A. Parus et al.
Ecotoxicology and Environmental Safety 208 (2021) 111595
Seddon, 2003) and combine it with a counter-ion to introduce additional
functionality (Hough et al., 2007; Pernak et al., 2011). As such, the
herbicidal anion may be paired with almost any cation which results in
improved properties, e.g. decreased dose of herbicide per hectare
(Syguda et al., 2018), high wetting properties (Cojocaru et al., 2013;
Ławniczak et al., 2015; Pernak et al., 2011), low toxicity (Pernak et al.,
2018, 2016, 2015) and improved work safety (Rogers and Seddon,
2003). Additionally, the use of a cation of natural origin, such as choline
or nicotinic acid, may possibly increase the biodegradability (Jordan
and Gathergood, 2015, Niemczak et al., 2017b). Another important
advantage of HILs is the possibility to adjust their surface activity by
using various cation structures, such as morpholinium or quaternary
ammonium. This may considerably limit or completely eliminate the use
of adjuvants in commercially available herbicidal mixtures (Cojocaru
et al., 2013; Ławniczak et al., 2015; Pernak et al., 2011).
Herbicides used for synthesis (MCPA; MCPP) were additionally pu-
rified prior to their use according to the procedure provided by Syguda
et al. (2018). Briefly, MCPA and MCPP were dissolved in hot toluene
with activated carbon and stirred intensely. Then, the solution was
subjected to filtration in order to remove any contaminants using acti-
vated carbon. Finally, the herbicides were recrystallized from cold
toluene.
2.2. Synthesis of studied compounds
The synthesis was conducted in accordance with the procedure
provided by Syguda et al. (2018). Each step was described below.
2.2.1. (2-methyl-4-chlorophenoxy)acetyl chloride
In order to prepare the (2-methyl-4-chlorophenoxy)acetyl chloride,
100 g of (2-methyl-4-chlorophenoxy)acetic acid was placed in a round
bottom flask (500 mL) which was supplied with a reflux condenser, a
dropping funnel and a magnetic stirrer. Thionyl chloride (with a 3-fold
excess) was slowly added with continuous mixing and the mixture was
heated at 75 ◦C for two hours. Then, the excess of thionyl chloride was
removed via evaporation and a colourless liquid was obtained (boiling
point at 145–146 ◦C, 17–18 hPa). The reaction yield of (2-methyl-4-
chlorophenoxy)acetyl chloride was equal to 95%.
A new group of herbicidal ionic liquids which include two herbicides
has been introduced recently (Syguda et al., 2018). The concept behind
such HILs is to combine one herbicide in the anionic form with a second
herbicide, which is incorporated in the structure of the cation. The
number of literature reports focused on the combination of herbicides
such as 2,4-D, MCPA, MCPP, 4-CPA, chloropicolinic-acid, chlor-
obenzoic-acid, glyphosate or dicamba in order to limit the frequency of
spraying or to increase the spectrum of targeted weed species is very
limited (Choudhary et al., 2017; Niemczak et al., 2018; Syguda et al.,
2018). Currently, there are no reports focused on HILs which incorpo-
rate both MCPA and MCPP, even though this combination of herbicides
is often used in commercial formulations. Despite structural similarities,
MCPA is characterized by a lower spectrum of controlled weed
compared to 2,4-D, hence MCPP is supplemented to extend the activity
(e.g. in case of chickweed or white clover). The combination of MCPA
and MCPP is commonly used in the UK and EU countries, since MCPA is
more popular in Europe than 2,4-D. Moreover, it is also used in the USA
in case of certain crop plants, since 2,4-D may cause damage to peas and
flax cultivars (Zimdahl, 2015).
