text: It is common in E1 and S1 reactions for a poor leaving group to be transformed into a good one by protonation or complexation with a Lewis acid. Thus, it is by protonation before departure that a molecule can formally lose such poor leaving groups as hydroxide.
  The same principle is at work in the Friedel-Crafts reaction. Here, a strong Lewis acid is required to generate either a carbocation from an alkyl halide in the Friedel-Crafts alkylation reaction or an acylium ion from an acyl halide.
  In the vast majority of cases, reactions that involve leaving group activation generate a cation in a separate step, before either nucleophilic attack or elimination. For example, S1 and E1 reactions may involve an activation step, whereas S2 and E2 reactions generally do not.