1. Organic Chemistry Chapter 6
Reaction Mechanism
- Substitution and Elimination -
Nanoplasmonic Research Group

2. Chemical Reactions in Organic Chemistry
So far, you’ve studied addition reactions
Now, we will consider substitution and elimination reactions
Interestingly, these two are in competition
Under a given situation, you can predict which reaction would occur !!!

3. 1. Substrate (steric, orbital overlap), 2. Nucleophile (strong, weak),
You need carefully consider 4 reaction variables for the prediction !!!!
1. Substrate (steric, orbital overlap), 2. Nucleophile (strong, weak),
3. Leaving group (good, poor),
4. Solvent (protic, aprotic)

4. Nucleophilic Substitution
In a typical reaction, a nucleophile displaces the leaving group binding to a substrate
Nucleophile & Leaving group
Substitution mechanisms
SN1 vs SN2, depending on how many molecules involve in transition state

5. The SN2 Mechanism
The reaction involves a transition state in which both reactants are together
The rate of the reaction depends on both the nucleophile and the substrate concentrations
Every SN2 displacement occurs with inversion of configuration

6. Order of Reactivity in SN2 - Steric Effect -

7. The SN1 Mechanism (I)
Distinct two steps while SN2 occurs with both events at the same time
The rate of the reaction does not depend on the concentration of the nucleophile

8. The SN1 Mechanism (II)
If the carbon bearing the leaving is stereogenic, the reaction occurs mainly with loss of optical activity (racemization)

9. Order of Reactivity in SN1 - Carbocation stability -

10. SN2
SN1

11. The Nucleophile Strong nucleophiles favor SN2
Negaive ions > corresponding neutral molecules
Elements low in the same column
In the same row, the less electronegative

12. The Leaving Group
A good leaving group reduces the barrier to a reaction
Stable anions that are weak bases are usually excellent leaving groups and can delocalize charge

13. The Solvent Polar vs Nonpolar Polar protic solvent
Stabilize carbocation, accelerating SN1 reaction
Polar aprotic solvent (acetone)
Generates more nucleopilic anion, accelerating SN2 reaction

14. Elimination (dehydrohalogenation)
The nucleophile acts as a base, removing a proton from the carbon next to the one that contains the halogen (leaving group)
Please refer to page 195
As with the substitution, there are two possible mechanisms for elimination reaction

15. The E2 Mechanism
A proton is transferred to the base as leaving group begins to depart
Transition state combines leaving of X and transfer of H
Product alkene forms stereospecifically!!!

16. The E1 Mechanism The same first step as the SN1 mechanism
Compete with SN1 and in some cases E2

17. E2 E1

18. Generic Reaction-Energy Diagrams

19. Predicting the products: Substitution versus Elimination