2.2.2. (2-dimethylamine)ethyl (2-methyl-4-chlorophenoxy)acetate
hydrochloride
First, 80 g of (2-methyl-4-chlorophenoxy)acetyl chloride was dis-
solved in 100 mL of chloroform and a solution of an equimolar amount
of DAE in 100 mL of chloroform was prepared. Then, the DAE – chlo-
roform solution was introduced to a reaction flask (500 mL) supplied
with a funnel, a magnetic stirrer and an ice-water bath. Next, the (2-
methyl-4-chlorophenoxy)acetyl chloride solution was added dropwise
to the DEA solution. The reaction was conducted for 20 min with
continuous mixing. Afterwards, chloroform was completely evaporated
and the flask with the product was placed in a refrigerator. After so-
lidification, the product was filtered under vacuum. Next, hexane was
added to the flask and the solution was filtered using a Büchner funnel,
additionally rinsed with hexane and dried. The reaction yield was equal
to 96%.
In the framework of this study, 5 novel HILs were prepared, which
incorporated a cation composed of MCPA in the form of a quaternary
ester (esterquat) and MCPP as a herbicidal anion. It was assumed that
this structural design will provide additional functionality, namely: i)
enhanced efficiency and decreased volatility, as a result of trans-
formation of herbicides into their respective ionic forms; ii) easier
preparation of the spray solution due to superior solubility of the ob-
tained cation-anion pairs; iii) displacement of surface active adjuvants
due to the presence of the esterquat moiety with a long alkyl chain (C8,
C9, C10, C11, C12, C14) as a substituent, which may contribute to lower
toxicity. The aim of the study was to confirm the above-mentioned as-
sumptions based on the evaluation of surface, herbicidal and antimi-
crobial activity. The obtained HILs were characterized in terms of basic
physicochemical properties (solubility and volatility) as well as biode-
gradability. Additionally, their phytotoxicity was assessed towards both
a model weed (cornflower (Centaurea cyanus)) and crop plant (maize
(Zea mays)).
2.2.3. (2-dimethylamine)ethyl (2-methyl-4-chlorophenoxy)acetate
In order to prepare the (2-dimethylamine)ethyl (2-methyl-4-chlor-
ophenoxy)acetate, 100 g of aminoester hydrochloride and 300 mL of
chloroform were placed in a flask (1000 mL), an equimolar amount of
triethylamine was added and the solution was mixed intensely. After 15
min, the reaction mixture was transferred to an evaporator and the
solvent was completely removed. After solidification, the product was
rinsed and dried similarly to the previous step. A colourless liquid was
obtained. The reaction yield was equal to 88%.
2.2.4. (4-chloro-2-methylphenoxy)ꢀ 2-
acetoxyethylalkyldimethylammonium bromides
At first, 5 g of (2-dimethylamine)ethyl (2-methyl-4-chlorophenoxy)
acetate was dissolved in 5 mL of acetone in a flask (100 mL). Next, an
appropriate alkyl bromide (CH3(CH2)7Br, CH3(CH2)8Br, CH3(CH2)9Br,
CH3(CH2)10Br, CH3(CH2)11Br, CH3(CH2)13Br) was added to the flask
with a 5% excess. The flasks were equipped with a reflux condenser and
2. Materials and methods
2.1. Chemicals
Reagents such as (±)ꢀ 2-(4-chloro-2-methylphenoxy)propionic acid
(MCPP) (93%), (2-methyl-4-chlorophenoxy)acetic acid (MCPA) (97%)
were purchased from Ciech-Sarzyna S.A. Thionyl chloride (97%), deanol
(DAE) (99%), triethylamine (99.5%), 1-bromooctane (99%), 1-bromo-
nonane (98%), 1-bromodecane (98%), 1-bromoundecane (98%), 1-bro-
mododecane (97%), 1-bromotetradecane (97%) were obtained from
Sigma-Aldrich (Germany). Other chemical reagents and solvents were
obtained from Sigma-Aldrich (Germany), Merck (Germany), Fluka
(Switzerland), POCH (Poland), Chempur (Poland) and International
Enzymes Limited (USA).
◦
heated for 24 h up to 55 C. Afterwards, the flasks were placed in a
refrigerator in order to crystallize the product. The product was then
filtered using the Büchner funnel, rinsed thoroughly with hexane and
cold acetone, and finally dried. The reaction was presented in Fig. S1
(ESI) and the product yields were listed in Table S1 (ESI).
2.3. Esterquats with herbicidal anions
Approx. 1 g of an appropriate (4-chloro-2-methylphenoxy)ꢀ 2-
2