Synthesis and characterization of herbicidal ionic liquids based on (4-chloro-2-methylphenoxy)acetate and phenoxyethylammonium

Article abstract of DOI:10.1007/s11696-021-01610-1

Ten ionic liquids containing the (4-chloro-2-methylphenoxy)acetate (MCPA) anion and domiphen derived phenoxyethylammonium cation were synthesized. The obtained compounds differed in terms of the substitution of the phenoxyethylammonium group in the ring (the presence of a methyl group in the meta or para positions and the presence of chlorine in the para position) as well as the length of the alkyl chain (from hexyl to tetradecyl). The basic physicochemical properties of the obtained ionic liquids (solubility and thermal stability) were characterized and their structures were confirmed. The herbicidal activity of the compounds was tested under greenhouse conditions using cornflower (Centaurea cyanus L.) as the test plant.

Full text of DOI:10.1007/s11696-021-01610-1

Chemical Papers
https://doi.org/10.1007/s11696-021-01610-1
ORIGINAL PAPER
Synthesis and characterization of herbicidal ionic liquids based
on (4‑chloro‑2‑methylphenoxy)acetate and phenoxyethylammonium
Kamil Ciarka1,2 · Radoslaw Olszewski1,2 · Tadeusz Praczyk3 · Juliusz Pernak4
Received: 3 November 2020 / Accepted: 16 March 2021
© The Author(s) 2021
Abstract
Ten ionic liquids containing the (4-chloro-2-methylphenoxy)acetate (MCPA) anion and domiphen derived phenoxyethyl-
ammonium cation were synthesized. The obtained compounds difered in terms of the substitution of the phenoxyethylam-
monium group in the ring (the presence of a methyl group in the meta or para positions and the presence of chlorine in the
para position) as well as the length of the alkyl chain (from hexyl to tetradecyl). The basic physicochemical properties of the
obtained ionic liquids (solubility and thermal stability) were characterized and their structures were confrmed. The herbicidal
activity of the compounds was tested under greenhouse conditions using cornfower (Centaurea cyanus L.) as the test plant.
Keywords Ionic liquids · Herbicides · Phenoxyethylammonium · MCPA · Domiphen
Introduction
compounds (Chatel and Rogers 2014; Swatloski et al. 2002;
Ohno and Fukaya 2009).
Ionic liquids (ILs), defned as ionic compounds with a melt-
ing point below 100 °C (Visser et al. 2012; Wasserscheid
and Welton 2002), are still the subject of intense interest and
research due to their unique properties and the vast amount
of cation–anion combinations, which is estimated at the level
of 1018Holbrey and Seddon (1999). New types of ILs are
described on a regular basis as a result of their recognized
properties and the search for potential applications. Initially,
they were an alternative to toxic solvents, and were used in
chemical synthesis, extraction (Wang et al. 2020), catalysis
(Dyson and Geldbach 2005) and electrochemical processes
(Wojciechowski et al. 2017). Important research concerns
the use of ILs as a solvent for biopolymers, e.g. in the pro-
cess of dissolving cellulose or extracting lignin from bio-
mass, and the processes of its transformation into aromatic
The appropriate selection of cations and anions during the
design of ILs determines their properties. A convenient divi-
sion of ILs is based on the idea of three generations (Hough
et al. 2007; Smiglak et al. 2014; Zając et al. 2018). The 1st
generation is focused on the physical quantities, the 2nd gen-
eration is focused on the physical and chemical parameters,
and the 3rd generation is the combination of 1st and 2nd
generations with an extension to the biological properties.
Examples of the 3rd generation include pharmaceuticals,
plant protection products and antibiotics, the bioavailabili-
ties of which were improved by their conversion into ILs
(Bica and Rogers 2010).
The transformation of carboxylic acids (cinnamic, ben-
zoic, salicylic, nicotinic, 2-methylphenoxyacetic, succinic,
malonic, oxalic, malic and citric) and phenoxyacetic acids
derivatives into ionic liquids is known (Kondratenko et al.
