The desired amount of aluminum metal, preferably in an alloy such as Aluminum Company of America Alloy 12 (8% Copper), is cut in clean, dry shavings
or chips. These are placed in a glass or steel reaction vessel equipted with: halide inlet and product discharge line, temperature and pressure
gauges, provision for heating and cooling, and a safety blow-out. The catalyst, about 0.1% of anhydrous aluminum chloride (or iodine, or the product
of a previous preparation) is placed in on spot on top of the aluminum and the reaction vessel closed. Air is pumped or flushed out, and methly
chloride gas introduced to a pressure of one atmosphere (or more, depending on the equiptment used). If the reaction fails to start in one hour at
room temperature, the charge is heated to 100C. Once started, the reaction is autocatalytic and highly exothermic. More methyl chloride is added,
and cooling is applied to maintain a suitable rate of reaction and to keep the temperature of the charge below 75C. At higher temperatures, the side
reaction may occur with the formation of gas.
When no more methyl chloride is absorbed, the reaction is completed. The reaction of the aluminum is quantitative, while any iron or copper in the
alloy remains unattacked. Side reactions are negligable. In a typical preparation, about 60 milimoles of aluminum in the form of alloy took up 88.9
milimoles of methyl chloride with the formation of only 0.9 milimole of methane; the non-volitile residue contained no aluminum and 0.4 miliatom of
chlorine; no iron or copper was detected in the distilled product.
The water-white, oily liquid is decanted or vacuum distilled, from the reamining metallic sludge into a suitable closed container.
.......
The product of the methyl chloride reaction is almost exactly an equimolecular mixture of dimethylaluminum chloride and methylaluminum dichloride. No
evidence of either trimethylaluminum or aluminum chloride is obtained on fractional distillation.
.......
When the individual compounds are desired, they may be separated from their mixture by fractional vacuum distillation, or the dimethylaluminum
chloride may be obtained in pure form by heating the mixture with an excess sodium chloride and distilling the dimethyl compound from the monomethyl
addition compound. |
