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 with (as far as I know) 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 $\ce{AlCl3}$?

  • 1
    $\begingroup$ 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 $\ce{HCl}$ comes from. However, the mechanism is similar if you use acetic anhydride. The first step consists in the electrophile activation by $\ce{AlCl3}$, which generates in situ the "acylium" ion ($\ce{H3C-C#O+}$) - 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 $\ce{(AcO)(Al)Cl2}$ with liberation of $\ce{HCl}$.

Here I have drawn the mechanism:

Friedel-Crafts Acylation - Mechanism

  • 1
    $\begingroup$ 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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