2020; Voronkov et al. 2014). Triethanolammonium salt of
2-methylphenoxyacetic acid (creasin, trecrezan) was intro-
duced with unique physiological properties, in particular
efcient adaptogenic and immunostimulatory efects were
exhibited. It is opened wide prospects for its application
in medicine and agriculture. A chloro-substituted analog
of creasin, tris(2-hydroxyethyl)ammonium (4-chloro-
2-methylphenoxy)acetate (chlorocreasin) turned out to be
even more efcient as physiologically active substance and
* Kamil Ciarka
kamil.ciarka@adob.com.pl
1
Faculty of Chemistry, Adam Mickiewicz University, ul.
Uniwersytetu Poznanskiego 8, 61-614 Poznan, Poland
2
PPC ADOB, ul. Kolodzieja 11, 61-070 Poznan, Poland
3
Institute of Plant Protection–National Research Institute,
ul. Wegorka 20, 60-318 Poznan, Poland
4
Department of Chemical Technology, Poznan University
of Technology, ul. Berdychowo 4, 60-965 Poznan, Poland
Vol.:(0123456789)
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Chemical Papers
it exhibited pronounced antitumor activity (Voronkov and
Rasulov 2007).
detectable concentrations, reaching aquatic environments at
signifcantly longer distances. The coefcient of absorption
and degradation of active substances depends on soil param-
eters: pH, organic matter content and air humidity. Also the
degradation of phenoxy acids also depends on the amount
of bacteria present in the soil (Paszko et al. 2016). The high
afnity of the positive charge contained in quaternary salts
can cause the binding of cations with the organic substances
present in the soil. Additionally, it is documented that the
infuence of the length of the alkyl substituent and the pres-
ence of an aromatic ring in ammonium salts adversely afects
the biodegradation process (Hora et al. 2020).
In 2011, it was proposed to transform known herbi-
cides into herbicidal ionic liquids (HILs), which results in
a decrease in the volatility of the herbicide, an increase in
the efectiveness of its action and, at the same time, allows
for regulation of the water solubility and toxicity (Pernak
et al. 2011). This approach has been demonstrated by the
transformation of popular herbicides (such as 2,4-D (Junfan
et al. 2018), MCPA (Pernak et al. 2011), glyphosate (Mar-
cinkowska and Łacka 2019), metsulfuron (Pernak et al.
2015), dicamba (Syguda et al. 2020) and pelargonic acid
(Turguła et al. 2020)) into HILs.
Domiphen bromide is an example of a commercial use of
this type of compound. This compound exhibits antibacterial
and antiseptic properties and is used in the pharmaceutical
industry (Yan et al. 2017). Additionally, domiphen mande-
late and prolinate have been described as antimicrobial and
antifungal ILs and employed for the asymmetric Michael
addition (Cybulski et al. 2011).
The (4-chloro-2-methylphenoxy) acetic acid (MCPA)
herbicide belongs to the group of synthetic auxins, also
known as growth regulators, due to its mechanism of action.
The same group of agents also includes herbicides such as
dicamba, mecoprop, and 2, 4-D. They are most often used
in the herbicidal protection of crops to combat many species
of dicotyledonous weeds (Zimdahl 2007).
The aim of this study is the synthesis of ILs with ion
acetate of MCPA as the anion and phenoxyamine derivatives
as the ammonium cations that may have a dual aspect of
action, herbicidal and bactericidal, and which can become an
alternative to the commercially used volatile MCPA esters.
MCPA in commercial products is most often applied in
the form of sodium–potassium or dimethylamine salts (Tom-
lin 2009). Numerous studies have been conducted regarding
the impact of pesticides on the natural environment (Syguda
et al. 2018; Gang et al. 2020; Parus et al. 2019; Pęziak-
Kowalska et al. 2019), and hence, special attention is paid
to the development of new formulations that would allow
for the reduction in the amount of active substances applied
while maintaining the same level of crop protection (Niem-
czak et al. 2020; Ten et al. 2020; Giszter et al. 2016). Direc-
tive 2009/128/EC of the European Parliament and the Coun-
cil from the 21st of October 2009 established a framework
for community action for the sustainable use of pesticides
and obliges all professional users to apply integrated pest
management, which emphasizes the production of healthy
crops with minimal disruption to the functioning of the agri-
cultural ecosystem. According to this idea, plant protection
products placed on the market should have a reduced risk
of a negative impact on human health and the environment.
