Tuning the Solubility of the Herbicide Bentazon: from Salt to Neutral and to Inclusion Complexes

Alessandra Azzali,Fabrizia Grepioni,Simone D’Agostino

doi:10.1021/acssuschemeng.1c02749

Alessandra Azzali, Fabrizia Grepioni + Show 1 more

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https://doi.org/10.1021/acssuschemeng.1c02749

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Abstract

We report on the mechanochemical synthesis of inclusion complexes obtained by reacting the neutral crystalline herbicide bentazon (HBtz) with native cyclodextrins (CDs). The reaction of HBtz with γ-CD resulted in the formation of the inclusion complexes [β-CD·HBtz]·6H2O and [γ-CD·HBtz]·8H2O, which were characterized via a combination of X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, and calorimetric measurements. No complexation, on the contrary, was achieved upon the reaction of HBtz with α-CD. The salt NaBtz·1.75H2O, widely used as a water-soluble salt of bentazon for the manufacturing of agrochemicals, was also synthesized and structurally characterized, and its solubility and dissolution properties were compared to those of neutral HBtz and of the β-CD and γ-CD inclusion complexes. It was found that the behavior of [β-CD·HBtz]·6H2O and [γ-CD·HBtz]·8H2O in water is similar, with dissolution rates of 28.4 ng·L–1·min–1 and 39.8 ng·L–1·min–1, respectively, and is intermediate between those of NaBtz·1.75H2O (228 ng·L–1·min–1) and neutral bentazon (3.5 ng·L–1·min–1). These results were also compared with those of the dehydrated inclusion complexes, which displayed intermediate dissolution rates between hydrous complexes and NaBtz·1.75H2O. All findings indicate that the inclusion of HBtz in β-CD and γ-CD might represent a viable alternative for the preparation of environmentally friendly agrochemicals with controlled bentazon release to be used in the formulation of herbicides.

Highlights

Bentazon (HBtz) is a cyclic sulfonamide employed as a selective, contact herbicide for postemergent weed control of broad-leaved weeds in a wide variety of cultivations.[1]
The aim of this research was twofold: on the one hand, we were interested in exploring the complexation ability of the whole series of native cyclodextrins toward the herbicide bentazon (HBtz), using a variety of preparative methods, including mechanochemistry and crystallization from solution; on the other hand, we wanted to compare the water dissolution properties of the inclusion complexes with those of neutral bentazon (HBtz) and of its sodium salt, NaBtz·1.75H2O, which was structurally characterized in this work for the first time
We have focused our attention mainly on preparing the inclusion complexes via mechanochemistry, viz. grinding and kneading

Summary

Introduction

Bentazon (HBtz) is a cyclic sulfonamide employed as a selective, contact herbicide for postemergent weed control of broad-leaved weeds in a wide variety of cultivations.[1] Since it must be applied aerially or through contact spraying on cultivations, its scarce aqueous solubility represents the main factor that limits its utility, and the formation of its sodium salt (known as sodium bentazon) is the only method employed, so far, to address this shortcoming (Scheme 1).[2] The solubility of sodium bentazon, is considerably increased with respect to pure bentazon, resulting in excess contamination of water resources. Herbicide Bentazon (Left) and its Sodium Salt (Right) number of pesticides detected in waters and to the establishment of strict directives in Europe, aiming at the protection of water sources used for the production of drinking water.[3−5]. Bentazon is classified as slightly toxic (class III) by World Health Organization (WHO)[6] and noncarcinogenic to humans,[7,8] it can undergo photodegradation in groundwater and soil.[8,9] The environmental protection agency (EPA) and the European Commission for Environmental and

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