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Why is AlCl3 or FeCl3 might be preferred for chlorination reactions and AlBr3 or FeBr3 for bromination reactions? Why not use AlCl3 for bromination reactions?

Source: https://www.masterorganicchemistry.com/2011/07/22/reagent-friday-aluminum-chloride-alcl3/

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  • $\begingroup$ Can you elaborate on your answer? $\endgroup$ – Jonathan Smith Feb 1 '18 at 4:39
  • $\begingroup$ This does not explain why FeBr3 would be a better catalyst for bromination reactions over FeCl3. $\endgroup$ – Jonathan Smith Feb 1 '18 at 14:39
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    $\begingroup$ yes, but by your logic, FeCl3 would be a better catalyst than FeBr3 for a bromination reaction, or any type of EAS reaction for that matter, which is not the case. $\endgroup$ – Jonathan Smith Feb 1 '18 at 14:47
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In Electrophilic Aromatic Substitution reaction (EAS), the choice of catalyst may influence the yield of the desired product.

First of all, note the following data:

Bond dissociation energy (in kJ/mol) -

Al–Br : 429.2

Al–Cl : 502

As the bond dissociation energy for Al–Cl bond is greater, it must be stronger.

Now, consider using aluminium bromide as catalyst for an EAS chlorination. In the first step, i.e. interaction of chlorine molecule with Lewis acid aluminium bromide, one can't possibly neglect the formation of bromonium ion which may act as electrophile - decreasing yield of chlorobenzene and giving unnecessary side product of bromobenzene. This possibility is supported by thermodynamics, as shown by the data mentioned above.

To make sure that yield of product is maximum and no unnecessary side reactions occur - aluminium chloride catalyst is employed for chlorination and aluminium bromide for bromination.

Hope this answers your question.

P.S. Correction to @AvnishKabaj's answer: If I had to give a reason it would be that when the AlCl3-Br2 complex is made there might be a teeny tiny chance that instead of the bromine breaking away the chlorine breaks away.

As per the bond energy data I have, from this exhaustive table ;it is easy to conclude that the bond strength of Al–Cl bond is greater than Al–Br bond. Hence, problems would arise when chlorination is done in presence of aluminium bromide catalyst. In the AlBr3-Cl2 complex, there'd be a teeny tiny chance that instead of the chlorine breaking away, the bromine breaks away. (based on thermodynamic favourability)

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    $\begingroup$ Oh! Yes! Also I just wanted to point out that aluminium bromide for chlorination is a bigger problem than using aluminium chloride for bromination; as I've mentioned in my edit in the postscript. $\endgroup$ – arya_stark Feb 3 '18 at 9:09
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    $\begingroup$ Why are you comparing using Al-Cl bond strength? Mind you, the chlorine atom that is abstracted from the complex is the terminal chlorine atom, which is attached to the central chlorine atom in the complex. $\endgroup$ – Tan Yong Boon Feb 4 '18 at 4:35
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Honestly speaking I think there won't be a difference as in clayden they have used $\ce{AlCl3}$ for bromination enter image description here

If I had to give a reason it would be that when the $\ce{ AlCl3-Br2}$ complex is made there might be a teeny tiny chance that instead of bromine breaking away the chlorine breaks away.

My basis of saying that the probability of chlorine breaking away is low is because there's a +tive charge on $\ce{Br}$ in the complex.Chlorine on the other hand is free from any such effects. There are no factors supporting the breakage of the $\ce{Al-Cl}$ bond. So the chances of $\ce{Br-Br}$ bond breaking are higher than the $\ce{Al-Cl}$ bond.

The same reasoning can be applied for $\ce{AlBr3}$ with $\ce{Cl2}$

That's why in Master Organic they've written preferred not necessary.

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