One of the methods to improve the ecotoxicological prop-
erties of plant protection products could be the transforma-
tion of active substances into ionic liquids. The appropri-
ate design of HILs allows products to be obtained with an
enhanced spectrum of action and, at the same time, a lower
impact on the natural environment. A review was recently
published on the synthesis, toxicity, biodegradation and ef-
cacy studies of HILs (Wilms et al. 2020). Bearing in mind
that about 30% of pesticides applied on leaves go directly
to the soil, biodegradation of plant protection products
becomes an important topic in the discussion on the use of
herbicidal liquids in agriculture. The agricultural pesticides
that applied to the land surface travel long distances and can
move downward until reaching the water table surface at
Experimental
Materials
Sodium hydroxide (≥ 97.0%), phenol (≥ 99%), 4-chloro-
phenol (≥99%), 4-chloro-3-methylphenol (99%), p-cresol
(99%), 2-chloro-N,N-dimethylethylamine hydrochloride
(99%), 3-dimethylamino-1-propyl chloride (96%), 1-bro-
mohexane (98%), 1-bromooctane (99%), 1-bromodecane
(98%), 1-bromododecane (97%), 1-bromotetradecane (97%),
dimethyl sulfoxide (≥99.9%) and domiphen bromide (97%)
were purchased from Sigma Aldrich. Solvents: 2-propanol
(99.9%), methanol (≥99.9%), acetonitrile (≥99.9%), acetone
(≥99.5%), ethyl acetate (≥99.5%), chloroform (≥99.8%),
toluene (≥ 99.5%), hexane (≥ 99%) were purchased from
Sigma Aldrich, whereas (4-chloro-2-methylphenoxy)acetic
acid (97%) was obtained from CIECH Sarzyna (Nowa Sar-
zyna, Poland). Anion Exchange Resin AmberTec® UP550
OH was purchased from Merck. Deionized water with con-
ductivity<0.1 μS cm−1 obtained using the HLP Smart 1000
demineralizer (Hydrolab, Poland) was used for synthesis and
measurements. All reagents and solvents were used without
further purifcation.
1 3

Chemical Papers
hydroxide solution was used directly in the next synthesis
step.
General
1
The H and 13C NMR analyses were performed using a
Synthesis of ILs
BRUKER ASCEND™ 400 MHz NANOBAY at 400 MHz
and 100 MHz, respectively. Tetramethylsilane (TMS) was
used as a standard and deuterated methanol (CD3OD)
was used as the solvent. The melting point was measured
using a Buchi Melting Point B-540. LC–MS/MS analy-
ses were conducted on Shimadzu Nexera X2 LCMS-8040
with SPD-M30A DAD detector, LC-30AD pump and SIL-
30AC autosampler. Eluent was 0.1 mL/min gradient fow
of 1:1 mixture of 0.1% TFA in water with acetonitrile. Vol-
ume of injection was equal to 1 μl. FT-IR analyses were
conducted on Thermo Nicolet FT-IR iS10 spectrometer in
A methanolic solution of 0.1 mol of the appropriate hydrox-
ide was placed in a reaction vessel equipped with a magnetic
stirrer. Then, a solution of 0.1 mol of (4-chloro-2-methyl-
phenoxy)acetic acid in 50 mL of methanol was added. The
neutralization reaction was carried out at 20 °C for 1 h. After
evaporating the solvent, the product was dried at 40 °C under
reduced pressure.
Solubility assay
range of 650–4000 cm−1 at resolution of 1 cm−1
.
