Nucleophilic Aromatic Substitution

ppt

1. Department of Chemistry University of kashmir Topic :- Nucleophilic Aromatic Substitution Aadil Ali Wani Roll no: 16062120001 Teacher Incharge Prof. Syed Wajahat Amin Shah

2. Nucleophilic Aromatic Substitution Substitution reactions in which the nucleophile displaces a good leaving group such as halide on an aromatic ring .

3. . Aryl halides are inert to both S N 1 and S N 2 nucleophilic substitutions. Mechanism must be different

4. Mechanism of nucleophilic aromatic substitution S N Ar Mechanism or Addition-Elimination Elimination-Addition or Benzyne Mchanism S N 1 S RN 1 Mechanism Most common is addition-elimination and benzyne mechanism Last two are not very common

5. I. S N Ar Mechanism or Addition-Elimination Mechanism Conditions: There must be an electron withdrawing group at the benzene ring which can stabilise the negative charge Ring must be activated by EWG like NO 2 , CN, CF 3 , COOR, COOH etc

6. Examples:- Pyridine is so much activated towards addition-elimination that it does not require any EWG

7. Proof for this argument comes from the fact that the order of reactivity for halogens is F > Cl > Br > I (and not the reverse of this i.e. I > Br > Cl > F based on their leaving group ability). Stronger bond dipoles associated with the more electronegative atom favour the addition step thus lowering the energy of activation of the nucleophilic addition step (which is r.d.s.).

8. II. Benzyne Mechanism (Addition –Elimination) No activation of the benzene ring by any EWG. Strong base is required. The rate of various aryl halides towards this reaction follows the order I > Br > Cl > F. So addition-elimination pathway is not possible.

9. Benzyne Mechanism (Addition –Elimination) The incoming group doesn’t always take the position vacated by the leaving group. “Triple bond” between sp 2 C p-p overlap sp 2 -sp 2 overlap - weak bond Benzynes unstable / reactive intermed . Slowest step may either be proton abstraction or leaving of leaving group.

10. Regioselectivity in Benzyne Mechanism w.r.t its formation Case 1: When there are ortho and para groups w.r.t the leaving group Case 2: When there are meta groups w.r.t the leaving group

11. Regioselectivity in Benzyne Mechanism w.r.t product formation Case 1: Symmetric benzyne Case 2: Unsymmetric benzyne

12. III. S N 1 MEECHANISM rate depends only on the electrophile and independent on the concentration of nucleophile

13. S N 1 Mechanism Not so common Only occurs when there is a superleaving group on benzene like N 2 and triflates

14. IV. S RN 1 MEECHANISM No activation of aromatic ring so no addition-elimination mechanism If benzyne mechanism then the above products should be formed in equal proportions SN1 not possible because only possible with superleaving groups like N2 So different mechanism called as S RN 1 mechanism

15. IV. S RN 1 MECHANISM Not so common mechanism Only occurs with aryl iodides

16. Thank You