.8, 61.6, 63.3, 66.1, 68.4, 71.1, 72.5, 72.6, 100.6, 169.7, 169.8, 170.1, 170.9. ESI-MS [M⁺] calcd: 434.2026, found: 434.2030.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N-butyl-N,N-dimethylammonium bromide (IVb). Bromides IVb–e were prepared according to the method described by Quagliotto et al. (2005).³⁷ 2-Bromoethyl-2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (III) (4.00 g, 8.78 mmol) was placed in a two-necked round-bottom flask, dissolved in anhydrous ethanol (20 mL) under nitrogen, mixed with N,N-dimethylbutylamine (0.89 g, 8.78 mmol) and refluxed. After 24 h the solvent was removed in vacuum, and the resulting viscous oil was purified by chromatography on basic alumina, first with ethyl acetate and subsequently with ethyl acetate/methanol (80 : 20 and 50 : 50). The resulting viscous pale yellow syrup solidified on standing and was further purified from trace of the amine by suspension in petroleum ether and a small quantity of chloroform, under stirring. A white powder was finally obtained: 4.20 g yield 86%.

¹H NMR (400 MHz, DMSO-d₆) δ ppm = 0.89 (t, 3H, J = 7.3 Hz), 1.21–1.30 (m, 2H), 1.56–1.62 (m, 2H), 1.90 (s, 3H), 1.95 (s, 3H), 1.98 (s, 3H), 1.99 (s, 3H), 3.01 (s, 6H), 3.26–3.30 (m, 2H), 3.53–3.54 (m, 2H), 4.00–4.15 (m, 5H), 4.74–4.79 (m, 1H), 4.87–4.92 (m, 2H), 5.23 (t, 1H, J = 9.6 Hz). ¹³C NMR (100 MHz, DMSO-d₆) δ ppm = 13.9, 19.6, 20.7, 20.8, 20.9, 21.0, 24.2, 51.0, 51.2, 61.9, 62.6, 62.9, 64.3, 68.4, 71.1, 71.2, 72.3, 99.3, 169.4, 169.7, 169.9, 170.5. ESI-MS [M⁺] calcd: 476.2496, found: 476.2498.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N-dimethyl-N-octylammonium bromide (IVc). The same procedure described for bromide IVb was applied, which allowed to obtain a white powder: yield 73%.

¹H NMR (400 MHz, CDCl₃) δ ppm = 0.87–0.91 (m, 3H), 1.29–1.37 (m, 10H), 1.73–1.76 (m, 2H), 2.00 (s, 3H), 2.04 (s, 3H), 2.07 (s, 3H), 2.10 (s, 3H), 3.34 (s, 3H), 3.37 (s, 3H), 3.46–3.53 (m, 3H), 3.81–3.85 (m, 1H), 3.94–3.98 (m, 1H), 4.10–4.28 (m, 4H), 4.69 (d, 1H, J = 8.1 Hz), 4.91–4.96 (m, 1H), 5.05 (t, 1H, J = 9.6 Hz), 5.22 (t, 1H, J = 9.6 Hz). ¹³C NMR (100 MHz, CDCl₃) δ ppm = 14.2, 20.6, 20.7, 20.9, 21.0, 22.7, 22.9, 26.3, 29.2, 29.3, 31.8, 51.9, 61.6, 63.4, 63.9, 66.4, 68.3, 72.4, 72.6, 100.5, 169.6, 170.1, 170.7. ESI-MS [M⁺] calcd: 532.3122, found: 532.3127.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N-dimethyl-N-dodecylammonium bromide (IVd). The same procedure as described for product IVb was applied, which allowed to obtain a white powder: yield 58%.

