1. OTHER POSSIBLE MECHANISMS
ELIMINATION
OTHER POSSIBLE MECHANISMS
Do some elimination reactions
occur in a different fashion?

2. Three types of elimination reactions are conceivable
just
studied
concerted E2
carbocation E1
halogen
first
proton
second
carbanion
E1cb
proton
first
halogen
second

3. E1 ALKYL HALIDES + WEAK BASE (SOLVOLYSIS)
The removal of a b-hydrogen becomes difficult without
a strong base and a different mechanism (ionization)
begins to take place
….. if the substrate is capable.

4. The E1 Elimination Reaction (two steps)
weak
base
carbocation
slow :X +
step one
3o > 2o > 1o
also favored
if a resonance
stabilized
carbocation
is formed
unimolecular
step
two
fast
rate = k [RX]
Works best in a
polar solvent.
IONS
FORMED

5. E1 ENERGY PROFILE two step reaction carbocation intermediate TS1 E N R
starting
material
step 1
step 2 DH
slow
product

6. STEREOSPECIFICITY

7. H H C C C C X C C C C H X H + + THE E1 REACTION IS NOT STEREOSPECIFIC
( THE OPEN CARBOCATION IS PLANAR AND CAN ROTATE) H
Carbocation is
sp2 hybridized
( planar )
and can react
from either side. H +
anti C C C C
rotation X
These two
carbocations
are equivalent
by rotation and
by symmetry.
syn C C C C H X H +
rate of C-C rotation
= to 1012 / sec
Elimination can be either syn or anti.

8. REGIOSELECTIVITY

9. E1 REACTION IS REGIOSELECTIVE
THE ZAITSEV RULE IS FOLLOWED
(stereochemistry is not a problem as in E2)
very dilute base
0.001 M
KOH / EtOH
major
minor
tertiary
trisubstituted
disubstituted
Zaitsev

10. DIFFERENCES BETWEEN
E1 AND E2

11. WITH INCREASING BASE CONCENTRATION
BEHAVIOR OF THE RATE
WITH INCREASING BASE CONCENTRATION E2
rate = k2 [RX] [B]
second order
E1 dominates
at low base
concentration
E2 dominates
at higher base
concentration
rate = k1 [RX]
first order
Rate E1
[RX] constant, [B] increasing

12. EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS
k2 [RX] [B]
At high base concentration
E1 never has a chance. E2
At low base concentration
E2 is nonexistent
k1 [RX]
tertiary RX, k’’ 1
secondary RX, k’ E1
Rate 1
primary RX, k 1
[RX] constant, [Base] increasing
For E1 elimination : k’’ (tertiary) > k’ (secondary) > k (primary).

13. EFFECT OF BASE CONCENTRATION ON E1/E2 REACTIONS
tertiary
k’’ 2
secondary k’ E2 2 k
k2 [RX] [B] 2
primary
k1 [RX]
tertiary RX, k’’ 1
secondary RX, k’ E1
Rate 1
primary RX, k
[RX] constant, [B]
For E2 elimination : line slopes k2 differ for 1o,2o,3o .
Different substrates react at different rates,

14. STRUCTURE OF SUBSTRATE R H H
R-C-X
R-C-X
R-C-X R
primary R
secondary H
tertiary
Obviously for E1 which forms a carbocation intermediate
rate : tertiary > secondary > primary > methyl
But this same order holds for E2 also.
tertiary has more
b -hydrogens
more opportunites
for reaction
EtO-

15. WHEN THE E1 MECHANISM OCCURS
E1 occurs only
1) at zero or low base concentration
2) with solvolysis (the solvent is the base)
3) with tertiary and resonance capable
substrates (alkyl halides)
If a strong base is present in moderate
to high concentration, or the substrate
is a primary halide, the E2 reaction
dominates.

16. ALKYL HALIDE + BASE E2 mechanism E1 mechanism strong base
high base conc.
weak base
low base conc. or
solvolysis
(solvent is base)
E2 mechanism
E1 mechanism
anti-coplanar
requirement
must be able to make
“good” carbocation
stereospecific
not stereospecific
regioselective
regioselective

17. rate = k [RBr] [OEt] rate = k [RBr] E2 E1 EXAMPLES .. :O E t C H B r C3 C H 2 N a O E t E t O H E1 C H 3 +
0.01 M C H B r 3 K O H E t O H
rate = k [RBr]

18. SOLVOLYSIS
The solvent is the thing !

19. SOLVOLYSIS SOLVOLYSIS = MANY E1 REACTIONS ARE SOLVOLYSIS REACTIONS
THE SOLVENT IS THE REAGENT (BASE)
competing product E1
EtOH adds to the
carbocation
EtOH solvent
acts as base -
no other base
is present

20. SOMETIMES E1 AND E2 RESULTS DIFFER

21. A COMPARISON OF E1 AND E2 major product E2 syn Anti-Zaitsev H NaOEt HBr H H
stereospecific
anti
EtOH / D H H
CH3
CH3
EtOH / D E1
anti
Zaitsev
not
stereospecific
CH3
E1 doesn’t require
anti-coplanarity