Reaction Friday: Friedel Crafts Acylation

by James

in Organic Chemistry 2, Organic Reactions

Today’s Reaction Friday is about the Friedel Crafts acylation of benzene and other aromatics, an important example of electrophilic aromatic substitution.


One thing I forgot to say at the bottom is that there’s several different ways of showing how the acyl halide is activated by Lewis acid – the convention I drew is common, but your textbook may vary. Also, for the final deprotonation, it’s also possible to show Cl(-) as the base to remove the proton from the carbocation, rather than the Fe-Cl bond.

 

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{ 10 comments… read them below or add one }

Manjunath Channapur

can we know the catalyst molar ration and the exact electronic enviornment of the benzene ring substrte

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james

This is more of a general introduction: on a practical basis, factors like catalyst ratio are going to be dependent on the exact substrate you’re using. I’d consult the references to Organic Syntheses contained in March’s Advanced Organic Chemistry.

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Med School Odyssey

I was reviewing Friedel-Crafts acylation this weekend actually and the book I was reading through made the point that H2O needed to be added afterward because the Lewis acid would wind up accepting electrons from the carbonyl if you didn’t. Is that a common step?

Also, if you do this in water, would the product actually be an enol?

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james

It’s pretty standard procedure in organic chemistry to follow a reaction with an aqueous workup to remove salts, water-soluble byproducts, etc. so it’s kind of a moot point.

You wouldn’t want to do this in water with FeCl3 or AlCl3 because those Lewis acids will hydrolyze in that solvent – as will (importantly) the acid halide. There are ways to do it in water if absolutely desired, but generally it’s performed in organic solvents. Re: enolization, aromatic ketones will favor the enol form more than non-aromatic ketones, but the keto form still outnumbers the enol by at least (I’d guess) about 1000:1 or therabouts. Water would increase the rate of keto-enol interconversion but would not lead to dominance of the enol over the keto form.

Good questions

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anang

Is the reaction temperature should be at 0 degree Celsius for both benzene and Acyl chloride + FeCl3?

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Aleksey

Just want to add some more info. on Friedel Crafts (F.C.)

To my knowledge:

All Activating groups are Electron Donating Groups (EDG) and are Ortho/Para directors.
Major product is Ortho, unless the Activating group or Electrophile is sterically hindered, then the Para is major product.

All Deactivating groups are Electron Withdrawing Groups (EWG) and are Para directors, except for Halogens. Halogens are Ortho and Para directors too, b/c Halogens can push electron density w/ resonance (lone pairs), making benzene rich in electrons at Ortho and Para positions.

All meta directing groups kill F.C. So N.R. Like an ex. in a video with NO2 group.

Resonance explains why Ortho, Para, or Meta attacks happen.

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ewok

What are some reasons why acetylferrocene predominated in a Friedel Crafts reaction over ferrocene and diacetylferrocene? Some possible ways are that it was forced to completion (ferrocene) and that the tube wasn’t oven-dried (though I don’t see how the second is applicable).

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sarah

what happens when you add 3 equivalent of aluminum chloride and 3 equivalent of acyl chloride on a benzene ring? do you pretty much form a benzene ring with three ketones attached two parra to each other and one ortho?

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Dan

Under the normal conditions for this reaction which don’t involve any heating if you got a tri-substituted (or even a di-substituted) product at all it would be a minor product. If you managed to carry out three acylation reactions in succession though I’m sure they’d most likely be meta to each other.

PS. I love this reaction, even on thiophenes with activating groups on 3 and 4 positions I’ve still not got 2,5-polyacylation carrying out the reaction at ~ 10 degrees C

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momo

I am very glad that I came across your website for organic chem. I am studying it after a very long time and your mechanisms help me understand the rxns better.

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