Disproportionation (intermolecular isomerization) of fluoro-, chloro-, and bromobenzene was investigated through the effect of water-promoted aluminum chloride and bromide. Investigations were carried out by the use of a gas-liquid chromatographic analytical method. It was observed that ring-bonded fluorine does not undergo intermolecular migration. The decreasing order of activity of halobenzenes in isomerization was found to be Br » Cl. The reaction mechanism of the intermolecular isomerization (disproportionation) of halobenzenes in terms of sigma complex type intermediate state formation is discussed as a consequence of the results obtained. The relative basicity of the halobenzenes determined from spectroscopic data is in agreement with this mechanism. Further proof of the suggested mechanism was obtained by using heavy water as the promoter. Halogen exchange and electrophilic arylations by the halobenzenes accompany the aluminum halide-induced isomerization of halobenzenes. In these reactions the relative order of activity of the halobenzenes was found to be F > Cl > Br, To explain the reversed order of activity as compared to that observed in isomerization and to account for the mechanism, intermediate complexing is suggested through the unshared pairs of the halogens to the acid catalyst (n-complex formation). Evidence for n-complexing of fluorobenzenes with aluminum bromide was obtained by infrared and high resolution nuclear magnetic resonance spectroscopy. Conductivity measurements were also in accordance with the proposed complex formation. Further proof of the n-complex mechanism was obtained by the chlorine exchange occurring between chlorobenzene and AbCV6.

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Individual peaks were identified by the retention times of pure halobenzenes and dihalobenzene isomers. Identification of separated fractions in a number of preparative scale runs waa made by infrared spectroscopy. Components were separated from the reaction mixture using a 3 m. X 1 in. aluminum preparative-scale column packed with polypropylene glycol supported on diatomaceous earth. Samples of up to 5 ml. were separated by this method.Acknowledgment.-The authors are indebted to D~. E. B, ~~k~~ for the n.m.r, andThe water-promoted, aluminum chloride-catalyzed isomerization of chlorofluorobenzenes takes place by an intramolecularThe equilibrium isomer mixture is composed of 470 ortho, 64% meta, and 327, paraIsomerization of ortho-chlorofluorobenzene is much slower than that of the meta and para isomers Halogen exchange and electrophilic arylation in this case overtake Isomerization of dichlorobenzenes also takes The equilibDifluorobenzenes are Halogen exchange and electrophilic arylation are the only reactions taking 1,Zshift of the chlorine substituent. chlorofluorobenzene. (because of the ortho deactivating effect of fluorine). isomerization, so that equilibrium cannot be reached from the ortho isomer.place by a predominantly intramolxular 1,2-shift mechanism (disproportionation is generally less than 1 %). rium mixture for dichlorobenzene was found to contain 8% ortho, 60% meta, and 32% para isomer. unable to undergo Friedel-Crafts isomerization. place. The mechanism of the investigated reactions is discussed based on the obtained experimental data.

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