¹H NMR (600 MHz, DMSO-d₆) δ ppm = 0.85 (t, 3H, J = 7.1 Hz), 1.25–1.29 (m, 18H), 1.63–1.66 (m, 2H), 1.94 (s, 3H), 1.99 (s, 3H), 2.01 (m, 3H), 2.02 (s, 3H), 3.03 (s, 6H), 3.27–3.31 (m, 2H), 3.53–3.59 (m, 2H), 4.02–4.19 (m, 5H), 4.79–4.82 (m, 1H), 4.93 (t, 2H, J = 9.3 Hz), 5.27 (t, 1H, J = 9.6 Hz). ¹³C NMR (150 MHz, DMSO-d₆) δ ppm = 14.0, 20.3, 20.4, 20.5, 20.6, 21.9, 22.2, 25.8, 28.6, 28.8, 28.9, 29.0, 29.1, 31.4, 50.7, 50.8, 61.6, 62.3, 62.6, 64.3, 68.2, 70.8, 70.9, 72.0, 99.0, 169.1, 169.3, 169.6, 170.1. ESI-MS [M⁺] calcd: 588.3748, found: 588.3746.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N-dimethyl-N-hexadecylammonium bromide (IVe). The same procedure as described for bromide IVb was applied, which allowed to obtain a white powder: yield 64%.

¹H NMR (600 MHz, CD₃OD) δ ppm = 0.90 (t, 3H, J = 7.1 Hz), 1.25–1.38 (m, 26H), 1.77–1.80 (m, 2H), 1.97 (s, 3H), 2.02 (s, 3H), 2.04 (s, 3H), 2.07 (s, 3H), 3.13 (s, 6H), 3.30–3.31 (m, 1H), 3.34–3.42 (m, 2H), 3.63–3.64 (m, 1H), 3.93–3.96 (m, 1H), 4.09–4.13 (m, 1H), 4.24–4.30 (m, 2H), 4.81–4.84 (m, 2H), 4.91–4.94 (m, 1H), 5.04 (t, 1H, J = 9.5 Hz), 5.28 (t, 1H, J = 9.5 Hz). ¹³C NMR (150 MHz, CD₃OD) δ ppm = 12.9, 18.9, 19.0, 19.1, 19.2, 22.1, 22.2, 25.8, 28.7, 28.9, 29.0, 29.1, 29.2, 31.5, 50.7, 61.2, 62.6, 62.8, 65.2, 68.1, 71.0, 71.1, 72.4, 99.9, 169.5, 169.6, 169.9, 170.6. ESI-MS [M⁺] calcd: 644.4374, found: 644.4374.

