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On catalysts for Aromatic Halogenation and Friedel Craft Reactions

On catalysts for Aromatic Halogenation and Friedel Craft Reactions - Chemistry Forum

On catalysts for Aromatic Halogenation and Friedel Craft Reactions - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 07-28-2003, 09:53 PM
Pedro
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Default On catalysts for Aromatic Halogenation and Friedel Craft Reactions



Hello to all,
Working through the chapter on aromatic compounds I found that the
catalyst used in the halogenation is normally FeBr3 and/or FeCl3.
Friedel Craft reactions on the other hand specify AlCl3 as the
catalyst. On several books I checked these Lewis acids are associated
with those specific reactions. Is there any reason why FeBr3 would not
work on a Friedel-Craft reaction or AlCl3 would not work on a
halogenation?
Thanks in advanced for your help!

Pedro
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  #2  
Old 07-29-2003, 02:38 AM
Joćo Antonio
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Default On catalysts for Aromatic Halogenation and Friedel Craft Reactions

theoretically, what you need is a Lewis acid, i.e., something that will
form a complex adding one halide thus rendering the reactant more reactive
(!?!)

eg: Br2 + FeBr3 -> "FeBr4(1-) + Br(1+)"
RX + AlCl3 -> "AlCl3X(1-) + R(+)"

the quotes meaning it's just an aproximated approach to what is really going
on.
Depending upon the book you are reading, it may give different examples,
like
phosphorus halides, HF, transition metal oxides, etc...

Solomons (5th ed) mentions AlCl3, BF3 and PCl5 for friedel-crafts and FeCl3
or FeBr3
for halogenation

Streitwieser, Heathcock and Kosower (4th ed) mention AlCl3, FeCl3, BF3 and
ZnCl2
for friedel-crafts and FeBr3, AlCl3 and protic acids for halogenation.

This last book has a good explanation of the reaction profile and the
function of the catalyst,
you may find it at an university library.


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  #3  
Old 07-29-2003, 06:24 AM
Eric Lucas
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Default On catalysts for Aromatic Halogenation and Friedel Craft Reactions

Ferric salts are generally not strong enough Lewis acids to promote
Friedl-Crafts reactions. Also, FeCl3 is a radical, and will probably
promote radical halogenations (for example, of benzylic positions). Often
you're trying to get away from radical halogenations when you use a Lewis
acid catalyst for aromatic halogenation.

As for AlCl3, it's is way, way more than is needed to promote an
electrophilic halogenation. It'll probably work, but there are practical
problems, like its extreme sensitivity to water and the fact that, in my
experience, it tends to catalyze rearrangements of both the incoming
halogen, and the other groups on the aromatic ring to start with. I also
believe it is a rather unselective catalyst, and is difficult to get
selective mono-halogenation. If you want the polyhalo benzene, and if you
aren't worried about rearrangements (or if you actually want them), AlCl3
might work just fine.

Eric Lucas

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aromatic , catalysts , craft , friedel , halogenation , reactions


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