COMPOSITION OF GRIGNARD COMPOUNDS .6. NATURE OF ASSOCIATION IN TETRAHYDROFURAN AND DIETHYL ETHER SOLUTIONS

Ebullioscopic data are presented for tetrahydrofuran and diethyl ether solutions of several Grignard and related magnesium compounds over a wide concentration range. Analysis of the data is accomplished by observing the change in association (i) with concentration and by consideration of the constancy of the equilibrium constants calculated for several possible descriptions of the associated system. The expected nonideality of the solutions studied was considered in the interpretation of the data. While all the compounds studied were found to be monomeric in tetrahydrofuran, the alkyl- and arylmagnesium bromides and iodides were found to be monomeric in diethyl ether only at low concentration (<0.1 m), exhibiting in general an increase in association with concentration. Interpretation of the data indicates that these compounds are associated in a polymeric fashion. In contrast the alkylmagnesium chlorides associate in diethyl ether to form stable dimers with the association insensitive to concentration changes. Comparison of the data for magnesium halides and dialkylmagnesium compounds in diethyl ether indicates that, except for the methyl compound, association is considerably stronger for the magnesium halides than for the dialkylmagnesium compounds. Thus, except for methylmagnesium halides, Grignard compounds associate with bridging mainly through the halogen atom. The methylmagnesium halides are exceptional since methyl bridging is strong enough in diethyl ether to permit association by bridging through either the methyl group or the halogen atom. Although the steric and electronic nature of the alkyl group has some effect on the association of Grignard compounds, the effect is generally small compared to the effect of halogen or solvent. © 1969, American Chemical Society. All rights reserved.