Quaternary ammonium salts (V, VI). The anion exchange reaction was carried out in semi-automated reactor system EasyMax 102 (Mettler Toledo). Appropriate N-[2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-alkyldimethylammonium bromide (5 mmol) was dissolved in methanol (10 mL) and potassium salts of 2,4-D (1.30 g, 5 mmol) or MCPA (1.19 g, 5 mmol) were added in the form of a methanolic suspension (2,4-D) or clear solution (MCPA). The obtained mixture was stirred for 30 min at RT and potassium bromide, which was formed during the reaction, was removed by filtration. Afterwards, the salts were purified by the addition of isopropanol (10 mL). The residual inorganic salt was filtered and the solvent was evaporated by rotary vacuum evaporator.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-trimethylammonium (4-chloro-2-methylphenoxy)acetate (Va). ¹H NMR (600 MHz, DMSO-d₆) δ ppm = 1.95 (s, 3H), 2.00 (s, 3H), 2.02 (s, 3H), 2.03 (s, 3H), 2.15 (s, 3H), 3.10 (s, 9H), 3.62 (m, 2H), 4.02–4.07 (m, 2H), 4.11 (dd, J = 12.3, 2.5 Hz, 2H), 4.16 (s, 2H), 4.20 (m, 1H), 4.83 (m, 1H), 4.95 (m, 2H), 5.28 (t, J = 9.6 Hz, 1H), 6.68 (d, J = 8.9 Hz, 1H), 7.07 (dd, J = 9.0, 3.0 Hz, 1H), 7.12 (d, J = 2.8 Hz, 1H). ¹³C NMR (150 MHz, 298 K, DMSO-d₆) δ ppm = 16.0, 20.2, 20.3, 20.5, 53.0, 61.5, 62.6, 64.3, 68.3, 70.7, 70.8, 71.9, 98.8, 112.9, 122.5, 125.9, 127.8, 129.3, 156.2, 169.1, 169.3, 169.5, 169.8, 170.1, calcd (%) for C₂₈H₄₀ClNO₁₃ (M = 634.07): C 53.04, H 6.36, N 2.21; found: C 53.51, H 6.70, N 1.76.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-butyldimethylammonium (4-chloro-2-methylphenoxy)acetate (Vb). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.93 (m, 3H), 1.29 (m, 2H), 1.66 (m, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 2.6 Hz, 6H), 2.15 (s, 3H), 3.04 (m, 6H), 3.32 (m, 2H), 3.57 (m, 4H), 4.07 (m, 2H), 4.17 (m, 3H), 4.21–4.35 (m, 1H), 4.82 (m, 1H), 4.95 (m, 2H), 5.29 (t, J = 9.6 Hz, 1H), 6.68 (d, J = 8.7 Hz, 1H), 7.07 (dd, J = 8.6, 2.4 Hz, 1H), 7.13 (m, 1H). ¹³C NMR (75 MHz, 298 K, DMSO-d₆) δ ppm = 13.5, 16.0, 19.1, 20.3, 20.4, 20.5, 23.7, 25.5, 50.7, 61.5, 62.0, 62.5, 63.9, 68.0, 68.3, 70.8, 71.9, 98.8, 112.9, 122.5, 125.9, 127.8, 129.4, 156.2, 169.2, 169.3, 169.5, 170.1, 170.5, calcd (%) for C₃₁H₄₆ClNO₁₃ (M = 676.15): C 55.07, H 6.86, N 2.07; found: C 54.73, H 9.25, N 2.64.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-octyldimethylammonium (4-chloro-2-methylphenoxy)acetate (Vc). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.87 (m, 3H), 1.27 (m, 10H), 1.64 (br, s, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 3.0 Hz, 6H), 2.15 (s, 3H), 3.04 (s, 6H), 3.29 (m, 2H), 3.58 (br, s, 2H), 4.07 (m, 3H), 4.13 (m, 1H), 4.17 (d, J = 4.7 Hz, 1H), 4.21 (s, 2H), 4.82 (m, 1H), 4.94 (m, 2H), 5.29 (t, J = 9.6 Hz, 1H), 6.69 (d, J = 8.7 Hz, 1H), 7.07 (dd, J = 8.6, 2.7 Hz, 1H), 7.13 (d, J = 2.7 Hz, 1H), ¹³C NMR (75 MHz, 298 K, DMSO-d₆) δ ppm = 13.9, 16.0, 20.2, 20.3, 20.4, 20.5, 22.0, 25.7, 28.5, 30.6, 31.2, 50.6, 61.5, 62.1, 62.6, 64.0, 68.0, 70.7, 70.8, 71.9, 98.9, 112.9, 122.6, 125.9, 127.8, 129.4, 156.1, 169.1, 169.2, 169.5, 170.0, 170.1, calcd (%) for C₃₅H₅₄ClNO₁₃ (M = 732.26): C 57.41, H 7.43, N 1.91; found: C 57.99, H 7.90, N 1.54.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-dodecyldimethylammonium (4-chloro-2-methylphenoxy)acetate (Vd). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.86 (m, 3H), 1.24 (m, 18H), 1.65 (br, s, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (m, 6H), 2.14 (s, 3H), 3.07 (m, 6H), 3.42 (m, 2H), 3.55 (s, 2H), 4.07 (m, 3H), 4.14 (m, 3H), 4.20–4.34 (m, 1H), 4.84 (m, 1H), 4.91 (m, 2H), 5.29 (t, J = 9.6 Hz, 1H), 6.66 (d, J = 8.7 Hz, 1H), 7.07 (dd, J = 8.7, 2.7 Hz, 1H), 7.12 (d, J = 2.7 Hz, 1H), ¹³C NMR (75 MHz, 298 K, DMSO-d₆) δ ppm = 14.0, 16.0, 20.2, 20.4, 20.5, 20.7, 21.8, 22.1, 25.7, 28.6, 28.7, 28.9, 29.0, 31.3, 50.8, 61.1, 61.5, 62.3, 64.3, 68.2, 70.7, 70.8, 71.9, 98.9, 112.8, 122.5, 125.9, 127.8, 129.3, 156.2, 169.1, 169.3, 169.5, 170.0, 170.2. calcd (%) for C₃₉H₆₂ClNO₁₃ (M = 788.37): C 59.42, H 7.93, N 1.78; found: C 59.01, H 7.57, N 1.36.