<P> Friedel--Crafts acylation involves the acylation of aromatic rings . Typical acylating agents are acyl chlorides . Typical Lewis acid catalysts are acids and aluminium trichloride . Friedel--Crafts acylation is also possible with acid anhydrides . Reaction conditions are similar to the Friedel--Crafts alkylation . This reaction has several advantages over the alkylation reaction . Due to the electron - withdrawing effect of the carbonyl group, the ketone product is always less reactive than the original molecule, so multiple acylations do not occur . Also, there are no carbocation rearrangements, as the acylium ion is stabilized by a resonance structure in which the positive charge is on the oxygen . </P> <P> The viability of the Friedel--Crafts acylation depends on the stability of the acyl chloride reagent . Formyl chloride, for example, is too unstable to be isolated . Thus, synthesis of benzaldehyde via the Friedel--Crafts pathway requires that formyl chloride be synthesized in situ . This is accomplished via the Gattermann - Koch reaction, accomplished by treating benzene with carbon monoxide and hydrogen chloride under high pressure, catalyzed by a mixture of aluminium chloride and cuprous chloride . </P> <P> The reaction proceeds via generation of an acylium center: </P> <P> The reaction is completed by deprotonation of the arenium ion by AlCl, regenerating the AlCl catalyst: </P>

In friedel crafts reaction anhydrous alcl3 is used. its function is to