in this part we discuss about SN1 reaction mechanism kinetics

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SUBSTITUTION REACTION
PRESENTED BY:
B.PHARMACY 2nd year team
(UCMS)
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Universal College of Medical Sciences
SUBSTITUTION REACTION
 Substitution reaction is the process in which
one group is substituted for another.
Nucleophilic substitution reaction:
 For nucleophilic substitution reaction , the reagent is
a suitable nucleophile and it displaces a leaving group.
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• Nucleophile are the electron rich species.
• They are negatively Charged or neutral species.
• For example: OH-,NH3,NC-,Br-,Cl-,H2O,etc.
NOTE:
 An anion is a better nucleophile than uncharged or
neutral species.
Strong bases are good nucleophile.
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Continued:-
•
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 Alkyl halides (R-X) are the main substrates in nucleophile
substitution reaction.
R-X +Nu R- Nu + X-
(Alkyl halide) (Nucleophile) (Substituted product) (Weak Nucelophile)
NOTE: Always, Strong nucleophile substitutes weak nucleophile.

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Types of nucleophile substitution reaction:
 Nucleophilic substitution Unimolecular
Reaction(SN1).
 Nucleophilic Substitution Bimolecular
Reaction(SN2).
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Universal College of Medical Sciences
Nucleophilic substitution Unimolecular
Reaction(SN1):
 This reaction is the displacement of one group. ie ; leaving
group , by another group i.e. ; nucleophile. It is
unimolecular
as kinetics data indicate that only one species involved in
the rate determining step.(Nucleophile is not involved).
 SN1 reaction is mostly favoured at tertiary alkyl halide
then secondary and lastly primary alkyl halide.
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Order:-
• For Example;
When tertiary alkyl halide reacts with potassium hydroxide
in presence of aqueous medium ,2-methyl propanol is
obtained.
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30>20>10>-CH3Cl

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Universal College of Medical Sciences
MECHANISM:-
 1ST Step: Formation of carbocation:
• Tertiary alkyl halide dissociates to form carbocation and
chloride ion . Nucleophile is not involved in this step.
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Universal College of Medical Sciences
 2nd Step: Attack of nucleophile;
• Thus, formed carbocation reacts with nucleophile,2-
methyl propanol is formed.
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Universal College of Medical Sciences
KINETICS
1st step of formation of carbocation is the rate determining step.
And it involves the substrate ie; tertiary alkyl halide only.so, rate of
reaction is of 1st order.
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Universal College of Medical Sciences
STEREOCHEMISTRY
Intermediate obtained in the reaction determines it’s
stereo –chemistry. Front attack of nucleophile in the
carbocation leads to retention of configuration and
back side attack favours inversion .So, the
stereochemistry of (SN1) reaction is
racemization(racemic mixture of retention and
inversion).
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NOTE: In some SN1 reaction , inversion is less greater than
retention. This is due to the fact that as the leaving groups departs
, it actually hinders front attack& favours back side attack.
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Universal College of Medical Sciences
EVIDENCES OF SN1 REACTION
• Rate of reaction depends only upon substrate.
• So, that SN1 reaction is unimolecular reaction.
• Stereochemistry of SN1 reaction is racemization.
• It involves rearrangement reaction.
• Reaction is favoured by polar solvents.
• Order of reactivity of alkyl halides in SN1 reaction is
30>20>10>-CH3Cl
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