SN1 Reaction

Pharmaceutical Organic Chemistry

1. MR.DHIRAJKUMAR S.NIKAM ASST.PROFESOR Pharmaceutical Chemistry Bharati Vidyapeeth College of Pharmacy Navi Mumbai India ‹#› SN1 Reaction

2. S N 1 Reaction Contents : Features of S N 1 reaction S N 1 Mechanism Stereochemistry Carbocation stability Carbocation rearrangement Factors affecting S N 1 reaction 2

3. S N 1 Reaction Features: Follows first order kinetics Racemization Non- stereospecific Carbocation rearrangement Reactivity sequence of substrate 3 o > 2 o > 1 o > CH 3 W 3

4. S N 1 Reaction Mechanism 4 Example Rate= k[ RBr ] follows first order kinetics Mechanism

5. S N 1 S tereochemistry Step 1 5 Step 2

6. S N 1 Reaction Mechanism 6 Energy diagram

7. Carbocation stability 7 Structure of carbocation : Trigonal Planar Stability of carbonations

8. Carbocation stability Three factors affects carbocation stability: Inductive effect Hyper conjugation effect Resonance 8

9. Carbocation stability 9 Stability order Resonance

10. Carbocation Rearrangement 10 Alkyl Shift Hydride Shift

11. Carbocation Rearrangement 11 Alkyl shift ring expansion Aryl Shift

12. Factors affecting S N 1 reaction All carbocations are very good electrophiles and even weak nucleophile will react quickly with them. e.g. H 2 O and CH 3 OH 12 S N 1 relative reactivity R 3 CX >R 2 CHX > RCH 2 X > CH 3 3 o 2 o 1 o Methyl Relative rate >10 6 1 < 10 -4 < 10 -5 I. Nature of substrate II. Nature of nucleophile

13. Factors affecting S N 1 reaction 13 III. Effect of leaving group IV. Solvent effect

14. Factors affecting S N 1 reaction 14 V. Order of leaving group V. Order of polar protic solvents

15. Summary S N 1 reaction 15 Overall summary Kinetics Follow first order kinetics Rearrangement Undergoes carbocation rearrangement Product 50 % Inversion 50 % Retention Forms racemic mixture Nucleophile Unimportant, any weak nucleophile can perform reaction Leaving group Important , requires best leaving group Solvents Important , requires polar protic solvent

16. References Organic Chemistry by R.T. Morrison and R. N. Boyd, 7 th edition. Organic Chemistry by Pine, Stanley H.; Hendrickson, James B.; Cram, Donald J.; Hammond, George S., 5 th edition. 16