PPT Describe elimination reactions, Evidences and mechanism of elimination reaction. Energy profile diagram. RELATIVE stabilities of intermediates. Comparison between E1 & E2 elimination reaction

1. Dr. Pravin N. Khatale
Professor
Dept. of Pharmaceutical Chemistry
Dr. Rajendra Gode Institute of Pharmacy, Amravati.
Email: pnkhatale@gmail.com
ELIMINATION REACTIONS

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.
C C
H
X
Base
C C

6. 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.

7. The E2 elimination
 = k [substrate] [base]
Reaction involves a single step: base pulls a proton away from
carbon; simultaneously a halide ion departs and the double
bond forms. Halogen takes its electron pair with it; hydrogen
leaves the electron pair behind, to form the double bond
In this transition
state, two bonds are
being broken: C-H
and C-X.

9. Effect of the LG on E2 Reactivity

10. Energy Diagram for the E2 Mechanism

11. Evidences For the E2 Mechanism
(a) Follow second-order kinetics; also
(a) Are not accompanied by rearrangements;
(a) Show large hydrogen isotope effect;
(a) Are not accompained by hydrogen exchane; and
(a) Show large element effect

12. (a) Follow second-order kinetics; also
(b) Are not accompanied by rearrangements;
Facts (a) and (b) are, of course, exactly what we
would expect for the E2 mechanism. The rate-
determining step (the only step) involves reaction
between a molecule of alkyl halide and a molecule of
base; the result is second-order kinetics. This single
step simply provides no opportunity for
rearrangement.
Evidences For the E2 Mechanism

13. (c) Show large hydrogen isotope effect;
A difference in rate due to a difference in the isotope
present in the reaction system is called an isotope effect.
If a particular atom is less tightly bound in the transition state of
a reaction than in the reactant, the reaction involving the
heavier isotope of that atom will go more slowly.
Here we will consider primary hydrogen isotope effect: a bond to
protium (H) is broken faster than a bond to deuterium (D)
Evidences For the E2 Mechanism

14. The labeled 2-phenylethyl bromide containing deuterium at both
β-positions was prepared. The rate constant (KD) for its
dehydrobromination by sodium methoxide was measured and
when compared to rate constant (KH) was found (KH)/ (KD) =7,
that is compound containing protium reacts seven times as fast as
compound containg deuterium.

15. (d) Are not accompained by hydrogen exchane;
Evidences For the E2 Mechanism

17. (d) Show large element effect;
Evidences For the E2 Mechanism

18. E2 Reaction: Orientation and Reactivity

19. In sec-butyl bromide attack by base at any one of three β-
hydrogens (on C-1)can lead to the formation of 1-butene; attack
at either of two β-hydrogens (on C-3) can lead to the formation of
2-butene. 2-Butene is preffered over 1-butene despite the
probability factor o 3:2.

20. Russian chemist Alexander Saytzeff in 1875 formuleted
rule: “ In dehydrohalogenation the preffered product is the
alkane that has the greater number of alkyl group attached
to the doubly bonded carbon atom.

22. 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.
The E1 elimination

23. The dehydrohalogenation of (CH3)3CI with H2O to form
(CH3)2C=CH2 can be used to illustrate the E1mechanism.

24. Energy diagram for E1 reaction

26. Evidences For the E1 Mechanism
(a) Follow First-order kinetics; also
(a) Are not accompanied by a primaary hydrogen
isotope effect;
(a) Where the structure permits, are accomapined by
rearrangements;
(a) Show the effect of structure on reactivity as SN1
reactions do.

27. Reactivity in E1
Tertiary> Secondary>Primary
In E1, as in SN1, reactivity is determined by the rate of
formation of the carbocation; and this depends upon
stability of the carbocation.
Where the structutes permits , first order eliminations
are accomapined by rearrangement.

30. The E1 reaction: Orientation

31. How fast the substrate reacts is determined by the rate of step
(1). But which alkane is produced is clearly determined by which
β-proton is lost faster from the carbocation in step (2). The 2-
pentyl cation, for example, can lose either a proton from C-3 to
form 2-pentene or a proton from C-1

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

33. 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.

34. 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!

38. 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.

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