In the mechanism for the acetylation of bromobenzene, how is $\ce{HCl}$ gas produced?

The reactants are acetic anhydride, bromobenzene, $\ce{AlCl3}$ and dichloromethane.

Initially, 4-bromoacetophenone is made, accompanied -by as far as I know - with acetic acid and the regeneration of $\ce{AlCl3}$.

How then is $\ce{HCl}$ produced?

To remove the $\ce{AlCl3}$ at the end, 3 $\ce{H2O}$ are added and $\ce{HCl}$ gas is produced then.

I'm trying to think of ways that $\ce{HCl}$ gas is produced prior to that, however. Could a chlorine of the aluminate ion deprotonate the arenium ion?

Could dichloromethane (solvent) donate a $\ce{Cl-}$ to $\ce{AlCl3}$?

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    $\begingroup$ I suspect your suggestion is correct. $\ce{[AlCl3(OOCCH3)]-}$ loses a chlorine instead of the acetate when deprotonating arenium ion to form $\ce{HCl}$ as a byproduct $\endgroup$ – bon Feb 2 '15 at 18:40

You could perform the electrophilic aromatic substitution with acetyl chloride as well instead of acetic anhydride - in the first case it is evident where HCl comes from. However, the mechanism is similar if you use acetic anhydride. The first step consists in the electrophile activation by AlCl3, which generates in situ the "acylium" ion - the actual acylating agent. The second step is the electrophilic aromatic substitution, in which the electrophile is preferentially "attacked" by the aromatic ring at the ortho or para position (ortho- and para-orienting effect of halides). The driving force for the reaction is the last step which leads to restore the aromaticity and it is assisted by (AcO)(Al)Cl2 with liberation of HCl.

Here I have drawn the mechanism.

enter image description here

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  • $\begingroup$ I have a question..Are you sure $AlCl_3$ doesn't attack on the middle oxygen? $\endgroup$ – user35508 Dec 6 '18 at 11:17

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