November 21, 2019

One compared to the single crystals of conventional chemistry

One of the main intentions of chemists in the modern day is to create and develop new ways of synthesizing molecules which are more sustainable due to the introduction of green chemistry. Usually, this has been done by searching for and selecting ideal solvents which are smart, practical and environmentally friendly. An even better approach involves proposing and developing the possibility of solvent-free or solvent-catalyzed reactions, hence, mechanochemistry. These solid-state reactions aim to achieve mechanical activation and chemical transformations of the solid reactants.· What is mechanochemistry?Mechanochemistry is a new technology with great significance in many applications, being developed due to its versatility for the synthesis of compounds as an environmentally friendly method compared to the popularly known conventional chemistry. It involves grinding of solid reactants with little or no solvent, usually with a mechanical force to give a chemical reaction. This includes mixing, milling and grinding of the solid particles to achieve chemical transformations. Mechanochemistry covers neat grinding in which no solvent is added, liquid-assisted grinding (LAG), which includes the addition of solvents in catalytic amounts, and ion- and liquid- assisted grinding (ILAG), involving the use of both a solvent and an ionic salt.· History of mechanochemistryMechanochemical processes originated from the basic mortar and pestle method as a cheap method of manual grinding and mechanical activation of chemical structures. With this type of grinding, the reactions could not be controlled properly and the reaction parameters such as temperature and humidity varied easily. It also had the disadvantages of major safety issues and limitations of compounds forming due to reduced reaction times. This led to the advancement of the grinding methods and the introduction of shaker and planetary automated mills. Compared to manual grinding, these automated mills create an enclosed environment for the reactions and so provide a more controlled environment for the reaction and safer working conditions for the operator by reducing exposure to hazardous chemicals.1 Using the automated ball mills also ensures proper control of parameters to maximize energy input by altering milling time, operating frequency and the size and quantity of the balls.2Characterization and structure determination of the resulting product phase, usually microcrystalline powders of the mechanochemical process has also proven to be challenging over theyears.3 Milling procedures by mechanochemical synthesis usually yield microcrystalline powders compared to the single crystals of conventional chemistry which is easier for determining the structure using single-crystal X-ray diffraction. Also, recrystallisation of the solid product usually forms materials with a different structure e.g. polymorphs, hence more difficult to exploit mechanistic pathway.3 Solid-state analysis has been greatly facilitated in more recent times due to the development and availability of high quality instrumentation such as infrared (IR) spectroscopy, solid-state nuclear magnetic resonance (ssNMR) spectroscopy and powder X-ray diffraction (PXRD).1 New techniques have also been developed for real-time monitoring and studying thermodynamics, reaction kinetics and the effect of particle size.4 These analyses and investigations have helped to explore and expand mechanochemistry in the past few years in organic chemistry as well as inorganic, organometallic, supramolecular and pharmaceutical applications.2

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