Download E2 reactions

E2 Reactions
 E2 = elimination, bimolecular
 Reaction is concerted (one step)
 Rate = k[substrate][base]
(a second order process)
 E2 can occur with SN2.
 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.
E2 Reaction Mechanism
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One-step and requires a strong base.
Best transition state is anti-coplanar.
Example: t-butyl bromide + methoxide
Why don’t SN2, SN1, or E1 occur?
E2 Reaction Mechanism
 Example: t-butyl bromide + methoxide
E2 Reaction Mechanism
 Anti-periplanar alignment allows pi
bond to form from sp3 orbitals.
E2 Reaction Profile
rate = ?
k=?
Factors Affecting E2 Reactions
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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 SN1 or SN2).
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?
H2O
CH2
OH
N
Factors Affecting E2 Reactions the Leaving Group
 The LG should be good. Which of
the following substrates have
good LGs?
Cl
OCH3
H
NH2
O
O S
O
CH3
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?
E2 Reactions - Summary
 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 SN2 for 2° alkyl halides.
E2 Reactions - Summary
 Predict the products
Cl
Cl
Br
RO-
CH3CH2OH
RO-
E2 Competes with SN2 for 2°
Alkyl Halides
E1 and E2 Reactions
Base
Substrate
Leaving group
Solvent
E1
E2
weak ones work
strong
3°>2°
3°>2°>1°
good
good
polar, ionizing
polar, aprotic is
better
E1 and E2 Reactions
E1
E2
=k[RX]
=k[RX][Base]
Y
N
Stereochemistry
Zaitsev’s rule
Zaitsev’s rule
Rearrangement
~H, ~ CH3
possible
No
rearrangements
Rate
Carbocation
intermediate?
How Do You Decide Which
Reaction(s) Happen?
 The strength and structure of
the nucleophile/base is
paramount.
 Strong ones always give a
second order reaction (SN2 or
E2).
 If the base is bulky, E2 will
occur instead of SN2.
How Do You Decide Which
Reaction(s) Happen?
 Now, look at the structure of
the substrate.
 1° alkyl halides will undergo SN2 and
perhaps E2 unless rearrangement is
possible.
 3° alkyl halides will NOT undergo SN2.
 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!
Elimination Reactions
 Predict the products
Br
Na+ -OCH2CH3
ethanol
CH3OH
OCH3
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
CH3
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 sp2C-H and C=C peaks show
alkene.