Aliphatic Nucleophilic substitution reactions

1. ALIPHATIC NUCLEOPHILIC SUBSTITUTION
Prof. A.Ramachandran
Assistant Professor in Chemistry
GCE, Salem-11

2. DEFINITION
• Nucleophilic substitution reaction involves
the displacement of a nucleophile by
another nucleophile
• The nucleophile furnishes an electron pair
to the carbon from which the leaving group
departs with the bonding pair of electrons.

3. TYPES OF NUCLEOPHILIC SUBSTITUTION
REACTIONS
• Unimolecular Nucleophilic Substitution reaction (SN
1)
• Bimolecular Nucleophilic Substitution reaction (SN
2)
• Internal Nucleophilic Substitution reaction (SN
i)

4. SN1 MECHANISM
• The SN1 reaction is a nucleophilic substitution reaction
where the rate determining step is unimolecular.
• The rate of the reaction is dependent on one of the
reactants, the reaction is a first order reaction.
Rate ∝ [R3C-X]
• The product is formed through formation of carbocation
intermediate. Hence it follows two step mechanism.

5. SN1 MECHANISM
Taking the hydrolysis of tertiary butyl bromide as an example,
Step 1
The carbon-bromine bond is a polar covalent bond. The
cleavage of this bond allows the removal of the leaving group
(bromide ion).
When the bromide ion leaves the tertiary butyl bromide, a
carbocation intermediate is formed.
It is important to note that the breaking of the carbon-
bromine bond is endothermic.
The first step is reversible and slow one. Hence it shows rate
determining step.

6. SN1 MECHANISM
• Step I: Overall

7. SN1 MECHANISM
• Step II:
In the second step of the SN1 reaction mechanism,
the carbocation is attacked by the nucleophile.
A rapid attack of OH– on the carbocation completes
the hydrolysis.
A carbocation is flat (SP2, trigonal planar) with the
vacant 2P orbital vertical to the plane bearing the
three groups, the attack of the reagent can occur
from either side of the plane with equal
probability, i.e. a racemic product should result if
the alkyl halide is chiral.

8. SN1 MECHANISM
• Step II:

9. SN1 MECHANISM
Effect of Solvent
• A solvent that can facilitate the formation of the carbocation
intermediate will speed up the rate determining step of the SN1
reaction.
• The preferred solvents for this type of reaction are both polar and
protic.
• The polar nature of the solvent helps to stabilize ionic
intermediates whereas the protic nature of the solvent helps
solvate the leaving group.
• Examples of solvents used in SN1 reactions include water and
alcohols. These solvents also act as nucleophiles.

10. SN1 MECHANISM
Effect of Substrate
• A tertiary carbocation being stabilized by three electron
releasing groups. Allylic and benzylic halides can also react
by an SN1 mechanism since these substrates can form
relatively stable carbocation.
• The order of hydrolysis of alkyl halides by SN1 path is:
Benzyl, Allyl> tert- > Sec-> primary > CH3

11. SN2 MECHANISM
• Nucleophilic substitution reactions which follow second-order
kinetics are called SN2
• The rate of hydrolysis of methyl bromide with NaOH has been
found to be of second order, i.e.,
Rate ∝ [CH3Br] [OH-]
• The reaction is a concerted one step reaction without any
intermediate.

12. SN2 MECHANISM
• A collision between the two reactants resulting in the direct
displacement of Br- by OH- occurs.
• While a new C-OH bond is being formed, the C-Br bond starts
breaking, i.e. the bond formation and the bond breaking are
simultaneous.
• During the collision an energetic hydroxide ion approaches the
methyl bromide molecule from the side opposite to bromine to
avoid repulsion, i.e. at 180° to the leaving group- a back- side
attack.

13. SN2 MECHANISM
• A state is reached when the OH and Br are partially bonded to
the central carbon and the non- participating groups lying in a
plane perpendicular to the line HO….C….Br. This state is called
the transition state.
• In the transition state partial negative charge of the hydroxide
ion is transferred to bromine via the carbon atom.
• With further approach of hydroxide, a complete C-OH bond is
formed and bromine departs with the bonding pair of electrons.

14. SN2 MECHANISM
Overall

15. SN2 MECHANISM
Effect of Substrate:
• The optimum substrate would be CH3-X. Each replacement
of hydrogen by a more bulky alkyl group should decrease
the rate of reaction.
• Consequently, the order of reactivity of alkyl groups is
expected to be
Methyl > primary > secondary > tertiary

16. SNI MECHANISM
• SN
i means Substitution Nucleophilic Internal mechanism
• It follows second-order kinetics with no change in the configuration of
the product.
• Best example is the esterification of chiral alcohols with thionyl
chloride, which results in the retention of configuration of the product
• The rate of the reaction is found to be dependent on both the
reactants,
Rate ∝ [ PhCH(Me)OH] [ SOCl2]

17. SNI MECHANISM
Overall