The solubility of the obtained ILs was tested based on the
Vogel Textbook of Practical Organic Chemistry (Furniss
et al. 1989). Ten solvents were selected for testing: water,
methanol, DMSO, acetonitrile, acetone, isopropanol, ethyl
acetate, chloroform, toluene and hexane. The test allowed
to classify the obtained ILs into one of the three groups for
each solvent. The frst group was defned as "good solubil-
ity" (+) which means that 0.1 g of the compound is dissolved
in 1 mL of the solvent. The second group, defned as "limited
solubility" ( ), was used when 0.1 g of the compound is dis-
solved in 2 or 3 mL of the solvent. The last group labelled
"poor solubility" (−) was employed when 0.1 g of the com-
pound was not dissolved in 3 mL of the solvent. The tests
were carried out at 25 °C and atmospheric pressure.
Synthesis of phenoxyamines
Initially, 0.1 mol of the appropriate phenol, 0.2 mol of
sodium hydroxide and 30 mL of isopropanol were placed
in a reaction vessel equipped with a magnetic stirrer, a
thermometer and a refux condenser. The mixture was
heated to 50 °C. Then, a solution of 0.11 mol of the
appropriate amine in 20 mL of water was added drop-
wise over 10 min. After stirring for 5 h, isopropanol was
evaporated and the remaining aqueous solution was cooled
and extracted three times with chloroform. The combined
organic layers were washed twice with water and then
chloroform was evaporated. The product was dried at
40 °C under reduced pressure.
Thermal analysis
The thermal transformation temperatures of the obtained
salts were determined by DSC using a Mettler Toledo Stare
TGA/DSCl apparatus (Leicester, UK). During the analy-
sis of the Phase Conversion Temperature values, samples
between 5 and 15 mg were placed in aluminium pans, heated
from 25 to 120 °C at a heating rate of 10 °C per min and
cooled using an intracooler at a cooling rate of 10 °C per min
to −100 °C. During the analysis of the decomposition tem-
perature values, samples between 2 and 10 mg were placed
in aluminium pans and heated from 30 to 450 °C at a heating
rate of 10 °C per min. Nitrogen was used as the carrier gas.
Quaternization reaction
To conduct the quaternization reaction, 0.1 mol of the appro-
priate amine and 0.11 mol of the appropriate bromoalkane
were placed in a reaction vessel equipped with a magnetic
stirrer, thermometer and refux condenser. The reactants
were dissolved in 50 mL of acetonitrile, and then, the mix-
ture was heated to 60 °C and stirred for 20 h. The product
appears as a white precipitate. It was cooled to−10 °C, and
then, the precipitate was fltered of and washed with hexane.
The product was dried at 40 °C under reduced pressure.
Herbicidal activity
The herbicidal activity of the obtained ILs was tested
under greenhouse conditions. Cornflower (Centaurea
cyanus L. 1753) was used as the test plant. All salts were
dissolved in a mixture of water and ethanol (0.4:0.6 v/v).
The commercial product Chwastox Extra 300 SL (300 g
of MCPA in the form of sodium and potassium salts per
1 L of solution) was used as a reference substance. Seeds
Ion exchange
To conduct the ion exchange, 0.1 mol of the appropriate
bromide and 100 mL of methanol were placed in a reaction
vessel equipped with a magnetic stirrer. After dissolution,
50 mL of ion exchange resin was added to the mixture. After
stirring for an hour, the resin was fltered of. The obtained
1 3

Chemical Papers
Scheme 1 Synthesis of phenoxyamines (1–5). For R1, R2, n see Table 1
Table 1 Synthesized phenoxyamines (1–5)
Results and discussion
Amine
R1
R2
n
Yield (%)
The Williamson reaction between sodium phenate, con-
taining a chlorine atom or a methyl substituent in the ring,
and amines yielded phenoxyethyl-N,N-dimethylamine and
phenoxypropyl-N,N-dimethylamine (Scheme 1).
