[COSTRIBUTION S O .
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DEPARTMENT O F CHEMISTRY,
USIVERSITP O F PITTSBURGH]
The Constitution of the Grignard Reagent BY RAYMOND E.DESSY,'GEORGEs. HAKDLER, JOHN H.
W O T I Z .4ND
c. A. HOLLINGSWORTH
RECEIVED JANCARY 12, 1957 The equilibria EtzMg.MgBr2-t EtzMg iMgBr2 i=? 2EtMgBr have beeu investigated by employing magnesium bromide labeled with radioactive magnesium. Only a small amount of exchange was noted between E t z M g and Mg*Br2. These results, in conjunction with the fact t h a t the same species seems to be present in a solution of EtzMg and MgBrz as is present in the Grignard reagent prepared in the usual manner, indicate that the ethyl Grignard reagent is better represented by a complex Et?Mg MgBr: than by EtMgBr.
Despite the fact that the Grignard reagent has been known and utilized for over fifty years, its structure, particularly in ether solution, is unknown. The most widespread hypothesis considers the equilibria2
ing diethylmagnesium in the supernatant liquid. The specific activity of the magnesium in an aliquot of the diethylmagnesium solution was determined as a function of time. Table I also lists these specific activities, corrected to the same time of counting as the corresponding magnesium bromide sample and corrected for the small amount of magnesium bromide left unprecipitated by the dioxane.
The addition of dioxane to a Grignard solution results in the precipitation of virtually all of the halogen and part of the magnesium, leaving R 2 X g in the supernatant liquid.2 It also has been TABLE I shown3m4that equimolar mixtures of diethylmag- KADIOACTIVITV EXCHANGE BETIVEES (CzH;)*LIg A S U nesium and magnesium bromide in ether yield alMg*Brr most instantaneously a solution having the same 5 Exchange relative rate of reaction and kinetics in a reaction (based on s t a with a given substrate, hexyne-1, as does the GriSpecific activity Contact ?: tistical gnard reagent prepared in the normal manner from (counts/sec.-mg, Mg) time, Activity AligraexRun Mg*Brz Et2Mga min. ratio tion change) ethyl bromide and magnesium. This would seem 1 1.65 0.089 10 19:l 3 10 to imply that the same species is 'are present in the 2a 13.05 .A44 10 29:l 3 6 two solutions. Recalling these two pieces of in2b 4.20 .178 36hr. 24:l 4 8 formation, it therefore seemed of interest to ina The specific activity of the magncsiutzi in the dietliylvestigate equation 1 [R = ethyl, X = B r ] employmagnesium sample has been adjusted to the same counting ing radioactive magnesium as a tracer, using, for time as t h a t of the companion magnesium in the magnesium example, blg*Bra. bromide sample by means of log (activity) verszis time plots.
Methods and Results Radioactive magnesium (?\IgtS,to.s = 21.23 hr.) was prepared by a spallation reaction involving proton irradiation of potassium chloride. The magnesium, diluted with carrier, was separated as the oxinate and ignited to the oxide. The latter was converted to magnesium chloride, and exchange between this and elemental magnesium was accomplished by heating the mixture t o SOO'. The resulting labeled magnesium metal was then treated with bromine in dry ether t o yield magnesium bromide. The specific activity of the magnesium in an aliquot was determined as a function of time. Table I lists the specific activities of the magnesium in the magnesium bromide used in two entirely separate experiments. Equimolar amounts of this magnesium bromide and diethylinagnesium (prepared from ordinary magnesium) were dissolved in ether so as to give a solution 1.0 -11 in magnesium. The resulting solution was permitted to stand for ten minutes or 36 hr. and an excess of dioxane added, so as to precipitate the elements of magnesium bromide, leav(1) U e p a r t m r n t of CheniistrS , T h e Ohio S t a t e Uniaersity, Columbus 10, Ohio. ( 2 ) hI. S Kharasch a n d 0. Reinmuth, "Grignard Reactions of Tolliilic Suhitances," Prentice Hall, Tnc., h7pw Y n r k , S U..l!%jl. ) J . IT. Wntiz, C. A H o l l i n g ~ w o r t h and I
