The E2 reaction mechanism involves the base-induced elimination of a hydrogen and halide atom from an alkyl halide, forming an alkene product. The reaction proceeds through a concerted bimolecular transition state where the C-H bond breaks as the C-X bond forms. Stereoselectivity is determined by whether the anti or syn orientation is preferred. Cyclic compounds exhibit varying degrees of stereoselectivity depending on ring size. The orientation of the double bond in the product can be predicted by either Hofmann or Saytzev rules based on the stability of the carbocation intermediate.

1. E2 REACTION MECHANISM
Immanuelle Orchidea
By PresenterMedia.com

2. Keywords :
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bimolecular
concerted
dehydrohalogenation
kinetics
mechanism
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periplanar ("syn"
and "anti")
regioisomers
second-order
reaction
staggered
conformation
stereoisomers

3. Overview
The term E2 stands for "elimination
bimolecular.―
The product of an E2 reaction has
one more degree of unsaturation
than the starting materials did.

4. conversion of tert- butyl bromide to isobutylene
the base-induced elimination of "HX"
(dehydrohalogenation) of an alkyl halide
gives rise to an alkene

5. Base (B: ) attacks a
neighboring C—H
bond and begins to
remove the H at the
same time as the
alkene double bond
starts to form and the
X group starts to leave
Transition State
Neutral alkene is
produced when C—H
bond is fully broken and
the X group has departed
with the C –X bond
electron pair

6. Stereoselectivity in E2

7. Stereoselectivity

8. Stereoselectivity
In most simple acyclic cases, ANTI elimination is
found to be very much preferred
The degree of stereoselectivity may be influenced
to some extent by :
-Polarity
-Ion solvating ability (solvent)

9. Stereoselectivity
In cyclic compounds the conformation from which
elimination can take place may to a considerable
extent be enforced by the relative rigidity of the
ring structure.
The following degrees of stereoselectivity were
observed for HY elimination from the cyclic
compounds (CH2)nCHY:
Ring Size
%SYN elimination
Cyclobutyl
90
Cyclopentyl
46
Cyclohexyl
4
Cycloheptyl
37

10. Orientation in E2
In substrates which have alternative bHydrogen atoms available, it is possible to
obtain more than one alkene on elimination.
How do we forecast which alkene is the more
likely to be produced?
Hofmann
rules
Saytzev rules

12. Orientation in E2
Saytzev vs Hofmann
Hofmann :
that alkene will predominate which has LEAST alkyl
substituents on the double bond carbons (karbo
kation yang diserang yang kurang stabil)
Saytzev :
that alkene will predominate which has MOST alkyl
substituents on the double bond carbons (karbo
kation yang diserang yang lebih stabil, yang lebih
banyak mengandung alkil substituen)

14. That increase in the size of Y and more
particularly branching in it, leads to increasing
proportion of HOFMANN elimination with the
same alkyl group.