About the Inhibition of Grignard Reagent Formation by p-Dinitrobenzene: Comparing the Mechanism of Grignard Reagent Formation and the SRN1 Mechanism

Both S RN 1-type reactions and Grignard reagent formation are inhibited by trace amounts (with respect to the halide) of p-dinitrobenzene (DNB) and other oxidising agents such as CuCl 2 or dioxygen. Both are believed to be triggered by an electron-transfer step. In this report we examine the patterns of reactivity shared by these two types of reaction. Although the two reactions display an amazing number of similarities, the chain character of the Grignard reaction mechanism has been rejected by major contributors to this field. We have performed new experiments to explore the inhibiting effect associated with the addition of trace amounts of p-dinitrobenzene during Grignard reagent formation. This inhibition involves very reactive nanoparticles of magnesium prepared by metal vapour synthesis. The magnesium slurries studied, in the absence of p-dinitrobenzene, display no induction period at all. If the p-dinitrobenzene is added to the solution containing magnesium slurries 25 min before adding the organic halide (here bromobenzene), the reaction is neatly inhibited but, if one waits long enough, the Grignard reagent is nevertheless formed. If, however, the same trace amounts of p-dinitrobenzene are diluted in the organic halide and this mixture is added to the magnesium slurries, the inhibiting effect is far less pronounced. The addition of the nBu 4 NBr salt drastically diminishes the inhibiting power of DNB. These observations, in addition to the ESR study of radical anions formed in THF by the reaction of magnesium nanoparticles and DNB with various polyaromatics, suggest a mechanism for the observed inhibitions that involves a series of reversible adsorptions of the present species. This adsorption would depend on both thermodynamic and kinetic factors. Thermodynamics would favour the adsorption of DNB radical anions but C-X bond cleavage of the less strongly adsorbed aromatic radical anions would provide a kinetic driving force for displacing the equilibria in the direction of Grignard reagent formation. Salt effects could operate by displacing the equilibrium - adsorbed DNB radical anion or dianion versus solvated radical anion (with the counterion nBu 4 N + ) - to the right, accelerating therefore the liberation of active sites on the magnesium surface. It appears that the inhibiting effect of the same compound finds its origin in two different molecular series of events in S RN 1 and in Grignard reagent formation. The heterogeneous character of the Grignard reagent makes it possible to envision the possibility of active sites even on nanoparticles, whereas the inhibition of the S RN 1 mechanism by DNB occurs in homogeneous solution.