Aluminum chloride catalyst promoted with water

Aluminum chloride. The reaction of aluminum chloride with water is, to some extent, similar to the one of aluminum bromide namely, the more water added, the less hydrogen chloride liberated. With a 1 molal ratio of water to aluminum chloride, 0.71 mole hydrogen chloride was formed when the water-aluminum chloride ratio was increased to 2, only 0.29 mole hydrogen chloride was generated per mole aluminum chloride reacted (Table XI). The catalysts produced from the actioii of water on aluminum chloride were less active than those obtained from aluminum bromide and therefore the isomerization experiments were carried out at 80-100°. Unlike aluminum chloride, catalysts produced by the action of water on aluminum chloride did not require the presence of olefins or hydrogen chloride to promote the isomerization of 7i-butane. 

Before the war, using one of the earliest samples of tritium from the Radiation Laboratory c.t Berkeley, he found the rate of isomerization of n-butane to isobutane, over aluminum chloride promoted with water, was proportional to the rate of exchange of hydrogens between the hydrocarbon and a catalyst promoted with tritiated water. This observation may have been part of the stimulus for the more detailed studies with isotopic tracers of acid catalysis after the war. 

The use of additional substances to increase the activity of a catalyst is a well known phenomenon. Hydrogen chloride or traces of water are known to promote aluminum chloride catalyzed reactions. In the same way the reaction of isoparaffins with olefins has been shown to be catalyzed by boron trifluoride in the presence of nickel powder and with water as the promoter (Ipatieff and Grosse, 76). Hydrogen fluoride can take the place of the water and thus serve as the promoter. 

Formation of the initial carbocation depends on the catalyst used (62). Pure cyclohexane, for example, does not undergo isomerization upon contact with pure aluminum chloride, even at refluxing temperature. However, isomerization does occur in the presence of a trace of water. HCI-AICI3 and HBr-AlBra were also demonstrated by careful studies to exhibit isomerization activity only in the presence of traces of alkene or alkyl halide promoters. The added alkene serves as a source of the intermediate carbocations, thereby initiating the chain reaction via hydride transfer. The effect of water in carbocation formation is attributed to its ability to generate the strong acid H[Al(OH)Hlgs]. 

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