1. Chapter 7 Elimination Reactions
Unimolecular Elimination, E1
Elimination Reactions
Removal of L.G. and H, but no substitution
Formation of a C==C double bond
Solvolysis of 2-bromo-2-methylpropane actually gives 2 products
Rate of reaction for both products = k[haloalkane] E1
E1 Mechanism
Step 1 is Dissociation (just like for SN1)
SN1 E1

2. b) Step 2 is Deprotonation to form the Alkene

3. Bimolecular Elimination, E2
E1 can give a mixture of products because any H can be removed
Bimolecular Elimination, E2
Strong Bases affect nucleophilic substitution reactions (OH- and RO-)
Rate = [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
SN1 Product
E1 Products

4. Mechanism
Orbital Picture

5. Comparison of E1 and E2
E1 is stepwise and involves a carbocation
E2 is concerted
Experimental Evidence for Concerted E2 Mechanism
Second order rate law shows base and RX involved in R.D.S.
Better L.G. is faster (Bond breaking is important in R.D.S.)
Stereochemistry is important
If anti Hydrogen is present, E2 is fast
If anti Hydrogen is not present E2 is very slow

6. Substitution and Elimination
SN2, SN1, E2, E1 all occur from RX + Nu
How do we predict what the reaction will produce?
Weakly basic strong nucleophiles give substitution products
Good Nucleophiles
Base weaker than OH-
I-, Br-, RS-, N3-, RCO2-, PR3, CN-
SN2 for primary and secondary RX
SN1 for tertiary RX
Weakly basic weak nucleophiles can only do SN1/E1 (H2O, ROH)
Strongly basic Nu’s give more E as Bulk increases (OH-, RO-, NH2-)
SN2 mostly
E2 mostly
E2 mostly

7. Sterically hindered bases favor Elimination
Too bulky to do substitution
E2 or E1 depending on RX
Reactivity of Haloalkanes
Primary Haloalkanes
SN2 mostly, can be slowed by bulk at C2
E2 only if Bulky base
Secondary Haloalkanes
SN2 with good nucleophiles, weak base, Polar Aprotic Solvent
SN1/E1 with good LG, weak Nu, Polar Protic Solvent
E2 with strong base, Polar Protic Solvent
Tertiary Haloalkane
SN1/E1 with good LG, no base (solvolysis)
E2 with strong base