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. SN2
SN1

6. Mechanism of Nucleophilic Substitution (SN1 and SN2 )
SN1 - Substitution Nucleophilic Unimolecular
• SN1 - 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
• SN1 - produce intermediate carbocation
• Carbocation - positive charged on carbon
• Carbocation formation - sp2 hybrid (Trigonal Planar)
• Nucleophile attack from both sides
• Racemic Mixture

9. SN2 - 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 - SN1 Primary Halogenoalkane - SN2
SN1 and SN2 due to Steric Hindrance and Inductive effect
Tertiary - 3 alkyl gp - SN1
• 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 SN2
Primary - 1 alkyl gp - SN2
• Primary - Low Steric Hindrance/Inductive effect
• Low Steric Hindrance - allows Nu to attack from one side - SN2 possible
• Low Inductive effect - 1 alkyl gp, result in less stable carbocation - prevent SN1

13. Examples for SN1 and SN2 reactions
SN1 reaction, Hydrolysis of 2 -Bromo 2- Methylpropane with warm aq dil NaOH
(CH3)3CBr + OH -
 (CH3)3COH + Br-
Tertiary - SN1
• 2 steps mechanism
• 1st step, slow/rds, Heterolysis (breaking C-Br bond) forming carbocation
• 2nd step, fast, nucleophile OH -
reacts with carbocation

14. SN2 reaction, Hydrolysis of Bromoethane with warm aq NaOH
CH3CH2Br + OH -
→ CH3CH2OH + Br -
• Primary - SN2
• One step mechanism
• Bond making/breaking
simultaneous in transition state

15. Factor affecting rate of Nucleophilic Substitution (SN1 / SN2)
1. Nature of Halogen
• Bond length increase, Bond strength
decrease from CI to I, easier for
nucleophile to attack by SN2
2. Nature of Halogenoalkane
• Tertiary (SN1) faster than Primary (SN2)
Formation Carbocation (SN1) faster than
formation of transition state (SN2)
3. Nature of Nucleophile
• Negatively charged more reactive than
neutral nucleophile

16. ANY QUESTION?