nucleophilic substitution..... types of nucleophilic substitution....examples..... stereochemistry of SN1... stereochemistry of SN2... mechanisms

1. Presented by : Mahroosh

2. CONTE
NTS
TERMS
USED SN COMP
RISON
SN¹ SN²
•Mechanis
m
• Example
•
Condition
•
Mechanism
• Example

3. SUBSTITUTION
REACTIONS
Substitution :
“A chemical reaction during which
one functional group in a chemical
compound is replaced by another
functional group”
Nucleophile :
“An atom or molecule that donates
an electron pair to make a covalent
bond”
Examples :
OH¯ , Cl¯ , RO¨H ,RS¨H , CN ¯etc.

4. Definition of SN :
“It is the reaction of an electron pair
donor (nucleophile) with an electron
pair acceptor (electrophile) or simply
the leaving group in a substrate
is replaced by nucleophile ”
General reaction :

5. TERMS USED IN SN
Substrate :
“ Molecule which undergoes SN
reaction ”
Leaving group (nucleofuge) :
“Specie which is being replaced by
incoming entering group”

6. SN REACTIONS FOR
ALIPHATIC COMPOUNDS
SN
SN²SN¹
(Based on
molecularity)

7. SN¹
Abbreviation :
“unimolecular nucleophilic substitution
reaction”
S
• Substitution
N
• Nucleophilic
1
• Unimolecular

8. SN¹ MECHANISM
FORMATION OF CARBOCATION AND ATTACK OF
NUCLEOPHILE:

9. EXAMPLES

10. NECESSARY CONDITIONS FOR
SN¹
• Substrate :
• Temperature :
 Temperature α rate of reaction (upto carbocation
prodctn)
 Temperature α bond breaking reaction
 Temperature α carbocation production

11. • Solvent :
Polar protic
solvent
(high
polarity,
high
dielectric)
Water
Formic
acid
Alcohols
Ammonia

12. • Rate determining step :
• Rate expression :
Rate = k[R –
LG]
Where : [R – LG] »»» substrate
slow

13. •Kinetics :
 1st order reactions.
 Rate = k[R – LG]¹
•Molecularity :
oUnimolecular (one molecule is
involved in RDS)
[R –

14. • REACTION
INTERMEDIATE :
o Tertiary carbocation
o Single intermediate
o Stability of carbocations
< <

15. • Transition state :
“Transition states are potential
energy
maximums”.
• Symbol : [ ]*
o Several transition states.
]*[
hhhhhh
[ ]*

16. • Energy profile diagram :

17. RY

18. SN²
Abbreviation :
“bimolecular nucleophilic substitution
reaction”
S
• Substitution
N
• Nucleophilic
2
• Bimolecular

19. SN² MECHANISM

20. EXAMPLES

21. SN²
• Substrate :
• Temperature :
Temperature α rate of reaction
Temperature α bond breaking
reaction
> > >

22. • Solvent :

23. • Rate determining step :
o Both nucleophile and substrate are involved .
• Rate expression :
Rate = k[R – LG][Nu]
Where : [R – LG] »»» substrate , [Nu] »»»
nucleophile

24. •Kinetics :
o2nd order reaction
oRate = k[R – LG]¹[Nu]¹
• Molecularity :
oBimolecular (substrate and
nucleophile is involved in RDS)
[R – [Nu

25. • Reaction intermediate
:
o No reaction intermediate
o Single step reaction
• Transition state :
o Single transition state

26. • Energy profile
diagram :

27. STEREOCHEMISTR
Y

28. CONCLUSION
 SN¹
• One molecule in RDS
• Molecularity = 1
• Rate α [substrate]¹
• Polar protic solvent
• Weak nucleophile
• Barrier carbocation stability
• 2 steps involved
• Mixing of retention and
inversion
• 1st order kinetics
• 2 transition states
 SN²
• Two molecule in RDS
• Molecularity = 2
• Rate α [substrate]¹[Nu]¹
• Polar aprotic solvent
• Strong nucleophile
• Big barrier is steric hindrance
• One step involved
• Only inversion of
configuration
• 2nd order kinetics
• 1 transition state

29. • Which is faster
SN¹/SN² ???
SN² will be faster if :
Reagent is strong base.
Carbon connected to LG is methyl or
primary.
Solvent is aprotic (DMF , DMSO ) .
 They need space (for entering into
molecule and to push LG).