1
2
3
4
5
H
Cl
CH3
Cl
CH3
H
CH3
H
H
H
2
2
2
2
3
63
78
72
75
61
The obtained phenoxyamines were purified from the
excess of unreacted reagents by extraction with an organic
solvent. The synthesized compounds are listed in Table 1.
The reaction yields ranged between 61 and 78%. The struc-
tures of the obtained amines were confrmed by 1H and 13
C
were sown in a pot (0.5 L) and kept under greenhouse
conditions at 20 °C, 60% humidity and a 16/8 h (day/night)
photoperiod. Cornfower plants were treated with herbi-
cides at the 4–6 leaf stage of development. The herbicide
application was performed using a cabin sprayer (APORO,
Poznań, Poland) with a movable spraying beam equipped
with a TeeJet 110/02 fat fan sprayer (TeeJet Technolo-
gies, Wheaton, IL, USA) delivering 200 L of herbicide
solution per 1 ha of cultivation at the pressure 0.2 MPa.
The distance between the sprayer and the plants was equal
to 40 cm. The experiment was performed in four replica-
tions. After two weeks, the plants were cut at soil level
and weighed (accuracy 0.01 g). Fresh biomass reduction
for the individual variants was compared to the control
crop to which no herbicide formulation was applied. The
synthesized ionic liquids containing ion acetate of MCPA
as the anion and the control preparation Chwastox 300
SL were applied at a dose of 400 g of MCPA per hectare,
calculated based on the active substance.
NMR.
Spectra of compounds 1–5 are included in ESI
(Figs. 1–10). One of synthesized phenoxy amines is N,N-
dimethyl-2-(4-methylphenoxy)ethylamine (3). In this mol-
ecule, protons bound to the methyl group in ring exhibit
chemical shift δ = 2.06 and protons from methyl groups
bonded with nitrogen have chemical shift δ = 2.10. Ethyl-
ene bridge forms two triplet signals of δ=2.49 for protons
from carbon bound to nitrogen, and δ=3.81 for protons from
carbon bound to oxygen. Signals of δ = 6.64 and δ = 6.86
correspond to protons from phenoxy ring. Their symmetric
appearance is result of symmetric substitution of the phenyl
ring.
The Menshutkin reaction between phenoxyethyl-N,N-
dimethylamines and 1-bromoalkane (Scheme 2) resulted
in the formation of crystalline quaternary ammonium bro-
mides with a melting point from 39 for 8 to 150 °C for 10,
which are summarized in Table 2. Due to their melting
point being lower than 100 °C, ammonium bromides 6,
Scheme 2 Synthesis of the quaternary ammonium bromides (6–14). For R1, R2, R3, n see Table 2
1 3

Chemical Papers
Table 2 Synthesized quaternary
ammonium bromides (6–14)
Bromide
R1
R2
R3
n
Melting point (°C)
Yield (%)
6
H
Cl
H
CH3
H
H
H
H
H
H
H
C6H13
C6H13
C6H13
C6H13
C8H17
C10H21
C12H25
C14H29
C6H13
2
2
2
2
2
2
2
2
3
63–65
131–133
38–40
93
87
82
83
82
83
80
82
81
7
8
CH3
Cl
CH3
CH3
CH3
CH3
CH3
9
97–99
10
11
12
13
14
149–151
119–121
74–76
75–77
43–45
8, 9 and 12 can be recognized as ionic liquids. All com-
pounds 6–14 appear in form of white solid.
The reaction was carried out in methanol. The obtained
products with high yields ranging from 93 to 99% are sum-
marized in Table 3. The compounds are new high viscos-
ity quaternary ammonium salts which can be classifed as
room-temperature ILs. Their structures were determined
by 1H and 13C NMR (Figs. 29–48 in the ESI), LC–MS/MS
(Figs. 49–68 in the ESI) and FT-IR spectra (Figs. 69–78 in
the ESI).
The reaction yield exceeded 80%. The structures of the
1
new bromides were confrmed by H and 13C NMR. The
spectra are summarized in the ESI (Figs. 11–28).
