Ch. 9
Substitution and Elimination: Reactions of Akyl Halides
I. Substitution Reactions
Nucleophile (Nu:): nucleus- loving
- e- rich (lone pair or π bond), can also have negative (-) charge
- seeks e- deficient species (δ+ or +)
Electrophile (E+): electron-loving
- e- poor (δ+ or +)
- seeks e- rich species
Leaving group (LG)
- group leaves and take 2 e- with it
- stable groups (weak bases) make good LG’s (e.g Cl-, Br-, I-)
General reaction:
R – LG + Nu:  R – Nu + LG:
Possible mechanism?
*simultaneous
* stepwise
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SN2: Substitution Nucleophilic Biomolecular Reaction (9.2)
E.g
Mechanism: one step
SN2 Kinetic
Rate = k [CH3O-] [CH3Cl]
 Rate depends on both Nu: and E+
 Biomolecular rxn
Sterics affect rate of SN2
R Group
Abbreviation
MeX
X = Cl, Br, I
Carbon type
EtX
Methyl
1o
2o
IPrX
TBuX
3o
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Relative rate
30
1
0.02
0.004
Relative stabilities of alkyl halides
(Fastest)
Methyl > 1o > 2o > 3o (NR)
SN2 E vs. POR diagram
*if LG is too sterically hindered (3o)
-
T.S is high energy
-
Ea is high
-
reaction is too slow (N/R)
SN2 summary

1 – step mechanism

Backside attack

inversion of stereochemistry

requires an unhindered LG and good Nu: (-charge or NH3)

Rate (RX) Methyl > 1o > 2o > 3o (slowest or NR)
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E.g
Predict the major product and show the mechanism
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SN1 Substitution Nucleophilic Unimolecular (9.4)
Could this be an SN2 ?........................
SN1 mechanism (2 key steps but usually 3 steps) (9.4)
1. Loss of LG
2. Addition of Nu:
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SN1 Kinetic
Rate = k [tBuCl]
 Rate determining step involves E+ only
 Rate is independent of [Nu:]
 A more stable carbocation will be formed faster
Rate of SN1: benzylic/allylic > 3o > 2o >> 1o, methyl
Stereochemistry of SN1
SN1 result in racemization
SN1 Summary
 Stepwise mechanism via carbocation
 More stable carbocation, faster reaction
 Racemization occurs
 Requires no strong Nu (usually with solvent: H2O or ROH)
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E.g
Predict the major product(s) and show the mechanism
E.g Predict all possible products for the following reactions and identify whether which
approach (SN1 or SN2) would be possible for the reactions to occur.
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