Aluminum chloride-nitrobenzene mixtures

John J. Eisch; Yun Qian
State University of New York, Binghamton

Chemical & Engineering News, (9 Feb 1998) Vol. 76, No. 6, pp. 2.

The frequency with which nitrobenzene is employed as a solvent for aluminum chloride-promoted organic reactions may give the reasonable impression that such mixtures are without hazard. However, a recent incident in our laboratory demonstrates that the admixing of nitrobenzene with aluminum chloride in the presence of diphenylacetylene can lead to a violent reaction.

The following experiment was carried out on a 10-mM scale. A dry, darkened 1:1 mixture of diphenylacetylene and sublimed aluminum chloride that was being magnetically stirred was to be dissolved in 2 mL of anhydrous and degassed nitrobenzene in a Schlenk flask maintained under an argon atmosphere at 25 C. Upon introduction of the first drop of nitrobenzene from a microsyringe onto the powdery solid, a violent, gas-generating exothermic reaction occurred that spewed and forced black solids throughout the double-manifold argon distribution system. Hydrolytic workup of the reaction residue showed that it was composed largely of carbon and other black organic matter. Such products and the sudden gas evolution indicate that a rapid redox reaction took place.

The interaction of aluminum chloride with nitrobenzene to form a 1:1 complex is known to occur cleanly and exothermically, but it has long been recognized that such mixtures are thermally unstable and may lead to explosive decomposition, especially in the presence of a third substance, such as phenol or hexamethyltetralin (Bretherick, L. "Reactive Chemical Hazards" 4th ed., Butterworths, 1990).

In fact, the admixing of aluminum chloride with nitrobenzene containing 5% phenol has caused a rise in temperature and a violent explosion. Since this particular mixing step was done in a 500-gal still, the physical damage was extensive. Subsequent tests done on a smaller scale showed that heating a mixture of aluminum chloride and nitrobenzene or a mixture of nitrobenzene and phenol at 150 C produced no reaction. In contrast, heating a mixture of all three components caused a violent reaction and considerable tar formation (C&EN, Nov. 23, 1953, page 4915). These observations underline the decisive kinetic role of the third component in unleashing the potentially explosive character of aluminum chloride-nitrobenzene mixtures.

Our recent observations demonstrate that even an apparently innocuous third component, such as diphenylacetylene, can act as the explosive trigger for such mixtures. The foregoing reports, along with our own observations, should serve as a warning to any chemist who undertakes to dissolve aluminum chloride in nitrobenzene in the presence of an oxidizable organic component. Any such dissolution process should always be conducted initially on a small scale and with proper shielding in the event of an explosion.

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