Aryl Halides

Preparation of Aryl Halides

1. By Halogenation :

X2 = Cl2, Br2  ;  Lewis acid = FeCl3, AlCl3, ZnCl2, etc.

2. By Decarboxylation :

Chemical reaction of Aryl halide (Bimolecular nucleophilic substitution SN2 Ar)

  • An electron withdrawing group at ortho or para positions with respect to a good leaving groups are necessary conditions for SN2 Ar.

Mechanism :

Intermediate ion is stabilized by resonance and are stable salts called Meisenheimer salts.

  • A group that withdraws electrons tends to neutralize the negative charge of the ring and this dispersal of the charge stabilizes the carbanion.

G withdraws electrons : stabilizes carbanion, activates the Ar-SN2 reaction.

(–(CH3)3, –NO2, –CN, –SO3H, –COOH, –CHO, –COR, –X)

  • A group that releases electrons tends to intensify the negative charge, destabilizes the carbanion, and thus slows down reaction.

G (–NH2, –OH, –OR, –R) releases electrons : destabilizes carbanion, deactivates  the Ar-SN2 reaction.

Element effect :- The fact that fluoro is the best leaving group among the halogens in most ArSN2 but in SN1 & SN2 mechanism where fluoro is the poorest leaving group among halogens.