Reaction of 3 with 1-bromohexane results in N,N-
dimethyl-N-hexyl-N-(4-methylphenoxy)ethylammonium
bromide (8). Protons from alkyl chain are represented
by 4 signals: triplet at δ = 1.02 for protons from terminal
methyl group, multiplet at δ = 1.48 represents 6 protons
from chain carbons C2–C4, multiplet at δ = 1.93 is associ-
ated with protons from carbon C5 and multiplet at δ=3.61
is representing protons from carbon bonded to nitrogen.
Methyl groups are represented by singlets at δ = 2.36 for
protons from methyl group in ring, and δ=3.35 for methyl
groups bonded to nitrogen. Two triplets at δ = 3.97 and
δ = 4.54 represent protons from ethylene group, from car-
bon bonded to nitrogen and from carbon bonded to oxy-
gen, respectively. Symmetry of signals from ring protons
at δ = 7.02 and δ = 7.20 was preserved compared to amine
3.
Final IL based on ammonium bromide 8 is N,N-dimethyl-
N-hexyl-N-(4-methylphenoxy)ethylammonium (4-chloro-
1
2-methylphenoxy)acetate (18). H NMR spectrum shows
signals both from the ion acetate of MCPA and ammonium
cation. Signals at δ = 0.66, δ = 1.09, δ = 1.50 and δ = 3.13
represent protons from alkyl chain carbons C6, C2–C4, C5
and C1, respectively. Protons from four methyl groups pre-
sent in this IL are represented by signals at δ = 1.96 and
δ=1.99 for ring methyl groups, and δ=2.89 for 6 protons
from 2 methyl groups connected to nitrogen. Singlet at
δ=4.18 is associated with protons from methylene group in
ion acetate of MCPA moiety. Ethylene bridge proton signals
have chemical shift of δ=3.50 and δ=4.50 for protons from
carbon bonded to nitrogen and oxygen, respectively.
The synthesized bromides were converted into hydrox-
ides using an ion-exchange resin according to the scheme
presented in Scheme 3.
Positive ionization MS spectrum of 18 confrms molecu-
lar mass of the ammonium cation. On the basis of selected
peaks from fragmentation spectrum structure of precursor
ion can be determined. Signal of m/z = 130.00 represents
N,N-dimethyl-N-hexylaminium ion. Signal of m/z=135.20
represents (4-methylphenoxy)ethylium ion. Signal of
The ion exchange efciency was equal to 98–99%. The
last stage of the synthesis was the neutralization reaction
of the synthesized hydroxides with (4-chloro-2-methylphe-
noxy)acetic acid (MCPA) (Scheme 4).
Scheme 3 Synthesis of hydroxides on ion exchange resin. For R1, R2, R3, n see Table 2
1 3

Chemical Papers
m/z=179.90 represents N,N-dimethyl-2-(4-methylphenoxy)
ethanaminium ion. Presence of these fragmentation ions
confrms structure of cation moiety of 18.
soluble in methanol and chloroform, and insoluble in water
and hexane. The tested ILs exhibit good or limited solubil-
ity in DMSO, acetonitrile, acetone and isopropanol. In the
case of ethyl acetate and toluene, all ILs are characterized by
good solubility with the exception of 17, 19 and 20, which
are insoluble therein.
