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I would like to know what is the function of nitrobenzene as a solvent in Friedel Crafts alkylation reaction.

Answer to this question suggests that Friedel Crafts reaction is not possible with nitrobenzene and I understand that it's because of the big loss of electron density of the benzene ring due to the highly deactivating nature of $\ce{-NO2}$

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The aluminium halide complexes with nitro compounds (e.g., nitrobenzene) are known to display catalytic properties in some organic reactions, including that in Friedel craft alkylations. Those complexes are believed to be more soluble in the solvent (nitrobenzene).

Reference: Russian Journal of Coordination Chemistry 2001, 27(7), 469–475.

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    $\begingroup$ Although I agree solubility of the $\ce{AlCl3}$ is good in nitrobenzene (they sell solutions of this), perhaps due to the formation of these complexes, I wouldn't go as far as saying that the complexes are catalytic. The referenced paper cites two references to support this claim, none of which actually say this (one doesn't even use nitrobenzene). If you have another references that shows this catalytic activity, by all means, would be very interesting to read! $\endgroup$ – ralk912 Mar 20 '18 at 7:17
  • $\begingroup$ @ralk912: I'm sorry for the late reply but like to verify my comment. The Russian group is talking about complexes of aluminium halide with nitrocompounds as a whole. They claim N...O...Al complexation. I didn't go through whole paper so I wasn't sure about how specific is this claimed catalytic activity. Yet, I'm thinking to do little literature survey and put them here if I found solid evidence. Thanks for your interest. $\endgroup$ – Mathew Mahindaratne Mar 20 '18 at 15:19
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Without any more information, it's just that, a solvent. Since it is a poor substrate for Friedel Crafts alkylations, it can be considered inert in these conditions. The choice of nitrobenzene would probably be to carry out the reaction at a relatively high temperature, since it boils at 210.9 °C.

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