RESPECTED SIR,
WHAT CAN BE THE EXACT DEFINITION OF AROMATICITY IN THE CASE OF POLYCYCLIC COMPOUNDS.? BECAUSE THERE ARE MANY POLYCYCLIC COMPOUNDS WHICH DONOT FOLLOW HUCKLE'S RULE BUT THEN TOO THEY ARE DIFINED AS AROMATIC. for e.g PYRENE.....HOWEVER IN I.L.FINAR I FOUND THAT THE IN SUCH CASES WE CONSIDER ONLY PERIPHERAL ELECTRONS FOR HUCLE RULE....NOW WHAT ARE PERIPHERAL e's.
the exact definition of aromatic compounds, in this case becomes, on the basis of experimental observations, as the compounds which on the basis of structure and theoretical calculation show high degree of unsaturation however when subjected to reactions show saturation to great extent. also these compounds tend to react more in substitution reactions than addition reactions which are charecteristic reaction of unsaturated cyclic compounds.
for more details, refer to the chapter 'aromaticity' in morrison boyd . in case of pyrene the lone pair of electrons is also considered for calculation in the huckle rule. by peripheral electrons, it probably means the electrons present in the pie bonds as well as the lone pair of electrons present in the member other than carbon
CYCLIC CONJUGATED PLANAR SYSTEM HAVING 4n+2 pie
DELOCALISED ELECTRONS THAT CAN SUSTAIN A RING CURRENT ARE CALLED AROMATIC COMPOUNDS.