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SN Reaction REVIEW!
What the heck is SN reaction?
• SN reaction stands for NUCLEOPHILIC • SUBSTITUTION reaction
• Substitution… so, something re-places
something…AB + C AC + B
• There are two types of Sn reac-tion:- Sn2 and Sn1
Before we discuss SN2 reaction…
• There are two main characters in this reactions
Alkyl Halide and Nucleophile
Alkyl Halide
A compound that contains carbon and hydrogen (carbohydrates) as well as
the halogens (Cl, Br, I) as a leaving group
SN 2 reaction!!• 1 step mechanism
AB + C AC + B
A= Alkyl halideB= Leaving groupC= Nucleophile
C attacks A, and B leaves A at the same time!!
Why do we call it SN2 then???
The rate (how easy the reaction goes) involves concentration of two characters
Alkyl Halide and Nucleophile
More Alkyl Halide, faster the reaction
More Nucleophile, faster the reac-tion
We call this Bimolecular reac-tion
SN1 Reaction!It is a 2 step reaction!
AB + C A + B + C AC+B
Leaving group B leaves A, THEN C attacks A
Mechanism of SN1 Reaction
Again, Why do we call it SN1 if there are 2 steps??
• The rate of SN1 reaction only in-volves the concentration of alkyl halide.
SN1 reaction = How fast the leav-ing group leaves and forms a carbo-cation?More Alkyl Halides = Faster reac-
tionMore nucleophile = NOTHING HAP-PENS
Unimolecular Reac-tion
Nucleophile
Oh, bye the way, good leaving groups are weak baseI>Br>Cl>>F
Nucleophile steals leaving group’s spot
Nucleophilicity is a kinetic property.Faster, the better.L.G. property depends on basicity (weak base, better L.G.)
How do you know which one is good nucleophile?
Negative Charge
What does it mean, nucle-ophile?
Nucleo:Nux – nutNucleus – kernel, core
Phile:Philia – love, affection
Molecule is satisfied when all atoms have octet (except hy-drogen) and 0 formal charge. If they don’t have this, they will find someone to share love.
Nuc. increase to the left of periodic table
This is same with basicity. many good nucleophiles are good base. EXCEPT:
I-, Br-, Cl-, N3-, HS-,
CH3CO2-
B E
AR A
Solvent Effect and nucleophilicity
F-I-
OH
H
OH
H
O
H
H
O H
H
O
H
H
O
H
H
O H
H
O
H
H
How come it is not same as basicity???
Although nonpolar solvent make Sn2 favors, we need to use polar aprotic (DMF, DMSO),Because they have high BP, and cannot be separated by liquid-liquid extraction
Solvent for Sn1
Polar protic is favore, because it stabilize carbocation inter-mediates.
Competitive reaction