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In halogenation reactions of benzene, $\ce{AlCl3}$ is used as a catalyst in order to form electrophile $\ce{E^+}$.

So I was wondering that $\ce{AlCl3}$ is a Lewis acid (that fact being also used in mechanism) and so an electrophile. Why it can't just do electrophilic substitution reaction on the benzene itself? Isn't it strong enough or any other problem there doing that?

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    $\begingroup$ The bonds in $\ce{AlCl3}$ are not ionic. Solid $\ce{AlCl3}$ is made of three covalent bonds $\ce{Al-Cl}$ around the aluminum atom. This does not respect the octet rule. This requirement may be obtained by making a dimer $\ce{Al2Cl6}$. Anyway, all $\ce{AlCl3}$ wants to do is getting and attaching a chloride $\ce{Cl-}$ ion by a covalent bond. The result would be a $\ce{AlCl4^-}$ ion, where $\ce{Al}$ is surrounded by 4 doublets, and so it respects the octet rule. Why and how should $\ce{AlCl3}$ attack the benzene ring ? $\endgroup$
    – Maurice
    May 7, 2021 at 19:47
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    $\begingroup$ @Maurice OP's point is entirely valid. AlCl3 should be in principle able to bind to rings, either via sigma or pi-complex. Probably it does to some extent, but 1) dimerisation you mentioned can make it harder 2) AlCl3 is a hard acid and arenes are soft 3) the binding doesn't make products, perhaps because resulting Al-C bond is too fragile? I'm not sure about details, but it's rather interesting. $\endgroup$
    – Mithoron
    May 7, 2021 at 22:56

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The reaction wouldn't happen because you forget that $\ce{Cl2}$ is also present in the solution (because $\ce{AlCl3}$ is a strong lewis acid). An acid-base reaction is way faster than an aromatic electrophilic substitution.

Once the acid base has happened I doubt that $\ce{AlCl4-}$ would still behave as an electrophile.

Footnote: $\ce{AlCl3}$ is a strong lewis acid, this is why we don't conduct friedel-craft reaction in water which is why we always see anhydrous $\ce{AlCl3}$ mentioned in this reaction. Most commonly preferred media is $\ce{CS2}$ .

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    $\begingroup$ +1 , Thanks a lot ; but I didn't get this "So even in case of absence of Cl2 this won't work" , Can you kindly explain this case briefly ? $\endgroup$ Jun 28, 2021 at 17:48
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    $\begingroup$ Would also like to mention that " Anhydrous" AlCl3 is generally mentioned with the reaction. $\endgroup$ Jun 28, 2021 at 17:51
  • $\begingroup$ @YashAgrawal yes you are right, I had overseen that we can't use water in friedel-craft reaction. I have made an edit in response to that $\endgroup$ Jun 28, 2021 at 18:20
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The OP asks why doesn't AlCl$_3$ react with benzene - without Cl$_2$!

So, what does aluminum chloride do in benzene? It is slightly soluble in benzene, forming an oily compound (Ref 1, 2). So if you mix AlCl$_3$ with benzene, the "reaction" just goes to a complex and awaits further reactant. Perhaps the Cl atom on an alkyl chloride or on a Cl$_2$ molecule is more electron-donating than the benzene ring and favors formation of an alkyl cation in some form.

C$_6$H$_6$ + AlCl$_3$ + RCl (or Cl-Cl) --> C$_6$H$_6$ + RAlCl$_4$ (or Cl$^+$ + AlCl$_4$$^-$)

A quick way to see that the reaction of benzene with AlCl$_3$ has nothing going for it, is to imagine that AlCl$_3$ (or Al$_2$Cl$_6$) does react with C$_6$H$_6$. I could postulate two possibilities:

  1. C$_6$H$_6$ + AlCl$_3$ --> C$_6$H$_5$AlCl$_2$ + HCl or
  1. C$_6$H$_6$ + AlCl$_3$ --> C$_6$H$_5$AlCl$_2$H$^+$ + Cl$^-$

When you look at the first equation, you see immediately that HCl is going to attack the phenyl-aluminum bond to regenerate the original reactants.

When you look at the second equation, you see that the complex cation is so acidic that it will either rearrange to form the original reactants, or will protonate some other phenyl group. The possibility that the chloride anion could lie above the phenyl-aluminum bond and preferentially attack the carbon to yield chlorobenzene is just preposterous.

Ref 1. https://pubs.rsc.org/-/content/articlelanding/1951/tf/tf9514701287/unauth#!divAbstract

Ref 2. https://en.wikipedia.org/wiki/Aluminium_chloride

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  • $\begingroup$ Thanks! , I think I got why AlCl3 wouldn't react with benzene alone as two possibilities you mentioned are pretty thermodynamically unfavourable. $\endgroup$ Jul 1, 2021 at 17:42

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