[monkey]

Can anyone help me out?
I was wondering if this step is Electrophillic Aromatic Substitution. Can NEt4Br act like FeBr3 in EAS. Also isn't NEt4Br an ionic bond? Does that affect EAS?

Here is the step:


Thanks in advance!

[agrobert]

NEt₄Br is a stabilized ionic organic soluble source of Br^-.  This is EAS but I don't know if NEt₄Br is necessary (possibly accelerates reaction).  It is not a Lewis Acid like FeBr₃ so you couldn't do similar Friedal-Kraft reactions.

[xc]

Yes, it's electrophylic aromatic substitution but, as agrobert said, NEt₄Br cannot play the role of FeCl₃, since it's not a Lewis acid.

The role of NEt₄Br is to form, by reaction with Br₂, NEt₄Br₃, which is a well known brominating agent. This reagent can be isolated in form of orange crystals, and is much easier to handle than liquid Br₂ (though in this case, since it's prepared in situ, liquid bromine is used anyway).

I think the reason to use it in this example is to obtain a good regioselectivity. In the molecule of the example, the selectivity for EAS is in ortho and para positions (refered to the amine) and the use of a Lewis acid such as FeCl₃ would direct the reaction preferably to the ortho positions, due to complexation of the Fe with the nitrogen.

In the other side, NEt₄Br₃ is known to have selectivity to the para position. Also, it's a mild brominating agent, so it just forms the monobrominated product.