Answer to Question #185849 in Organic Chemistry for sadaf

Question

               

HOW IS THE DELOCALISED PI ELECTRON SYSTEM WITHIN THE BENZENE RING AFFECTS BY EACH REACTION?

Friedel-Crafts reaction and Hydrogenation benzene?

Answer 1

If the pi electrons were not delocalized, the benzene ring would be an irregular hexagon with 3 short double bonds and three relatively longer single bonds. This structure would be highly strained and easily susceptible to rupture and attack by various reagents, even the dilute halogen acids which add to the double bonds in linear alkenes.

Delocalization however makes all bonds in the ring equal and thus indistinguishable. The bonds in the real benzene ring are not exactly double or single bonds but in between, a resonant structure. This removes any strain and instead makes the benzene ring especially stable. It also becomes resistant to attack by agents which easily operate on multiple bonds.

Friedel-Crafts fails when used with compounds such as nitrobenzene and other strong deactivating systems. ... This places a positive charge next to the benzene ring, which is so strongly activating that the Friedel-Crafts reaction cannot occur. Lastly, Friedel-Crafts alkylation can undergo polyalkylation.