Metal fluoride stability
Robert Waymouth; Eric J. Moore
Chemical & Engineering News (17 Mar 1997) Vol. 75, No. 11, pp. 6.
We wish to report an explosion that occurred in our laboratory during the synthesis of a trifluoromethyl-substituted Grignard compound, 4-trifluoromethylphenylmagnesium bromide (4-CF3PhMgBr). In the procedure involving the explosion, 4-bromobenzotrifluoride was treated with magnesium in ether to form the Grignard derivative.
At the completion of the reaction, large pieces of unreacted magnesium were removed from the Grignard suspension by passing the mixture through a narrow bore cannulae. The ether was then removed in vacuo to afford a viscous oil, because a subsequent reaction necessitated the use of a second solvent (in this case, toluene).
It was this product that detonated, obliterating the reaction vessel (a 250-mL round bottom flask) and releasing noxious fumes when pyrophoric compounds were exposed to the atmosphere. Fortunately, no personnel were present during the explosion and only minor damage was done to the laboratory area. We believe that highly reactive magnesium particulates passed through the cannulae and initiated exothermic redox reactions with the trifluoromethyl group of the concentrated Grignard reagent.
It should be noted that the lattice energies of metal fluorides are
high and that caution should always be exercised in handling
organolithium or organomagnesium reagents that might decompose to
yield the metal fluorides. The concentration of such species should
be avoided, and they should always be handled as dilute solutions on
as small a scale as practicable. A letter to the editor by Peter G.
Urben speaks to the kinetic versus the thermodynamic stability of
organofluorine reagents, the need for avoiding isolation of these
compounds, and the thermodynamic fury that can be unleashed when
their kinetic stability is compromised (C&EN, July 8, 1996, page 3).
page last revised December 7, 1998