Chapter 7 Elimination Reactions
I.
Unimolecular Elimination, E1
A.
Elimination Reactions
1) Removal of L.G. and H, but no substitution
2) Formation of a C==C double bond
H
R
B.
R
R
B-
R
R
+
R
R
X
BH + X -
R
Solvolysis of 2-bromo-2-methylpropane actually gives 2 products
Br
1)
2)
H2O
OH
+
+ Br- + H+
SN1
E1
Rate of reaction for both products = k[haloalkane] E1
E1 Mechanism
a) Step 1 is Dissociation (just like for SN1)
Br
b)
Step 2 is Deprotonation to form the Alkene
CH2
=
CH3
C
CH2
H
+ H2O
CH3
CH3
+ H3O+
C
CH3
C.
H
E1 can give a mixture of products because any H can be removed
CH3 H
Et
C
Et
Br
H
H2O
CH3
Et
CH3
H
Et
CH2 H
C
Et
CH3
+
C
Et
OH
H
CH3
Et
CH3
CH3 H
CH3
CH3
CH3
C
Et
Br
SN1 Product
Et
C
+
CH3
CH3 H
Et
Br
CH3
CH3
E1 Products
II.
Bimolecular Elimination, E2
A.
Strong Bases affect nucleophilic substitution reactions (OH- and RO-)
1) Rate = [B-][RX] (second order or bimolecular)
2) Must occur before a carbocation forms, or we would just have SN1, E1
B.
E2 Mechanism
1) Occurs in a single, concerted step
2) Must have strong base
3)
Mechanism
HO
-
H
R
4)
R
R
R
R
+
R
Orbital Picture
X
R
R
OH2 + X
-
C.
Comparison of E1 and E2
1) E1 is stepwise and involves a carbocation
CH2
Br
CH3
CH2
H
C
CH3
+ H2O
CH3
2)
CH3
E2 is concerted
HO
-
H
R
D.
+ H3O+
C
R
R
R
R
+
R
R
X
OH2 + X
R
Experimental Evidence for Concerted E2 Mechanism
1) Second order rate law shows base and RX involved in R.D.S.
2) Better L.G. is faster (Bond breaking is important in R.D.S.)
3) Stereochemistry is important
a) If anti Hydrogen is present, E2 is fast
b) If anti Hydrogen is not present E2 is very slow
Br
fast
H
H
CH3O-
slow
CH3O
H
H
Br
-
III. Substitution and Elimination
A.
SN2, SN1, E2, E1 all occur from RX + Nu
B.
How do we predict what the reaction will produce?
1) Weakly basic strong nucleophiles give substitution products
a) Good Nucleophiles
b) Base weaker than OHc) I-, Br-, RS-, N3-, RCO2-, PR3, CNd) SN2 for primary and secondary RX
e) SN1 for tertiary RX
2) Weakly basic weak nucleophiles can only do SN1/E1 (H2O, ROH)
CH3
Br
CH3
C
CH2
H2O
CH3
CH3
C
OH
+
CH3
Strongly basic Nu’s give more E as Bulk increases (OH-, RO-, NH2-)
3)
HO
-
HO- +
HO:-
-
Br
+
OH + Br
+ Br-
Br
+
Br
SN2 mostly
E2 mostly
+ Br-
E2 mostly
4)
Sterically hindered bases favor Elimination
a) Too bulky to do substitution
b) E2 or E1 depending on RX
O-
C.
+
Br
O-
+ Br-
Reactivity of Haloalkanes
1) Primary Haloalkanes
a) SN2 mostly, can be slowed by bulk at C2
b) E2 only if Bulky base
X
2)
Secondary Haloalkanes
a) SN2 with good nucleophiles, weak base, Polar Aprotic Solvent
b) SN1/E1 with good LG, weak Nu, Polar Protic Solvent
c) E2 with strong base, Polar Protic Solvent
3)
Tertiary Haloalkane
a) SN1/E1 with good LG, no base (solvolysis)
b) E2 with strong base