Question

When performing a Friedel-Crafts alkylation reaction, not only does one have the problems of multiple alkylation and rearrangement of the cation, but the presence of strong electron donating groups (yes, donating is correct) prevents reaction. Strong electron withdrawing groups also prevent reaction. Provide explanations for these facts.

Answer 1

Strong electron withdrawing groups prevents Friedel Crafts reaction because when a withdrawing group attracts the electrons decrease the availability of the electronic pair and the enough electronic density of it in order to make possible the aromatic electrophilic substitution.

Strong electron donating groups like NH2 doesn’t undergo Friedel Crafts reaction because NH2 is a Lewis base that means an electron donor. Due to Friedel Crafts reaction is an acid base reaction, the AlCl3 will be attacked by the lone pair available from NH2 producing a salt. The same occurs with phenol because the lone pair of electrons available in the OH group so Friedels Crafts doesn’t undergo with benzene attached to the strong electron donating groups NH2 or OH.