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-hexadecyldimethylammonium (4-chloro-2-methylphenoxy)acetate (Ve). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.85 (m, 3H), 1.24 (m, 26H), 1.63 (br, s, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 3.2 Hz, 6H), 2.15 (s, 3H), 3.03 (s, 6H), 3.28 (m, 2H), 3.58 (br, s, 2H), 4.08 (m, 3H), 4.13 (m, 1H), 4.17 (m, 3H), 4.82 (m, 1H), 4.95 (m, 2H), 5.29 (t, J = 9.5 Hz, 1H), 6.67 (d, J = 8.7 Hz, 1H), 7.07 (dd, J = 8.6, 2.7 Hz, 1H), 7.12 (d, J = 2.7 Hz, 1H), ¹³C NMR (75 MHz, 298 K, DMSO-d₆) δ ppm = 13.9, 16.0, 20.2, 20.3, 20.4, 20.5, 21.8, 22.1, 25.8, 28.6, 28.7, 28.9, 29.0, 29.1, 30.6, 31.3, 50.6, 61.5, 62.1, 62.6, 64.0, 68.0, 68.1, 70.8, 71.9, 98.9, 112.9, 122.5, 125.9, 127.8, 129.3, 156.2, 169.1, 169.2, 169.5, 169.8, 170.8, calcd (%) for C₄₃H₇₀ClNO₁₃ (M = 844.48): C 61.16, H 8.36, N 1.66; found: C 60.63, H 8.02, N 1.99.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-trimethylammonium (2,4-dichlorophenoxy)acetate (VIa). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 2.01 (s, 3H), 2.06 (s, 3H), 2.08 (s, 6H), 2.10 (s, 3H), 3.19 (s, 9H), 3.72 (m, 2H), 4.10–4.29 (m, 6H), 4.36 (s, 2H), 4.87 (m, 1H), 4.98–5.06 (m, 1H), 5.33 (t, J = 9.9 Hz, 1H), 6.91 (d, J = 9.0 Hz, 1H), 7.30 (dd, J = 9.0, 2.4 Hz, 1H), 7.53 (d, J = 2.7 Hz, 1H), ¹³C NMR (75 MHz, DMSO-d₆) δ ppm = 20.3, 20.4, 20.5, 53.0, 61.5, 62.7, 64.2, 68.3, 70.7, 70.8, 71.9, 98.8, 115.0, 121.7, 123.1, 127.5, 128.8, 153.6, 169.2, 169.3, 169.6, 170.1, calcd (%) for C₂₇H₃₇Cl₂NO₁₃ (M = 654.49): C 49.55, H 5.70, N 2.14; found: C 49.83, H 5.39, N 1.71.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-butyldimethylammonium (2,4-dichlorophenoxy)acetate (VIb). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.93 (t, J = 7.3 Hz, 3H), 1.29 (m, 2H), 1.64 (m, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 3.5 Hz, 6H), 3.05 (s, 6H), 3.32 (m, 2H), 3.59 (m, 2H), 4.07 (m, 3H) 4.13 (m, 1H), 4.17–4.21 (m, 1H), 4.26 (s, 2H), 4.81 (m, 1H), 4.94 (m, 2H), 4.94 (m, 2H), 5.28 (t, J = 9.6 Hz, 1H), 6.65 (d, J = 9.0 Hz, 1H), 7.25 (dd, J = 8.9, 2.6 Hz, 1H), 7.46 (d, J = 2.6 Hz, 1H), ¹³C NMR (75 MHz, 298 K, DMSO-d₆) δ ppm = 13.5, 19.1, 20.2, 20.4, 20.5, 23.7, 25.5, 50.7, 61.5, 62.1, 62.5, 63.9, 68.0, 68.4, 70.8, 71.9, 98.8, 115.0, 121.7, 123.0, 127.4, 128.7, 153.7, 169.1, 169.3, 169.5, 170.0, calcd (%) for C₃₀H₄₃Cl₂NO₁₃ (M = 696.57): C 51.73, H 6.22, N 2.01; found: C 52.05, H 6.87, N 1.69.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-octyldimethylammonium (2,4-dichlorophenoxy)acetate (VIc). ¹H NMR (400 MHz, DMSO-d₆) δ ppm = 0.87 (m, 3H), 1.27 (m, 10H), 1.65 (m, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 3.8 Hz, 6H), 3.05 (s, 6H), 3.31 (m, 2H), 3.59 (m, 2H), 4.08 (m, 3H), 4.13 (m, 1H), 4.17–4.21 (m, 1H), 4.24 (s, 2H), 4.82 (m, 1H), 4.94 (m, 1H), 5.28 (t, J = 9.6 Hz, 1H), 6.86 (d, J = 9.0 Hz, 1H), 7.25 (dd, J = 8.8, 2.7 Hz, 1H), 7.46 (d, J = 2.5 Hz, 1H), ¹³C NMR (100 MHz, 298 K, DMSO-d₆) δ ppm = 13.9, 20.2, 20.4, 20.5, 21.8, 22.1, 25.5, 25.7, 28.5, 31.2, 50.7, 61.5, 62.1, 62.6, 64.0, 68.0, 68.6, 70.7, 71.9, 98.9, 115.1, 121.6, 123.0, 127.5, 128.7, 153.7, 169.1, 169.2, 169.3, 169.5, 170.0, calcd (%) for C₃₄H₅₁Cl₂NO₁₃ (M = 752.68): C 54.26, H 6.83, N 1.86; found: C 53.75, H 7.27, N 1.31.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-dodecyldimethylammonium (2,4-dichlorophenoxy)acetate (VId). ¹H NMR (400 MHz, DMSO-d₆) δ ppm = 0.86 (m, 3H), 1.25 (m, 18H), 1.64 (m, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (d, J = 4.3 Hz, 1H), 3.03 (s, 6H), 3.29 (m, 2H), 3.57 (m, 2H), 4.07 (m, 3H) 4.12 (m, 1H), 4.17–4.21 (m, 1H), 4.28 (s, 2H), 4.81 (m, 1H), 4.94 (m, 2H), 4.94 (m, 2H), 5.28 (t, J = 9.5 Hz, 1H), 6.85 (d, J = 9.0 Hz, 1H), 7.26 (dd, J = 9.0, 2.7 Hz, 1H), 7.47 (d, J = 2.5 Hz, 1H), ¹³C NMR (100 MHz, 298 K, DMSO-d₆) δ ppm = 14.0, 20.2, 20.4, 20.5, 21.8, 22.1, 25.7, 28.6, 28.7, 28.9, 29.0, 31.3, 50.6, 50.7, 61.5, 62.0, 62.1, 62.5, 64.0, 68.0, 68.2, 70.7, 70.8, 71.9, 98.9, 115.0, 121.7, 123.1, 127.5, 128.7, 153.6, 168.8, 169.1, 169.3, 169.5, 170.0, calcd (%) for C₃₈H₅₉Cl₂NO₁₃ (M = 808.78): C 56.43, H 7.35, N 1.73; found: C 56.91, H 6.89, N 2.05.

