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Substitution and Elimination. Reactions of Alkyl Halides. Substitution, Nucleophilic, Bimolecular – S N 2. Reaction Profile for S N 2 Reaction (Wade). Stereochemistry of S N 2 Reaction Inversion of Configuration. Proof of Inversion of Configuration at a Chiral Center.

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Substitution and elimination

Substitution and Elimination

Reactions of Alkyl Halides



Reaction profile for s n 2 reaction wade
Reaction Profile for SN2 Reaction (Wade)


Stereochemistry of s n 2 reaction inversion of configuration
Stereochemistry of SN2 ReactionInversion of Configuration



Acetate approaches from 180 o behind leaving group
Acetate Approaches from 180o Behind Leaving Group



Substrate reactivity
Substrate Reactivity

Since the energy of the transition state is significant in determining the rate of the reaction, a primary substrate will react more rapidly than secondary (which is much more rapid than tertiary).


1 o 2 o 3 o bulkiness of substrate
1o > 2o >> 3oBulkiness of Substrate





Nucleophiles preferably non basic
Nucleophiles(preferably non-basic)




S n 2 and e2
SN2 and E2


Bimolecular elimination e2 nucleophile acts as bronsted base
Bimolecular Elimination - E2Nucleophile acts as Bronsted Base


S n 2 competes with e2
SN2 Competes with E2


S n 2 competes with e21
SN2 Competes with E2


S n 2 competes with e22
SN2 Competes with E2





H and br anti coplanar orientation
H and Br Anti-coplanar orientation


In a cyclohexane leaving group must be axial
In a Cyclohexane, Leaving Group must be Axial




Hyperconjugation p bond associates with adjacent c h s bond
Hyperconjugationp bond associates with adjacent C-H s bond


With bulky base hofmann product forms
With Bulky Base, Hofmann Product Forms




E2 reaction of r r 2 iodo 3 methylpentane
E2 Reaction of(R,R) 2-iodo-3-methylpentane





S n 1 mechanism wade 1 st step is rate determining
SN1 mechanism(Wade)1st step is rate determining


Reaction profiles wade s n 1 s n 2
Reaction Profiles (Wade)SN1 SN2


Hammond s postulate
Hammond’s Postulate

  • Related species that are close in energy are close in structure.

  • In an endothermic reaction, the transition state is similar to the product in structure and stability.

  • In an exothermic reaction, the transition state is similar to the reactant in structure and stability.

  • i.e. the structure of the transition state resembles the structure of the most stable species.


Endo transition state looks like product exo transition state looks like reactant
Endo- transition state looks like productExo- transition state looks like reactant


S n 1 transition state
SN1 Transition State


S n 1 solvent effects
SN1 Solvent Effects



Carbocation stability more highly substituted lower energy
Carbocation Stabilitymore highly substituted, lower energy



Carbocations can rearrange 1 2 hydride shift
Carbocations can Rearrange1,2-Hydride Shift


Carbocations can rearrange 1 2 methide shift
Carbocations can Rearrange1,2-Methide Shift






E1 and s n 1 compete
E1 and SN1 Compete









Cis reacts more rapidly
Cis Reacts more Rapidly