SN 1&SN 2 reactions

brief discussion of SN reactions of alkyl halides

2. Presentation For ChemistryPresentation For Chemistry
Assigned By : Assigned By : Mr.ShahbazMr.Shahbaz
Topic: Nucleophilic SubstitutionTopic: Nucleophilic Substitution
Presenter: Mahr Muhammad UmarPresenter: Mahr Muhammad Umar

3. Alkyl halides - Hydrocarbons with
• Halogen attached (F, CI, Br, I)
• (1°, 2°, 3°) halogenoalkane - number of alkyl groups attach to carbon bonded to
halogen
• Polar bonds due to high EN of halogen

4. Nucleophilic Substitution -
• Nucleophile (non bonding electron) attack the partial positive charge carbon
(nucleus)
• Chloride (halogen) - leaving group and substituted by nucleophile
• Nucleophile replace/substitute the halogen

5. S
N
2
S
N
1

6. Mechanism of Nucleophilic Substitution (S
N
1 and S
N
2 )
S
N1
  - Substitution Nucleophilic Unimolecular
• S
N
1 -  2 steps, unimolecular ( first order)
• 1st step - slow/rds, Carbocation formation 
• 2nd step - fast, Nucleophilic attack 
carbocation 
• Rate = k [substrate],  First order overall
• Rate depend on conc. substrate 
NOT conc. nucleophile
• Nature of the nucleophile doesn’t
 affect the rate

8. Stable Intermediate Carbocation
• S
N
1 - produce intermediate carbocation
• Carbocation - positive charged on carbon 
• Carbocation formation -  sp2 hybrid (Trigonal Planar)
• Nucleophile attack from both sides
• Racemic Mixture 

9. S
N
2 - Substitution Nucleophilic Bimolecular
• 1 step mechanism, Bimolecular collision
• Rate = k[substrate][nucleophile], Second order overall
• Rate depend on conc of substrate and nucleophile
• Bond making/breaking occur together result in trigonal bipyramidal shape
• Inverted configuration (backside attack by Nucleophile)

11. Tertiary Halogenoalkane - S
N
1      Primary Halogenoalkane - S
N
2
S
N
1 and S
N
2 due to Steric Hindrance and Inductive effect
Tertiary - 3 alkyl gp - S
N
1
• Tertiary - High Inductive/Steric hindrance
• High Inductive effect - 3 alkyl gp
donates/push e towards carbocation
• Stabilizes positive charge on it
• High Steric Hindrance - 3 alkyl gp
hinder/blocks Nu from attacking,
prevent S
N
2
Primary - 1 alkyl gp - S
N
2
• Primary - Low Steric Hindrance/Inductive effect
• Low Steric Hindrance - allows Nu to attack from one side - S
N
2 possible
• Low Inductive effect - 1 alkyl gp, result in less stable carbocation - prevent S
N
1

13.  Examples for S
N
1 and S
N
2 reactions
S
N
1 reaction, Hydrolysis of 2 -Bromo 2- Methylpropane with warm aq dil NaOH
(CH3)3CBr  +  OH
-
   (CH3)3COH  +  Br
-
Tertiary - S
N
1 
• 2 steps mechanism
• 1st step, slow/rds, Heterolysis (breaking C-Br bond) forming carbocation
• 2nd step, fast, nucleophile OH
-
 reacts with carbocation

14. S
N
2 reaction, Hydrolysis of Bromoethane with warm aq NaOH
CH3CH2Br  + OH
-
 →  CH3CH2OH +  Br
-
 
• Primary - S
N
2
• One step mechanism
• Bond making/breaking 
simultaneous in transition state

15. Factor affecting rate of Nucleophilic Substitution (S
N
1 / S
N
2)
1. Nature of Halogen
• Bond length increase, Bond strength 
decrease from CI to I, easier for 
nucleophile to attack by S
N
2
2. Nature of Halogenoalkane
• Tertiary (S
N
1) faster than Primary (S
N
2)
Formation Carbocation (S
N
1) faster than 
formation of transition state (S
N
2)
3. Nature of Nucleophile
• Negatively charged more reactive than 
neutral nucleophile

16. ANY QUESTION?