N-[2-(2,3,4,6-Tetra-O-acetyl-β-D-glucopyranosyloxy)ethyl]-N,N,N-hexadecyldimethylammonium (2,4-dichlorophenoxy)acetate (Ve). ¹H NMR (300 MHz, DMSO-d₆) δ ppm = 0.86 (m, 3H), 1.24 (m, 26H), 1.65 (br, s, 2H), 1.95 (s, 3H), 2.00 (s, 3H), 2.03 (s, 6H), 3.03 (s, 6H), 3.41 (m, 4H), 3.57 (br, s, 2H), 4.09 (m, 4H), 4.26–4.36 (m, 3H), 4.81 (m, 1H), 4.94 (m, 2H), 5.29 (t, J = 9.6 Hz, 1H), 6.85 (d, J = 8.9 Hz, 1H), 7.26 (dd, J = 9.0, 2.6 Hz, 1H), 7.45 (d, J = 2.6 Hz, 1H), ¹³C NMR (100 MHz, 298 K, DMSO-d₆) δ ppm = 13.9, 20.2, 20.5, 20.5, 20.7, 21.8, 22.1, 25.8, 28.6, 28.7, 28.9, 29.0, 29.1, 31.3, 50.8, 50.9, 61.1, 62.0, 62.3, 64.3, 68.4, 70.0, 73.3, 76.7, 77.2, 102.6, 115.0, 121.7, 123.1, 127.5, 128.7, 153.7, 169.2, 169.5, 169.8, 170.0, 170.3, calcd (%) for C₄₂H₆₇Cl₂NO₁₃ (M = 864.89): C 58.33, H 7.81, N 1.62; found: C 58.87, H 7.35, N 1.91.

