Passage: Upon heating a primary amine with the N-2,4-dinitrophenyl-pyridinium salt (2), the addition of the amine leads to the opening of the pyridinium ring.  A second addition of amine leads to the displacement of 2,4-dinitroaniline (5) and formation of the König salt  (6a and 6b).  The trans-cis-trans isomer of the König salt (6a) can react by either sigmatropic rearrangement or nucleophilic addition of a zwitterionic intermediate to give cyclized intermediate (7).  This has been suggested to be the rate-determining step.  After proton transfer and amine elimination, the desired pyridinium ion (9) is formed.

Question: What happens when you add a primary amine?
the opening of the pyridinium ring