A remarkable difference in the deprotonation steps of the Friedel–Crafts acylation and alkylation reactions

Friedel–Crafts acylation and alkylation reactions were investigated using density functional theory calculations. The reaction systems studied were (benzene + acetyl chloride + Al2Cl6 (or AlCl3)) and (benzene + 2‐chloropropane + Al2Cl6). In the acylation reaction, the acylium ion intermediate is reached either via a MeC(Cl)OAl2Cl6 complex or via direct Cl transfer: MeC(O)ClAl2Cl6 → MeCO⌉+Al2Cl . The ion adds to benzene electrophilically to form a Wheland intermediate containing a strong CHCl hydrogen bond, which leads to deprotonation and the subsequent formation of acetophenone. The resulting HClAl2Cl6 fragment is subjected to a nucleophilic attack by the carbonyl oxygen of the acetophenone, and recovery of the Al2Cl6 bridge is unlikely. Attack of the Al2Cl6 moiety by MeC(Cl)O gives the complex MeC(Cl)O–AlCl3, whose reactivity toward acylation is similar to that of the MeC(Cl)O–Al2Cl6 complex. In the alkylation reaction, deprotonation does not take place, but rather a [1,2] H‐shift from the Wheland intermediate. The resulting α‐protonated cumene undergoes deprotonation, with subsequent recovery of the Al2Cl6 bridge. In addition, the Al2Cl6‐catalyzed isomerization of the n‐propyl to the isopropyl cation was found to be a dyotropic shift.