NIDHI SANKET Sem 1

1. ALIPHATIC NUCLEOPHILIC SUBSTITUTION REACTION
2020-2021
Submitted By – NIDHI SANKET
M.Sc Chemistry Semester I
Department Of Chemistry
S.P.C. Govt. College, Ajmer

2. CONTENTS
 INTRODUCTION
 NUCLEOPHILIC SUBSTITUTION REACTION
 MECHANISM OF SN REACTION
 SN2 REACTION - MECHANISM
STEREOCHEMISTRY
 SN1 REACTION – MECHANISM
STEREOCHEMISTRY
 REFERENCES

3. NUCLEOPHILIC SUBSTITUTION REACTION
• At saturated carbon, Nucleophilic reaction involves the displacement of a nucleophile by another.
• A classic example is the hydrolysis of alkyl halides,
HO- + R-X HO-R + X-

4. MECHANISMS OF SN REACTION
• The two main mechanisms are,
1. SN1 Reaction
2. SN2 Reaction
• S stands for chemical substitution, N stands for nucleophilic and the number represents the
molecularity of the reaction.

5. SN2 REACTION
• Nucleophilic substitution reaction in which concentration of two molecules get change in rate
determining step is called SN2 reaction.
• The designation SN2 stands for bimolecular nucleophilic substitution .
• SN2 occurs where the central carbon atom is easily accessible to the nucleophile.
• The rate of reaction depends on both, the concentration of substrate and the concentration of
nucleophile.
Rate = k [substrate] [nucleophile]

6. SN2 REACTION
• Reaction between primary alkyl halide and aqueous base is an example of SN2 reaction .
R-X + -OH R-OH + X-
• Since hydroxyl ion is strong nucleophile than halide (X-) ion , hence halide ion is easily replaced by
hydroxyl ion.
• Rate = k [RX] [-OH]
• Rate of hydrolysis of alkyl halide by SN2 reaction is CH3X > 1o > 2o > 3o .

7. MECHANISM OF SN2 REACTION
• It is a single step process which involves no intermediate.
• It involves only one transition state which is of low polarity.
• The nucleophile attacks from the backside.
• An inverted product is formed and this inversion of configuration is called the WALDEN
INVERSION.

8. STEREOCHEMISTRY OF SN2 REACTIONS / EVIDENCES
FOR THE FORMATION OF TRANSITION STATE
• A method has been suggested by Hughes and Ingold to establish the inversion of configuration.
•The method involves the conversion of (+)2 iodooctane with potassium radioiodide (K129I) in
acetone to (-)2 iodooctane.
• The exchange of ordinary iodide with the radioactive iodide was accompanied by the loss of optical
activity.
• Inversion of configuration is always indicative of SN2 reaction.

9. SN1 REACTION
• Nucleophilic Substitution reaction in which concentration of only one molecule get change in
rate determining step is called SN1 reaction.
• The designation SN1 stands for unimolecular nucleophilic substitution.

10. • It is a two step process and since the slow step or rate determining step involves only the
substrate, the rate of reaction depends only on the concentration of substrate.
• Rate = k [substrate]
• Typical example of SN1 reaction is hydrolysis of tertiary butyl chloride.
(CH3)3C-Cl + -OH (CH3)3C-OH + Cl-
a. This reaction takes place in three steps.
b. Rate = k [(CH3)3C-Cl ]
SN1 REACTION

11. MECHANISM OF SN1 REACTION
It involves formation of carbocation intermediate .

12. STEREOCHEMISTRY OF SN1 REACTION / EVIDENCES
FOR THE FORMATION OF CARBOCATION
INTERMEDIATE
The SN1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of
3-iodo-3-methylhexane.

13. STEREOCHEMISTRY OF SN1 REACTION
• Nucleophile can attack from both the sides. Nucleophile is free for attack on
carbocation from either of the side .
• RACEMIZATION occurs i.e. Retention in configuration and Inversion in
configuration take place and two types of product are formed.
• 100% racemization or pure racemization is rarely observed in SN1 mechanism.
• Inverted product is always predominates over retention in configuration product i.e.
racemic mixture as well as some invert product are formed in SN1 mechanism .

14. REFERENCES
 MARCH`S ADVANCED ORGANIC CHEMISTRY (7th EDITION)- by
Michael B. Smith - Wiley Student Edition
 ADVANCED ORGANIC CHEMISTRY- by Dr. Jagdamba singh and Dr. L.D.S.
Yadav – A Pragati Edition
 Special Thanks to Dr. Alok Chaturvedi Sir

15. THANK YOU