E1 and E1cB Reactions

1. Hafiz Muhammad Athar Rizwan
Organic Chemistry

2. In an elimination reaction some fragments of a
molecule are removed.
Elimination reaction are classified into two general
headings
 α-eliminations
 β-eliminations i

3.  α-eliminations in which two groups are eliminated from
the same atom.
In this case unstable species are formed which undergo
further reactions.

4.  β-eliminations in which groups on adjacent atoms
are eliminated with the formation of multiple bonds.
H
Base
C C C C
X

5. β-eliminations proceeds through two mechanisms which
are
 E2 bimolecular elimination reactions
 E1 unimolecular elimination reaction
The E2 and E1 mechanisms differ in the timing of
bond cleavage and bond formation, analogous to the SN2 and
SN1 mechanisms.

6. E1 indicates a elimination, unimolecular reaction.
The E1 reaction proceeds via a two-step mechanism:
the bond to the leaving group breaks first before the
π bond is formed. The slow step is unimolecular,
involving only the alkyl halide.

7. The dehydrohalogenation of (CH3)3CI with H2O to form
(CH3)2C=CH2 can be used to illustrate the E1 mechanism.

8. Energy diagram for E1 reaction

9. 1. Dehydrohalogenation (-HX) and
2. Dehydration (-H2O)

11. CH3 CH3
slow H
rapid CH3
CH3 C Br CH3 C + + B H + Br-
- Br- C C
CH3 H C H
CH3 H
H B:-

12. R H R R
strong acid
R C C R C C + H2O
OH R R R

13. 1) CH3 CH3
+
CH3 C CH3 + H CH3 C CH3
OH + OH2
2)
CH3 CH3
slow
CH3 C CH3 CH3 C CH3 + H2O
+
+ OH2
3)
CH3 CH3
+
CH3 C CH3 C CH2 + H
+ CH3

15. 1) Substrate effect
2) Base effect
3) Isotope effect
4) Orientation of elimination

16. The order of the reactivity of the alkyl groupsis
Tertiary> Secondary>Primary
This is because the rate determining step is the
formation of carbocation and the stability of
these ions increases.

17. Tertiary (3o) > secondary (2o) > primary (1o)
It is hard (but not impossible) to get primary
compounds to go by E1. The reason for this is
that primary carbocations are not stable!

21.  Bulky Bases Favor Elimination

22.  E1 reactions do not show an isotope effect:
kH/kD = 1
 This tells us that the C-D or C-H bonds are not
broken in the rate determining step
(step 1). They are broken in the fast step (step 2) in
the mechanism).

23.  E1 reactions faithfully follow Zaitsev’s rule!
 This means that the major product should be the
product that is the most highly substituted.

24.  In reactions of removal of hydrogen halides
from alkyl halides or the removal of water
from alcohols, the hydrogen which is lost will
come from the more highly-branched b-
carbon.

25. 1. Kinetics
2. Stereochemistry
3. Rerangement

26.  E1 reactions follow first order (unimolecular)
kinetics:
Rate = k [R-X]1
The solvent helps to stabilize the carbocation,
but it doesn’t appear in the rate law!!

27. 1)
H H
slow _
C C C C + Br
rate determining +
Br step
H .. +
H ..
2) O: O
H H
H
H fast
+ C C
C C
+
 Rate law: rate = k [R-Br]1

28.  E1 reactions do not require an anti coplanar orientation of H
and X.
 Diastereomers give the same products with E1 reactions,
including cis- and trans products.
 E1 reactions usually give the thermodynamically most stable
product as the major product. This usually means that the
largest groups should be on opposite sides of the double bond.
Usually this means that the trans product is obtained.

29.  Alkyl groups and hydrogen can migrate in
rearrangement reactions to give more stable
intermediate carbocations.

30. CH3 H CH3 CH3
+
CH3 C C CH3 CH3 C C CH3 CH2 C CH CH3
+
CH3 CH3 H CH3
secondary carbocation tertiary carbocation minor
CH3 CH3
C C
CH3 CH3 CH3
CH3 C CH CH2 major
CH3
trace

31. Factors to Consider:
1. How Basic is the Nucleophile?
2. 2. Steric Hindrance at Reacting Carbon
3. 3. Steric Hindrance at Nucleophile

33.  E1cB mechanism (E-elimination, 1cB-first order with
respect to conjugate base) is one of the three limiting
mechanisms of 1,2-elimination. It is a two-step
mechanism.

34.  Step 1
Step 2

35.  Step two is first order and its reactant is the conjugate base of
the substrate, hence the notation 1cB.
 A 1,2-elimination occurring via E1cB mechanism is called and
E1cB reaction. Stand-alone E1cB reactions are not common,
and they have a complex rate law, meaning that the rate-
limiting step is the second step.
 However, in the most common E1cB reactions, the base is
‾OH and the solvent is water, in which case the rate law
simplifies to

 rate = k[substrate][base]

36.  When the base is strong enough and the leaving group
is very poor(such as fluorine or hydroxyl groups),
E1CB is preferred.