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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