Experiments probing how force accelerates chemical reactions have provided new insights into mechanochemical kinetics. The findings by US-based scientists could inform the development of chemical manufacturing methods that are more sustainable and less wasteful than current approaches.
In mechanically activated organic chemistry, force drives the making and breaking of covalent bonds. This helps to minimise the waste and energy cost associated with organic synthesis because the reactions are run neat or with minimal solvent, and energy is provided through mechanical, rather than thermal, means.
However, as materials scientist and organic chemist Adam Braunschweig from the City University of New York explains, mechanochemistry has not yet been widely adopted, in part because of the substantial gaps that remain in our understanding of reaction kinetics during these reactions.
Aiming to provide answers, they experimentally and computationally studied the reaction kinetics of mechanically activated Diels–Alder cycloaddition reactions between four dienophiles and surface-confined diene monolayers to measure how force affects reaction rates.
The reactions were carried out on monolayers so that the complexities associated with grinding powders during milling and reactant availability were not factors in the kinetic analysis. This meant that the effects of force on the free energy of activation and reaction trajectories would be isolated.
The researchers then used elastomeric arrays containing 900 pyramidal tips to bring fluorescently labelled dienophiles into contact with monolayers of the tethered diene, anthracene, that was immobilised covalently onto the surface of a silica wafer.
‘We attached [the tips] to a piezo actuator, which just allows you to press very controllably into the surface,’ explains Braunschweig. ‘We pushed down, for one second, two seconds, three seconds, so we got the full-time course. Then, what’s great about these tips is you can tilt them with respect to a surface; so one bit touches with a different force than another bit.’
‘In a single experiment, you get the entire time course and all the forces plus each of these patterns is repeated 30 times, so you get really good, high fidelity, low-error data.’
They then used fluorescence microscopy to track the Diels–Alder adduct formation as a function of applied force and reaction time under force.
‘We discovered that the moderate mechanical energy that is applied along the right trajectory, can substantially accelerate the rate of organic reactions,’ adds Yerzhan Zholdassov , a doctoral student in Braunschweig’s lab and lead researcher on the study.
‘Moreover, different molecules have different responses to the applied mechanical energy,’ he says. ‘These findings have major implications in chemical industry, which allows us to produce chemical products without the harmful solvents and high energy input.’
International Conference on Organic Chemistry
https://organic-chemistry-
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