1. Electrophilic Aromatic Substitution Dr. Marwa Eid 1

2. No addition reactions Aromatic substitution: One of the H atoms is repalecd by some groups. 2

3. 3 3 A Resonance Picture of Bonding in Benzene resonance 6  -electron delocalized over 6 carbon atoms The Stability of Benzene benzene exhibits special stability due to resonance delocalization of  -electrons.

4. 4`j  In chemistry, an electrophile (electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nuclephile.  Because electrophiles accept electrons, they are Lewis acids.

5. 5 Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic.Benzene does not undergo addition reactions like other unsaturated hydrocarbons, because addition would yield a product that is not aromatic. Substitution of a hydrogen keeps the aromatic ring intact.Substitution of a hydrogen keeps the aromatic ring intact.

6. 6 The characteristic reaction of benzene is electrophilic aromatic substitution —a hydrogen atom is replaced by an electrophile.The characteristic reaction of benzene is electrophilic aromatic substitution —a hydrogen atom is replaced by an electrophile.

7. 7 Electrophilic Aromatic Substitution HE+ EY + HY ++++ ––––

8. 8 NitrationSulfonation Halogenation Friedel-Crafts alkylation Friedel-Crafts Acylation Electrophilic Aromatic Substitution

9. 9

10. 10 Nitration and sulfonation introduce two different functional groups into the aromatic ring.Nitration and sulfonation introduce two different functional groups into the aromatic ring. Nitration is especially useful because the nitro group can be reduced to an NH 2 group.Nitration is especially useful because the nitro group can be reduced to an NH 2 group.

11. H 11 + + H2OH2OH2OH2O H 2 SO 4 HONO 2 NO 2 Nitrobenzene

12. H 12 + + H2OH2OH2OH2O heat HOSO 2 OH SO 2 OH Benzenesulfonic acid

13. 13 In halogenation, benzene reacts with Cl 2 or Br 2 in the presence of a Lewis acid catalyst (, such as FeCl 3 or FeBr 3, to give chlorobenzene or bromobenzene, respectively.In halogenation, benzene reacts with Cl 2 or Br 2 in the presence of a Lewis acid catalyst (electron pair acceptor), such as FeCl 3 or FeBr 3, to give chlorobenzene or bromobenzene, respectively. Halogenation reactions with I 2 and F 2 are not synthetically useful because I 2 is too un-reactive and F 2 reacts too violently.

14.  Friedel - Crafts alkylation ArH + RCl/AlCl 3 Ar-R + HCl  Friedel - Crafts acylation ArH + RCOCl/AlCl 3 Ar-COR + HCl

15. H 15 + + HCl AlCl 3 C(CH 3 ) 3 tert-Butylbenzene (CH 3 ) 3 CCl 2-chloro, 2-methyl propane

16. 16 In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl 3 ) forms an alkyl benzene.In Friedel-Crafts alkylation, treatment of benzene with an alkyl halide and a Lewis acid (AlCl 3 ) forms an alkyl benzene. Friedel-Crafts Alkylation of benzene

17. H 17 + + HCl AlCl 3 1-Phenyl-1-propanoneO CH 3 CH 2 CCl CCH 2 CH 3 O

18. 18

19. 19 HH HH HH E+E+E+E+ HH H HHHE +

20. this step restores aromaticity of ring 20

21. 21