CONDUCTA.NCE OF SODXTJM ALUMINUM ALKYLSIN TOLUE~NE AND DIETHYL ETHER
2595
The Conductance of Sodium Aluminum Alkyls in Toluene and Diethyl Ether'
by M. C. Day, Harold M. Barnes, and Alfred J. Cox Department of Chemistry, Louisiana State Uninersity, Baton Rouge, Louisiana (Received Aprzl 18, 156.4)
The conductances of NaA1Et4 in toluene and NaA1Et4 and N ~ A ~ ( ~ - in B udiethyl )~ ether have been measured in the concentration range of approximately 0.1-1.2 N . The conductance of XaAIEtr is observed to be enhanced by very nearly a factor of 10 in the presence of ether. Temperature dependent studies of the conductances in ether show the dependence to be inverted from that which is normally expected in solvents of low dielectric constant. These effects can be at tributed to specific ion-solvent interactions.
Introduction
Experimental
In recent years there has been a renewed interest in the nature of ionic species in solvents of low dielectric constant. This is, to some extent, attributable to the recent Fuoss-Onsager conductance equation. The use of nonaqueous solvents permits one to study the effects of the dielectric constant on the solute behavior. But at the same time it results in a variety of ion-solvent interactions of indeterminate extent and affect. These are not included in the theoretical models, but they obviously play an important role in the nature of ionic behavior in solution. Due to the limited solubilities of ionic species in purely nonpolar solvents, the study of ionic species in such solvents has been very nearly nonexistent. Outside of the study of the alkali metal picrates and substituted ammonium thiocyanates in benzene which have been summarized by Kraus, a experimental work has been conducted in solvents which should have some significant ion-dipole interactions. The most desirable system would be one in which the only ion-solvent interactions are of the ion-induced dipole variety. With the long range purpose of studying ion-solvent interactions, we have recently carried out a series of preliminary studies of the conductances of sodium aluminuni alkyls in solvents of low dielectric constant such as toluene, hexane, and diethyl ether. It is the purpose of this paper to report of our preliminary findings* The accuralcyOf the data is not intended to be adequate for theoretical considerations. Rather, general trends are to be noted.
Preparation of Compounds. NaA1Et4 was preparedl according to the method of Frey and Kobetza4 Tri-. ethylaluminum is added to a sodium dispersion using sodium-dried toluene as a reaction medium. The system is refluxed under a nitrogen atmosphere for approxi-. mately 2 hr. The resultant mixture is filtered, and the NaAlEt4 is crystallized from the toluene solution on cooling. The NaA1Et4 is separated by filtration, washed with sodium-dried hexane, and dried under vacuum. The conductance was measured in the fused state after each of several recrystallizations as a test O F purity. I t was found that no significant improvement was obtained as a result of recrystallization. I n subsequent preparations of NaAlEt,, one recrystallization was assumed to be adequate. The melting point of NaA1Et4was observed to be 126'. N~iAl(i-Bu)~ was prepared by the addition of triisobutylaluminum to a sodium dispersion using sodiumdried hexane as a reaction medium. The system is refluxed under a nitrogen atmosphere for approximately 8 hr. The N ~ A ~ ( & BisU obtained )~ by filtration and vaporization of the hexane under vacuum. A cleair crystalline compound with a melting point of 28' is ob-
(1) Taken from a portion of the thesis submitted to the Louisiana State University by H. M. Barnes in partial fulfillment of the requirements for the degree of Master of Science. (2) R. M. Fuoss and L. Onsager, J.Phys. Chem., 61, 668 (1957). (3) c. A. Kraus, ibid., 58, 673 (1954) ; 60, 129 (1956). (4) F. W. Frey and P. Kobetz, J . Org. Chem., 26, 2950 (1961).
Volume 68, Number 5 September, 156.4
2596
taiiied. It was observed to decompose if kept in the fused state. Handling of Materials. All sample preparations and solution transfers were carried out in a nitrogen drybox. A circulating system attached to the box passed the drybox atmosphere through an oven containing copper to remove oxygen and through a Dry Ice trap to remove moisture. When it was necessary to make measurements outside the drybox, the system was kept in an enclosed container with a continuous flow of dry nitrogen above the sample. Conductance Measurements. The magnitudes of the conductances in the concentration rqnge of this work were such that a slight modification of a conventional aqueous type cell can be used. The cell constant was determined with a 0.1 N aqueous solution of KC1. The cell was filled in the drybox and transferred to a constant temperature bath outside the drybox. By nieans of a cell cap containing a capillary connection to the solution and an inlet and outlet for gas flow, a slight nitrogen gas flow was maintained above the cell solution. Measureiiients were made over a concentration range of approxiniately 0.1-1.2 N . The constant temperature bath was filled with a mineral oil, Marcol, and the temperature was maintained within &0.05'. Measurements of the resistance were made a t 1000 and 60 a.p.s. using a Model RCl6 conductivity bridge made by Industrial Instrument co. Viscosities and Densities. The viscosities of the solutions were determined with an Ostwald viscometer. For the measurements with ether solutions, diethyl ether was used as a standard. o-Xylene was used as a standard for the viscosity measurements of the toluene solutions. Densities were measured with a pycnometer using oxylene as a standard.
Results and Discussion Conductance in Toluene. Due to the limited solubility of KaAIEta in toluene, it was necessary to carry out the concentration dependent studies a t an elevated temperature. In Fig. 1, the concentration dependence of the equivalent conductance of NaAIEtr at 88.5' is shown. The order of magnitude of the conductance is very similar to that observed by Kraus and Fuoss6 for concentrated solutions of (i-Am)dNNOa in dioxane. In comparison, the equivalent conductance of a 0.300 N solution of (i-Am)4NnT03in dioxane at 25' was ohm-' equiv.-'; that of a 0.321 reported as 0.1005 N solution of KaA1Et4in toluene at 88.5' was observed to be 0.189 c i i ~ohm-'equiv. .~ -I. The dielectric constants The Journal of Physical Chemistry
M. C. DAY,H. 31. BARNES,AND A. J. Cox
Figure 1. Equivalznt conductance of NaAIEtl in toluene a t 88.5".
,5c
c 4E
A€
A4
60
70
80
t°C
Figure 2. Equivalent conductance of a 0.5 N NaAlEtr solution a8 a function of temperature. ( 8 ) R. M. Fuoss and
C.A. Kraus, J . Am. Chem. Soc., 5 5 , 21 (1933).
2597
CONDUCTANCE OF SODIUM ALUMINUM ALKYLSIN TOLUENE AKD DIETHYL ETHER
3 1.c
