# Why use glacial acetic acid in bromination of anisole?

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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[...] 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.

Edit3

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