The solubility of the synthesized ILs was determined
according to the methodology presented by Vogel. The
obtained results (Table 4) indicate that all compounds are
Scheme 4 Synthesis of ILs (15–24). For R1, R2, R3, n see Table 3
Table 3 Synthesized ILs
(15–24)
IL
R1
R2
R3
n
Yield (%)
State at 20 °C
15
16
17
18
19
20
21
22
23
24
H
Cl
Cl
CH3
CH3
CH3
CH3
CH3
CH3
H
H
CH3
H
H
H
H
H
H
H
C6H13
C6H13
C6H13
C6H13
C8H17
C10H21
C12H25
C14H29
C6H13
C12H25
2
2
2
2
2
2
2
2
3
2
97
99
99
96
96
94
95
94
93
98
liquid
liquid
liquid
liquid
liquid
liquid
liquid
liquid
liquid
liquid
H
Table 4 Solubility of prepared ILs (15–24) at 25 °Cb
IL
Water
9.0[a]
Methanol
6.6
DMSO
6.5
Acetonitrile
6.2
Acetone
5.1
Isopropanol
4.3
Ethyl acetate
4.3
Chloroform
4.1
Toluene
2.3
Hexane
0
[b]
15
16
17
18
19
20
21
22
23
24
−
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
−
+
−
−
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
−
+
−
−
+
+
+
+
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
[a] Snyder polarity index [b]+high solubility (more than 0.1 g can be dissolved in 1 mL of solvent), moderate solubility (0.1 g can be dissolved
in 2–3 mL of solvent),–low solubility (less than 0.1 g can be dissolved in 3 mL of solvent)
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Chemical Papers
The efect of the length of the alkyl substituent on the
solubilities of the compounds in acetone, isopropanol, ethyl
acetate and toluene can be observed. The IL with the short-
est chain, 18, exhibits good solubility in these solvents. In
turn, ILs 19 and 20 with the octyl and decyl alkyl chains,
respectively, are characterized by limited solubility in ace-
tone and isopropanol, while they are completely insoluble
in ethyl acetate and toluene. With the increase in the alkyl
chain length to dodecyl for 21 and tetradecyl for 22, an
improvement in the solubility in the abovementioned sol-
vents is observed. After a comparison of the efect of the
ring substitution of the phenoxy group in the cation, it can
be observed that ILs 15, 16 and 18 exhibit good solubility in
all of the tested solvents except hexane. On the other hand,
17, which possesses chlorine in the phenoxy group, is char-
acterized by limited solubility in acetone and isopropanol
and low solubility in ethyl acetate and toluene.
is within the range of 176–179 °C. In the case of 15, which
does not possess any ring substituents, T5% = 191 °C. The
melting point and the crystallization temperature could only
be determined for 24 and are equal to 72.5 °C and 45.9 °C,
respectively. For all of the synthesized ILs, it was possible
to determine the glass transition temperature, which is in the
range from −29.6 for 22 to −11.2 °C for 16. A decrease in
the glass transition temperature with increasing length of the
alkyl substituent in the cation can be observed.
The herbicidal activity of the synthesized ILs was tested
against cornfower (Centaurea cyanus L.). The herbicidal
activity was determined based on the results of biological
tests carried out in a greenhouse. The commercially avail-
able product Chwastox Extra 300 SL, which contains MCPA
sodium and potassium salt as the active substance, was used
as a control. The products were used at a dose of 400 g of
MCPA per hectare. The obtained results indicate that all of
the tested ILs exhibit high herbicidal activity. Ion acetate
of MCPA was the anion in all ILs, and therefore, their use
caused symptoms typical for phenoxyacetic acid herbicides.
In the initial stage, epinasty and the inhibition of growth
cone activity occurred, followed by chlorosis, eventually
leading to the necrosis of plant tissues (Coob and Reade
2010). Photographs of plants 14 days after the application
of compounds are included in the ESI, (ESI Fig. 79). The
plant fresh biomass reduction results are shown in Fig. 1.
All ILs showed activity greater than 95%, which is at the
commercial product level.
