[a student]

hi all
as you know when you have an electrophile and a pyridine the electrophile will attack the 3 or 5 position of the pyridine ring but sometimes the lone pair of pyridine can attack the electrophile. I don't know how I should recognize that which electrophiles will attack the 3 or 5 positions and which one will be attacked with the lone pair of pyridine?
every feed back will be appreciated

[KritikalMass]

Please correct me if I am mistaken but think that the lone pair on the nitrogen in pyridine allows pyridine to act as a good nucleophile. Additionally, the lone pair on the nitrogen is not delocalized into the ring. As a result of this, the electrophile will always preferentially be added to the nitrogen to form the pyridinium ion.

So it's not really a matter of which electrophiles will add to the ring, and which will add to the nitrogen- both events happen so you end up with a mixture of products with the major product being the electrophile added to the nitrogen.

http://users.ox.ac.uk/~mwalter/web_06/year2/arom2/hetarom_rxn_mech.shtml

Please correct me if I am mistaken on this issue.

[nj_bartel]

I'm pretty sure EAS reactions on pyridine are typically done under pretty rough conditions (couple hundred + oC under pressure, conc. acid, etc.) to overcome the inherent difficulties (still with pretty poor yield).  I'd expect the conc. acid to tie up the basic N-LP, and I'd expect substitution to take place on 3/5 due to higher e- density than 2/6 and better resonance structures than on 4.