SN1 vs SN2 vs E1 vs E2
Name______________________
Comparison of Reactions:
Sn2 vs Sn1 vs E1 vs E2
„
„
„
Substrate Good Nuc, Good Nuc,
(RX)
Weak Base Strong
Base
I- Br- HS- OH- RONH3 PH3
NH2methyl
SN2
SN2
Species that are Good Nucleophiles but
Weak Bases promote SN2 reactions
I-, Br-, Cl-, HS-, NH3, PH3,
Species that are Good Nucleophiles but
Strong Bases promote both SN2 and E2
(depends on substrate)
OH-, RO-, H2NSpecies that are Poor Nucleophiles and
Weak Bases promote SN1/(E1) reactions
H2O, ROH
primary
SN2
secondary
tertiary
Predict the Products:
CN
DMF
Cl
H
b)
I-
H
H
OH-
I
H2O
(SN2)
(E2)
OH
d)
(SN2)
No Rxn
No Rxn
SN2
Mostly
SN2 (some
E2)
E2
No Rxn
E2
SN1
SN1
(SN1)
OH-
I
f)
CH3OCl
OCH3
g)
Cl CH3OH
No Reaction
+
Br
(E1)
(E2)
e)
I
acetone
Cl
H2O ROH
Predict the Products:
CN-
Br
a)
Poor Nuc
Weak Base
h)
H
H
OCH3
(SN2)
OCH3 H
CH3OH
racemic mixture
(SN1)
(E1)
1
Sn1 vs Sn2 vs E1 vs E2
Unimolecular Elimination, E1
Name______________________
EI elimination reaction
Elimination Reactions: Removal of
Leaving Group (L) and H, but no substitution
Formation of a C=C double bond
Solvolysis of 2-bromo-2-methylpropane
With H2O actually gives 2 products
(SN1=alcohol, E1=alkene)
Rate of reaction for both products
K= k[haloalkane]
Step 1 is Dissociation (just like for SN1)
Bimolecular Elimination, E2
carbocation
intermediate
2nd transition
state
alkene
product
E2 elimination reaction
Strong Bases affect nucleophilic
substitution reactions (OH- and RO-)
Rate K= [B-][RX]
(second order or bimolecular)
Must occur before a carbocation forms,
or we would just have SN1, E1
E2 Mechanism
Occurs in a single, concerted step
Must have strong base
substrate
transition
state
alkene
product
1
Sn1 vs Sn2 vs E1 vs E2
Name______________________
Substitution vs. Elimination
Comparison of E1 and E2
SN2, SN1, E2, E1 all occur from RX + Nu
E1 is stepwise and involves a carbocation
Br
CH3
C
CH2
H
CH2
CH3
+ H2O
CH3
-
R
H
R
+
CH3
E2 is concerted
HO
+ H3O
C
R
R
R
R
X
R
R
+
OH2 + X
How do we predict what the reaction will produce?
-Weakly basic, strong nucleophiles give substitution products
-Good Nucleophiles
-Base weaker than OHSN1 for tertiary RX
-I-, Br-, RS-, N3-, RCO2-, PR3, CN-SN2 for primary and secondary RX
Weakly basic, weak nucleophiles can only do SN1/E1 (H2O, ROH)
Br
CH3
C
CH3
CH2
H2O
CH3
C
CH3
Experimental Evidence for Concerted E2 Mechanism:
Second order rate law shows base and RX involved in RDS
Better L is faster (Bond breaking is important in RDS)
Stereochemistry:If anti-H is present, E2 is fast, if not present E2 is
Br
very slow
fast
H
CH3O
slow
CH3O
-
H
+
OH
CH3
Strongly basic Nu’s give more E as Bulk increases (OH-, RO-, NH2-)
HO-
+
Br
HO -
+
Br
+
Br
OH + Br
E2 mostly
+ Br-
Br
-
HO :
+ Br
SN2 mostly
-
E2 mostly
H
H
Sterically hindered bases favor Elimination
-too bulky to do substitution
-E2 or E1 depending on RX
O-
+
Br
Reactivity of Haloalkanes
Primary Haloalkanes
SN2 mostly, can be slowed by bulk at βC
E2 only if bulky base
Secondary Haloalkanes
SN2 with good nucleophiles, weak base, Polar
Aprotic Solvent
SN1/E1 with good L, weak Nu, polar protic solvent
E2 with strong base, polar protic solvent
Tertiary Haloalkane
SN1/E1 with good L, no base (solvolysis)
E2 with strong base
O-
+ Br
Likely mechanism of reaction of haloalkane with base
-
X
2