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Arthur T Knackerbracket has found the following story:

The Grignard reaction is used to synthesize carbon-carbon bonds, a crucial step for making new molecules for academic and industry uses. Finding efficient and selective methods for this reaction, using low cost materials and minimal energy resources has been the target of the research activity for more than 100 years. Incredibly enough, the way the Grignard reaction works has been unknown—until now. As we finally understand it, ways to its improvement can now open up.

[...] Eisenstein and Cascella decided to tackle the problem using computer simulations. Modelling both the reagent and the solvent in a realistic manner, they were able to detect the multiple chemical species during the Schlenk equilibrium[*]. Importantly, their study identified that the whole process is determined by solvent molecules that combine to, or detach from, the magnesium atoms. Thus, the dance of solvent drives the exchange of partners for the magnesium atom, giving rise to the Schlenk equilibrium, and resulting in the different compounds present in the solution.

[*] Wikipedia entry on the Schlenk equilibrium.

[...] By computer simulations accompanied with high-level quantum chemistry data, thanks to a collaboration with Professor Jürgen Gauss (Johannes Gutenberg-University Mainz, Germany), it was possible to establish a series of key points. First, almost all the dancing couples will end up forming stable carbon-carbon bonds, meaning that all the molecules produced by the Schlenk equilibrium promote the formation of carbon-carbon bonds, although at different rates. Second, different partners in the dance request different dancing steps; meaning, different substrate molecules will react following different mechanisms characterised by either heterolytic or homolytic splitting of the magnesium-carbon bond (the two electrons of the bond go to the carbon, or are equally shared between the magnesium and the carbon).

"What has always been known as the Grignard reaction is, in reality, a group of reactions that occur simultaneously in the same sample," says Cascella.

Their studies demonstrated that unlike other common reactions, in this case the solvent drives the whole chemical process. This was also one of the reasons why the Grignard reaction remained mysterious for so many years: "Systems dominated by the solvent are hard to study, points Eisenstein. Their structure is ever changing, and most experimental methods are not (yet) good enough to see what actually happens. Just like trying to take a photograph of a flock of birds having a shutter speed that is too slow. All you can see in the photo is a blurred mess of feathers and bird-like shapes, but you cannot decide how many birds you have, how they fly, or even which species it is. We cannot determine anything from that. That is where computational methods have an edge."

More information: Raphael M. Peltzer et al. How Solvent Dynamics Controls the Schlenk Equilibrium of Grignard Reagents: A Computational Study of CH3MgCl in Tetrahydrofuran, The Journal of Physical Chemistry B (2017). DOI: 10.1021/acs.jpcb.7b02716

Raphael Mathias Peltzer et al. The Grignard Reaction – Unraveling a Chemical Puzzle, Journal of the American Chemical Society (2020). DOI: 10.1021/jacs.9b11829

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