This document discusses nucleophilic substitution reactions, specifically SN1 and SN2 mechanisms. SN1 is a two-step reaction that proceeds through a carbocation intermediate, depending on the concentration of the substrate. SN2 is a one-step bimolecular reaction where bond breaking and formation occur simultaneously. Tertiary halogenoalkanes typically undergo SN1, while primary halogenoalkanes usually undergo SN2 due to steric and inductive effects. Factors like the nature of the halogen, halogenoalkane, and nucleophile affect the rate of these substitution reactions.

1. Nucleophilic Substitution,
SN1, SN2, Nucleophile, Halogenoalkane
In Organic Chemistry

2. IB Chemistry, Nucleophilic Substitution, SN1, SN2, Halogenoalkanes,
Nucleophile in Organic Chemistry
Halogenoalkanes - Hydrocarbons with
• Halogen attached (F, CI, Br, I)
• (1°, 2°, 3°) halogenoalkane - number of alkyl gps attach to carbon bonded to
halogen
• Polar bonds due to high EN of halogen

3. 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

4. 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
Click HERE for more info

5. 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

6. 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)

7. 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

8. IB 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
Click HERE to view

9. 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
Click HERE to view

10. 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
Click HERE to view

11. Substitution with Nucleophile (Ammonia and potassium cyanide)
• NH3 /CN- acts as nucleophile
• SN2 - form amine and nitrile
• Substitute X with NH3 -amine
• Substitute X with CN- nitrile
Click HERE to view

12. Bromoethane with aq ethanolic ammonia/ potassium cyanide
CH3CH2Br + NH3 ---> CH3CH2NH2 + HBr
• NH3 as nucleophile
• SN2 pathway, one step
• Product - Alkyl amine
CH3CH2Br + CN- ----> CH3CH2CN + Br-
• CN- as a nucleophile
• Product - Alkyl nitrile , addition 1 carbon
• Reduction of nitrile with H2/catalyst nickel - produce amine

13. Reaction Pathway with SN2 substitution (OH-, NH3, CN-)
CH3CH2Br + OH-  CH3CH2OH + Br-
CH3CH2Br + NH3  CH3CH2NH2 + HBr
CH3CH2Br + CN- ---- > CH3CH2CN + H2/Ni  CH3CH2CH2NH2
(2 carbon) ( 3 carbon ) ( 3 carbon )
Products act as Nucleophile, producing 1°, 2°, 3° Amines
Keynotes
Rxn 1 : Product C2H5NH2 acts as nucleophile for reaction 2
Rxn 2 : Product C2H5NH2 reacts with C2H5Br , producing (C2H5)2NH
Rxn 3: Product (C2H5)2NH acts as nucleophile reacts with C2H5Br
producing (C2H5)3N

14. Video, SN2 reaction with NH3 and KCN
More Video on SN1 and SN2
Click HERE to view
Thanks to all pictures and video contributors for the above post

15. Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenses
http://creativecommons.org/licenses/
Prepared by Lawrence Kok
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