Apparatus

The ¹H and ¹³C NMR spectra were obtained using the Varian Mercury spectrometer (operating at 300 MHz and 75 MHz), the Varian VNMR-S spectrometer (400 MHz, 100 MHz) and the Bruker Avance spectrometer (600 MHz, 125 MHz). Tetramethylsilane (TMS) was used as an internal standard for all samples. Elemental analyses were performed at the Adam Mickiewicz University, Poznan. High resolution mass spectrometry analyses were performed using a Waters Xevo G2 Q-TOF mass spectrometer (Waters Corporation) equipped with an ESI source operating in positive-ion modes. Full-scan MS data were collected from 100 to 1000 Da in positive ion mode with a scan time of 0.1 s. To ensure accurate mass measurements, data were collected in centroid mode and mass was corrected during acquisition using leucine enkephalin solution as an external reference (Lock-Spray™), which generated reference ion at m/z 556.2771 Da ([M + H]⁺) in positive ESI mode. The accurate mass and composition for the molecular ion adducts were calculated using the MassLynx software (Waters) incorporated with the instrument.

Thermal analysis

Thermal transition temperatures of the obtained salts were determined by DSC, using a Mettler Toledo Star TGA/DSC1 apparatus (Leicester, UK). Samples between 5 and 15 mg were placed in aluminum pans and heated from 25 to 150 °C at a heating rate of 10 °C min⁻¹ and cooled using an intracooler at a cooling rate of 10 °C min⁻¹ to −100 °C. Thermogravimetric analysis was performed using a Mettler Toledo Stare TGA/DSC1 unit (Leicester, UK) under nitrogen. Samples between 2 and 10 mg were placed in aluminum pans and heated from 30 to 450 °C at a heating rate of 10 °C min⁻¹. Nitrogen was used as a carrier gas.

Surface activity

The surface tension was determined using the pendant drop method. Basically, the principle of this method is to form an axisymmetric drop at the tip of a needle of a syringe. The image of the drop is taken from a CCD camera and digitized. The surface tension (γ in mN m⁻¹) is calculated by analyzing the profile of the drop according to the Laplace equation. Surface tension and CA measurements were carried out by the use of DSA 100 analyzer (Krüss, Germany), at 25 °C. Temperature was controlled using Fisherbrand FBH604 thermostatic bath (Fisher, Germany). The values of critical micelle concentration (CMC) and the surface tension at the CMC (γ_CMC) were determined from the intersection of the two straight lines drawn in low and high concentration regions in surface tension curves (γ vs. log C curves) using a linear regression analysis method. The CA was measured using the sessile drop method (Young–Laplace), i.e. drop of liquid as deposited on a solid surface (paraffin). The drop was produced before the measurement and had a constant volume during the measurement. In this method, the complete drop contour was evaluated. After the successful fitting of Young–Laplace equation the CA was determined as the slope of the contour line at the 3-phase contact point.

Surface active parameter, such as the adsorption efficiency pC₂₀ is defined by using the eqn:

pC₂₀ = −log C₂₀,

where is the molality of compound which leads to a reduction of the surface tension of the solvent by 20 mN m⁻¹.

