organic chemistry, E2 mechanism, mechanism

1. LOGO
ELIMINATION REACTION OF
ALKYL HALIDES

2. ELIMINATION REACTIONS
 Involve in loss of elements from the starting material
to form a new  bond in the product.

3. ELIMINATION REACTIONS
 Removal of the elements of HX is called
dehydrohalogenation.
 Dehydrohalogenation is an example of β
elimination.

4. Leaving group:
 Leaving groups are the fragments that retain the
electrons in a heterolytic bond cleavage.
 Weaker bases are more stable with the extra pair
of electrons and therefore make better leaving
groups.
 H2O > OHˉ
 Iˉ > Brˉ > Clˉ > Fˉ

5. E2 MECHANISMS
 The E2 mechanism (Bimolecular Elimination)
 The most common mechanism for dehydrohalogenation
is the E2 mechanism.
 E2 eliminations are promoted by a strong base.

6.  The reaction is bimolecular and it involved “second-
order kinetics” because two molecules must come
together for the reaction to occur.
 Second order kinetics is the rate of reaction depends on
the concentration of both the base and the substrate.
 Since this is a one step mechanism, this is the slow step,
and therefore controls the rate of reaction.
 Where,
Rate = k [base] [substrate]

7.  rate = k[(CH3)3CBr][¯OH]
 An energy diagram for an E2 reaction:
In the transition
state, the C-H
and C-Br bonds
are partially
broken and the
O-H and  bonds
are partially
formed.

8. Lithium Hydroxide, LiOH
Rubidium Hydroxide, RbOH
Sodium Hydroxide, NaOH
Potassium Hydroxide, KOH
Strontium Hydroxide, Sr(OH)2 Barium Hydroxide, Ba(OH)2
Examples of strong base:
Cesium Hydroxide, CsOH Calcium Hydroxide, Ca(OH)2

9.  The identity of the Alkyl Halide :

10. The mechanism of an E2 elimination reaction:
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.
Neutral alkene is produced
when the C=H bond is fully
broken and the X group has
departed with the C-X bond
electron pair.

11.  Notice that the hydrogen that is removed is on
the carbon that is adjacent to the one bearing the
halogen
 Likewise, the “H” and the “X” atoms that are
eliminated during the dehydrohalogenation of an
alkyl halide must be on the carbon atoms.

12. Characteristics of the E2
Mechanism

13. SAYTZEFF’S RULE
 The major product of dehalogenation is the most stable
alkene.
 The most stable alkene is the most substituted C=C due
to the electron donating properties of the alkyl group.

14. MARKOVNIKOV’S RULE
 The hydrogen (H) is attached to the C with less alkyl
substituents and the halide (X) attached to the C with
more alkyl substituents.
 Major product has the most stable carbocation
intermediate during the addition process.

15.  The most stable carbocation is the more substituted
carbocation due to induction and hyperconjugation.
 A carbon rich in subtituents will gain more substituents
and the carbon with more hydrogens attached will get the
hydrogen.
 Inductive effect is an experimentally observable effect of
the transmission of charge through a chain of atoms in a
molecule by electrostatic inductions.
 Hyperconjugation is the interaction of the electron in a
sigma bond (usually C–H or C–C) with an adjacent empty
(or partially filled) non-bonding p-orbital or antibonding π
orbital or filled π orbital, to give an extended molecular
orbital that increases the stability of the system.

16. ANTI MARKOVNIKOV’S
REACTION
 Anti-Markovnikov is exactly opposite of Markovnikov
reaction.
 The hydrogen (H) is attached to the C with less alkyl
substituents and the halide (X) attached to the C with
more alkyl substituents.
 Anti-Markovnikov behavior extends to other chemical
reactions than just additions to alkenes.
 Another famous example of anti-markovnikov addition is
hydroboration.

17. STEREOCHEMISTRY OF THE
E2 REACTION
 In formation of a pi bond, the C-H and C-LG bonds all
aligned in a plane to be coplanar.
 When C-H bond and C-LG bond at 1800 with respect
each other (opposite side), it will called anti-periplanar
transition state that has staggered conformation with
lower energy.
 When C-H bond and C-LG bond at 00 with respect each
other (same side), it will called syn periplanar transition
state which is in eclipsed conformation with higher
energy.
 The staggered conformation (anti-periplanar) has more
stable than eclipsed conformation (syn-periplanar).

18.  X is a leaving group(LG).

19.  E2 often occur in anti-periplanar.
 Anti-periplanar - involves a base reacting with the proton
anti-periplanar to the leaving group (that simultaneously
leaves) in a single step to give an alkene.
 The order in stability of alkene according to Zaitsev’s
rule:
<--- More stable alkene --- --- Less stable alkene --->

20.  In cyclohexyl halide such as bromocyclohexane, the more
stable conformation has bromine in an equatorial position.
As shown at left below, in this conformation, there are no
β hydrogens anti to the bromine. In this conformation, an
E2 elimination of HX to form cyclohexane is not possible.
 In the less stable chair conformation illustrated above
right, the bromine is in an axial position.There are
hydrogen atoms anti to the bromine on both of the
adjacent (β) carbons, so E2 elimination of HX is possible.

21.  For cyclohexyl halides, the requirement for an
anti-periplanar transition state geometry means
that the halide leaving group must be axial, never
equatorial. This has important consequences for
how cyclohexane derivatives react.

22.  Explanation: Exists as two conformations (A and
B), each of which has one group axial and one
group equatorial.
 E2 occur from conformation B.

23.  Explanation: Exists as two conformations, C
having two equatorial substituents and D having
two axial substituents.
 E2 occur from conformation D.

24. STEREOSELECTIVITY
 Eliminations often favor the more stable trans-
product over the cis-product.

25.  The product of E2 reaction as predicted by the Zaitsev
rule (Saytzeff’s rule)
 Examples:

26.  This reaction prefers an anti orientation of the
halogen and the beta-hydrogen which is attacked
by the base. These anti orientations are colored
in red in the above equations.
 In conclusion, the substituted cyclohexanes, E2
elimination must occur with a trans diaxial
arrangement of H and LG to gives the product.

27. GROUP MEMBERS
Hani Liyana Binti Rahmat UK26352
Nurul Amalina Anati Binti Abdullah UK26346
Amy Madzirah Binti Ramlan UK26347
Norshafiqah Binti Mohamad Saidi UK26360
Wan Azwira Binti Ab Ghani @ W.Ahmad UK26257
Nurul Syazdiana Binti Mohd Zuki UK26258
Sharifah Nurul Aina Binti Sayed Burhanudin UK26335
Nur Hananun Binti Mohd Azimi UK26314
Nurul Huda Binti Alias UK26294
Nadia Farahana Binti Muhammad UK26339

28. REFERENCES
 http://www.personal.psu.edu/the1/e2.htm
 http://library.tedankara.k12.tr/carey/ch5-7.html
 http://www.enc.edu/~timothy.t.wooster/courses/CH321/le
cturenotes/chapter6/E2.htm
 http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/al
halrx3.htm
 http://www.mhhe.com/physsci/chemistry/carey/student/ol
c/graphics/carey04oc/ref/ch05eliminationreactions.html

29. LOGO