 E2 = elimination, bimolecular
 Reaction is concerted (one step)
 Rate = k[substrate][base]
(a second order process)
 E2 can occur with S
N
2.
 Occurs by abstraction of H + from a C adjacent to the C with the LG.
 Products follow Zaitsev’s Rule.
 If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.

 One-step and requires a strong base.
 Best transition state is anti-coplanar.
 Example: t-butyl bromide + methoxide
 Why don’t S
N
2, S
N
1, or E1 occur?

 Example: t-butyl bromide + methoxide

 Anti-periplanar alignment allows pi bond to form from sp 3 orbitals.

rate = ?
k = ?

Factors Affecting E2 Reactions
 Structure of the substrate
 Strength of the base
 Nature of the leaving group
 The solvent in which the reaction is run.

Factors Affecting E2 Reactions -
Structure of the Substrate
 Structure of the alkyl halide:
3° > 2° >1°
 This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).

Factors Affecting E2 Reactions -
Structure of the Substrate
Decide whether the following substrates could react by E1 or E2
(and by S
N
1 or S
N
2).
Br
Cl
I
Br

Factors Affecting E2 Reactions -
Strength of the Base
 The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?
CH
2
OH
H
2
O
N

Factors Affecting E2 Reactions the Leaving Group
 The LG should be good. Which of the following substrates have good LGs?
Cl
NH
2
OCH
3
H
O
O S
O
CH
3

Factors Affecting E2 Reactions -
Solvent Effects
 Polarity is not so important because negative charge is spread over the transition state.

Zaitsev’s Rule
 When two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.

Zaitsev’s Rule
 Show the E2 elimination products that could come from:
Cl
Br
 Which E2 reaction would occur more quickly?

 The structure of the substrate affects the rate.
 Relative rates for E2: 3°>2°>1°.
 The base must be strong.
 The LG should be good.
 The solvent should be polar.
 Coplanar (usually anti) transition state is required.
 Products will follow Zaitsev’s Rule.
 Can occur with S
N
2 for 2° alkyl halides.

 Predict the products
Cl RO
-
Cl
CH
3
CH
2
OH
Br
RO
-

E2 Competes with S
N
Alkyl Halides
2 for 2°

Base
Substrate
Leaving group
Solvent
E1 weak ones work
E2 strong
3°>2° 3°>2°>1° good good polar, ionizing polar, aprotic is better

E1
Rate =k[RX]
E2
=k[RX][Base]
Carbocation intermediate?
Y
Stereochemistry Zaitsev’s rule
Rearrangement ~H, ~ CH
3 possible
N
Zaitsev’s rule
No rearrangements

How Do You Decide Which
Reaction(s) Happen?
 The strength and structure of the nucleophile/base is paramount.
 Strong ones always give a
2 or second order reaction (S
E2).
 If the base is bulky, E2 will occur instead of S
N
2.
N

How Do You Decide Which
Reaction(s) Happen?
 Now, look at the structure of the substrate.
 1° alkyl halides will undergo S possible.
N
2 and perhaps E2 unless rearrangement is
 3° alkyl halides will NOT undergo S
N
2.
 2° alkyl halides are the toughest to predict.

How Do You Decide Which
Reaction(s) Happen?
 Evaluate, in order,
 Nucleophile, strength and structure
 -or- Base, strength and structure
 Structure of the alkyl halide/carbocation
 Leaving group
 Solvent
 YOU MUST BE ABLE TO DRAW THE
MECHANISM!

 Predict the products
Br
Na
+ -
OCH
2
CH
3 ethanol
CH
3
OH
OCH
3

Did this product come from the reaction of 1-chlorobutane with a) sodium acetate or b) sodium t-butoxide?

This product came from the reaction of 1-chlorobutane with sodium acetate. The C=O and C-O peaks show ester.
O
O
C
CH
3

Did this product come from the reaction of 1-chloropentane with a) sodium acetate or b) sodium t-butoxide?

This product came from the reaction of 1-chloropentane with sodium t-butoxide. The sp 2 C-H and C=C peaks show alkene.