The maximum surface excess concentration at the air/water interface can be calculated following Gibbs adsorption isotherm to the experimental surface tension data.

The surface area occupied by a single molecule of compound at the air/water interface is calculated by using the eqn:

where R is the gas constant (8.314 J mol⁻¹ K⁻¹); T is the absolute temperature; is the slop of γ vs. log C profile at the point CMC; N_A is Avogadro's constant (6.022 × 10²³ mol⁻¹).

The effectiveness of the surface tension reduction, also called surface pressure at the saturated air/solution interphase, is also an important surface parameter which can be obtained from the surface tension curve by using the eqn:

π_CMC = γ₀ − γ_CMC,

where γ₀ is the surface tension of purity solvent and γ_CMC is the surface tension of solution at CMC. π_CMC is a measure of effectiveness of compound to decrease the surface tension of the solvent.

Biological activity

Biological activity was evaluated during greenhouse experiments. The herbicidal efficacy of the obtained HILs was tested using white mustard (Sinapis alba L.) and cornflower (Centaurea cyanus L.) as the test plants. The plants were grown in 0.5 L plastic pots containing commercial peat-based potting material (pH 6). All salts were dissolved in water at the amount corresponding to the dose of 400 g of MCPA or 2,4-D per 1 ha. Commercial herbicides Chwastox Extra 300 SL (300 g L⁻¹ of MCPA as sodium and potassium salts) and Aminopielik Standard 600 SL (600 g L⁻¹ of 2,4-D as dimethylammonium salts) were used as reference products and they were applied at the same doses as the HILs. The plants were treated by herbicides at 4–6 leaf growth stage. The applications were made using a moving sprayer (APORO, Poznan, Poland) with a TeeJet 110/02 flat-fan nozzle (TeeJet Technologies, Wheaton, IL, USA) delivering 200 L ha⁻¹ of spray solution at 0.2 MPa operating pressure. After treatment, the plants were placed again in a greenhouse at a temperature of 20 ± 2 °C, humidity of 60% and photoperiod (day/night hours) of 16/8. Fresh weight of plants was determined two weeks after treatment using a technical balance (Sartorius BP 2000 S, Sartorius Göttingnen, Germany) with 0.01 g accuracy. The reduction of plant fresh weight was determined in comparison to the untreated plants. The study was carried out in 4 replications in a randomized setup. To compare the studied treatments, a multiple Tukey's post hoc test (α = 0.05) was applied.

Conclusions

In summary, new HILs based on D-glucose and MCPA or 2,4-D anions were synthesized via glycosylation of penta-O-acetyl-β-D-glucopyranose with 2-bromoethanol, subsequent quaternization and anion exchange in order to obtain the final products. The analysis showed a high thermal stability of obtained HILs with decomposition temperatures (T_onset50) in the range of 235–340 °C. All the synthesized ILs presented high surface activity and good wettability of the paraffin surface (which is similar to the surface of plants). This property could make them potentially useful in the future as herbicides.

All the synthesized HILs have demonstrated herbicidal activity. The influence of ILs on the efficacy against cornflower and white mustard showed that their efficacy was dependent both on the plant species and the structure of the cation. HILs may be an alternative for the presently applied preparations, which consist of herbicides and supporting compounds. HILs are composed of ions, with the anion acting as the herbicide and the cation as support. Through their use only a single chemical compound will be introduced into the environment, not a mixture of several substances. Furthermore, HILs exhibit negligible vapor pressure and their structure may be modified in order to control their water solubility, which allows to limit their mobility in the environment and solve the issue of contamination due to unintended transport. This, along with the fact that the presented HILs may be synthesized from natural renewable resources, corresponds well with the principals of green chemistry and the policy of sustainable development. The literature data suggest that the newly discovered D-glucose-based HILs have the potential for application as environmentally friendly herbicides. The toxicity and biodegradability of the described HILs are currently under detailed study. The costs and hazards associated with the use of HILs versus the benefits should be carefully reviewed before HILs move from laboratory tests to large-scale industrial applications.

Acknowledgements

This work was supported by National Science Centre, Poland, (grant No. DEC-2012/07/B/ST5/00806).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06703d

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