The thermal stability was measured as both the onset
temperature needed for 5% sample decomposition (T5%) and
the 50% decomposition temperature (T50%). Comparison of
the thermal stability between all prepared compounds was
performed by assessing the onset of thermal decomposition
for the frst 5% weight loss (T5%), which provides a more
accurate assessment of the thermal stability than the 50%
onset of thermal decomposition. The thermal decomposi-
tion of ILs begins in the range of 170–200 °C, as shown in
Table 5. IL 23 exhibits the highest thermal stability. It is the
only compound among the tested ionic liquids that exhibits
a T5% above 200 °C as well as the only one that includes a
propyl group in the cation and not an ethyl group. The ther-
mal stability of the tested ILs increases with the increase in
the length of the alkyl substituent in the cation, from 176
for 18 to 192 °C for 21 and 22. The presence of substitu-
ents (chlorine and methyl group) in the phenoxy group ring
negatively afects the temperature stability. For 16–18 T5%
In case of ring substituent confguration in obtained ILs,
the alkyl substituent and the presence of the chlorine atom
or methyl group in the ring in the cation practically do not
afect the herbicidal activity. The activity of the anion, ion
acetate of MCPA, was preserved in the tested ILs. New HILs
were obtained, i.e. MCPA in the form of ILs as derivatives
of domiphen with recognized antibacterial and antiseptic
properties.
The frst stage of research on ionic liquids with domiphen
derivatives as a cation has been completed. The prepared
ionic liquids are efective at the same level as commercially
available MCPA in the form of sodium or potassium salt.
The chemical characteristics of the obtained ionic liquids
have been presented. However, for full understanding of
the biodegradation pathway and microbial degradation of
chemical compounds in the environment, the new investiga-
tion is necessary.
Table 5 DSC and TG analysis of the synthesized ILs (15–24)
IL
Tg (°C)
Tc (°C)
Tm (°C)
T5% (°C)
T50% (°C)
15
16
17
18
19
20
21
22
23
24
−17.8
−11.2
−12.5
−11.8
−19.7
−25.8
−23.4
−29.6
−14.4
−25.6
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
190
179
176
175
190
190
192
192
200
186
224
211
204
205
223
225
232
240
235
224
Conclusions
–
45.9
–
72.5
Ten novel ILs were obtained. They consisted of a (4-chloro-
2-methylphenoxy)acetate anion and an N-alkyl-N,N-dimethyl-
phenoxyammonium cation with an alkyl substituent length from
Tm—melting point; Tc—temperature of crystallization; Tg—glass
transition temperature; T5%—decomposition of 5% of sample; T50%
—
decomposition of 50% sample
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Chemical Papers
Fig. 1 Herbicidal activity of
the synthesized ILs (15–24),
C–control
hexyl to tetradecyl and the substitution of the phenol ring at
the para and meta positions with a chlorine atom and a methyl
group. They are also new derivatives of domiphen, a quaternary
ammonium bromide with well-known antibacterial and anti-
septic properties. The four-step synthesis strategy involved the
preparation of phenoxyamines in the Williamson reaction with
yields in the range of 61–78%, which were alkylated with 1-bro-
moalkanes in the Menshutkin reaction with yields of 80–93%.
After the ion exchange, the synthesis of ILs was performed
by neutralizing the synthesized hydroxides with (4-chloro-
2-methylphenoxy)acetic acid (MCPA) with yields of 93–99%.
The synthesized ILs exhibit thermal stability up to 175 °C and
are characterized by high herbicidal activity against cornfower
(Centaurea cyanus L.). The fresh weight reduction exceeded
95% in all cases. The obtained results allowed the tested com-
pounds to be classifed as HILs, in which the activity of the
MCPA anion was preserved. They are also new derivatives of
domiphen, a quaternary ammonium bromide with well-known
antibacterial and antiseptic properties. However, it cannot be
clearly stated whether the efectiveness of the synthesized ionic
liquids (15–24) is related solely to the presence of the MCPA
ion or also to the presence of the phenoxyethylammonium
cation. Therefore, in the second stage of this research, on this
group of compounds, additionally the herbicidal and bacteri-
cidal activity of the raw materials used for synthesis, quater-
nary bromides (6–14) and hydroxides used for their synthesis
should be checked. The obtained results will make it possible
to unequivocally assess which part of the ionic liquids (cation
or anion) is responsible for biological activity and may have an
impact on the use of the tested products in agriculture.
Declarations
Conflict of interest Authors declare that there is no confict of interest.
Open Access This article is licensed under a Creative Commons Attri-
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tion, distribution and reproduction in any medium or format, as long
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copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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