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In my organic chemistry lab (next week, the experiment has not been conducted yet) we are being asked to react anisole with bromine to get 2-bromoanisole (as well as 2,4-dibromoanisole and all the others). In our lab instructions we are told to add anisole, bromine and glacial acetic acid to our mixture. No mention of other compounds (such as $\ce{FeBr3}$) was made. I was under the impression that bromine is not reactive enough to brominate an aromatic without $\ce{FeBr3}$. What purpose does the acid serve, and how does it make this reaction work without the iron compound?

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up vote 8 down vote accepted

[...] bromine is not reactive enough to brominate an aromatic without FeBr3

That's true for benzene, but the methoxy-group in anisole isn't just o/p-directing. It is activating as well. Actually, it's that much activating that anisole will react with bromine even without the presence of a Lewis acid.

So, why glacial acetic acid? It's a polar solvent and thus nice to stabilize the Wheland intermediates. Moreover, it's easily removed in aqueous workup.

Edit 1 Does the acetic acid react with the (mostly undissociated) acetic acid? At least a polarization of bromine ($\ce{Br^{\delta+}-Br^{\delta-}...HOAc}$) is conceivable and I remember that $\ce{BrOAc}$ has been postulated as the electrophilic species.

Edit 2 I remembered some brominations on dimethoxy-substituted hydroxymethylnaphthalines I did back in the days. The reaction was performed at 60 °C in $\ce{CHCl3}$ in the presence of about 3 equivalents of anhydrous sodium acetate. Excess of bromine was reduced by addition of aqueous sodium sulfite. The yield was in the 85% range and I don't find any notes on dibrominated products.


And with excess Bromine you would (theoretically) react all of the anisole?

Pretty much, with 4-bromoanisole as the main product.

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So you don't actually get any intermediate compound like you would with $FeBr_3$? And with excess Bromine you would (theoretically) react all of the anisole? – Dannnno Feb 27 '14 at 0:28
No not polybrominated products - just if the reaction would go to completion – Dannnno Feb 27 '14 at 1:02
Two things: ‘It is activating as too’ (1st paragraph); ‘Does the acetic acid react with the […] acetic acid?’ (edit 1). You might want to clear up what you wrote there. Otherwise +1. – Jan Oct 2 '15 at 21:53

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