Subsititution Nucleophilic reaction mechanism

1. Dr. L. Sakthikumar M.Sc., M.Phil.,ph.d
Assistant professor in Chemistry,
Saiva Bhanu Kshatriya College,
Aruppukottai – 626101,
Tamilnadu
E-mail : lsakthisbk@gmail.com
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2. NUCLEOPHILIC SUBSTITUTION REACTIONS
NUCLEOPHILES
 The word nucleophile comes from nucleus, the
positive part of an atom, plus -phile from Greek word
philos meaning to love.
 A nucleophile is any negative ion or any neutral
molecule that has at least one unshared electron
pair. Eg:
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4. MECHANISM OF SN2 REACTION
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5.  The SN2 reaction is a Substitution,
Nucleophilic, bimolecular reaction. The
transition state involves both the nucleophile and
the substrate, it accounts for the observed second-
order reaction rate.
 The mechanism for SN2 reaction was proposed
by Edward D. Hughes and Sir Christopher Ingold
(the University College, London) in 1937.
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6.  The nucleophile attacks the carbon bearing the leaving group from the back
side.
 The orbital that contains the electron pair of the nucleophile begins to overlap
with an empty (antibonding) orbital of the carbon bearing the leaving group.
 The bond between the nucleophile and the carbon atom is forming, and the
bond between the carbon atom and the leaving group is breaking at the same
time.
 Because bond formation and bond breaking occur simultaneously in a single
transition state, the SN2 reaction is a concerted reaction.
 The configuration of the carbon atom becomes inverted during SN2 reaction.
 The reaction is a exothermic reaction where the free energy value G is negative.
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8. A free-energy diagram for a hypothetical SN2 reaction that
takes place with a negative value of G°
1. Most of the SN2 chemical reactions occur much more rapidly at higher
temperatures.
2. A reaction with a lower free energy of activation will occur very much
faster than a reaction with a higher one.
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9. THE STEREOCHEMISTRY OF SN2 REACTIONS
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10. MECHANISM OF SN1 REACTION
 The SN1 reaction is a Substitution, Nucleophilic and
Unimolecular. The reaction of tert-butyl chloride with
sodium hydroxide in a mixture of water and acetone to
form tert-butyl alcohol is a good example for SN1
reaction.
 The rate of formation of tert-butyl alcohol is dependent
only on the concentration of tert-butyl chloride and
independent of the concentration of hydroxide ion.
The rate equation for this substitution reaction is first
order with respect to tert-butyl chloride.
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12. A free-energy diagram for the SN1 reaction of tert-
butyl chloride with water
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13. STEREOCHEMISTRY OF SN1 REACTIONS
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14. FACTORS FAVOURING SN1 &SN2
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