[acrolein]

I started watching the heterocyclic chemistry lectures of Scripps on youtube as of yesterday, and cannot get around
with the first Problem of the Day https://www.youtube.com/watch?v=H7ZiI8IlUK0&t=1473s.

The premise is how the meta bromination of pyridine occurs with fuming sulfuric acid. Why would the protonation (or addition of SO3) enables the addition? In the book which Baran has published, there is a video which he says addition of SO3 quenches the electron-withdrawing character of nitrogen which I cannot get around with.

I have found the reaction on the original paper that describes the transformation and Heterocyclic Chemistry by Joule and Mills but neither suggest a reason of why this would happen.

Thank you in advance for explaining!

[rolnor]

Maybe the SO3 in the acid forms some very reactive brominating species with bromine?
If the electron-withdrawing properties of nitrogen changes it should be possible to see a shift in NMR?