1. Elimination Reactions

2. • Elimination reactions involve the loss of elements from
the starting material to form a new π bond in the
product.
General Features of Elimination

3. • Equations [1] and [2] illustrate examples of elimination
reactions. In both reactions a base removes the
elements of an acid, HX, from the organic starting
material.
Elimination Reaction with Strong Bases

4. • Removal of the elements HX is called
dehydrohalogenation.
• Dehydrohalogenation is an example of β elimination.
• The curved arrow formalism shown below illustrates
how four bonds are broken or formed in the process.
Dehydrohalogenation

5. • The most common bases used in elimination reactions
are negatively charged oxygen compounds, such as HO¯
and its alkyl derivatives, RO¯, called alkoxides.
Bases used to Promote Elimination Rxns.

6. • To draw any product of dehydrohalogenation—Find the
α carbon. Identify all β carbons with H atoms. Remove
the elements of H and X form the α and β carbons and
form a π bond.
Products of Elimination

8. • Recall that the double bond of an alkene consists of a σ
bond and a π bond.
Alkenes—The Products of Elimination

9. • Alkenes are classified according to the number of
carbon atoms bonded to the carbons of the double bond.
Classifying Alkenes

10. • Recall that rotation about double bonds is restricted.
No Free Rotation Around a Double Bond!

11. • Because of restricted rotation, two stereoisomers of 2-
butene are possible. cis-2-Butene and trans-2-butene are
diastereomers, because they are stereoisomers that are
not mirror images of each other.
Diastereomers

13. • In general, trans alkenes are more stable than cis
alkenes because the groups bonded to the double bond
carbons are further apart, reducing steric interactions.
Cis/Trans-Alkenes (E/Z)
(E) Alkene
(Z) Alkene

14. • The stability of an alkene increases as the number of R
groups bonded to the double bond carbons increases.
Order of Alkene Stability
• The higher the percent s-character, the more readily an atom
accepts electron density. Thus, sp2
carbons are more able to
accept electron density and sp3
carbons are more able to
donate electron density.
• Consequently, increasing the number of electron donating
groups on a carbon atom able to accept electron density
makes the alkene more stable.

15. • trans-2-Butene (a disubstituted alkene) is more stable
than cis-2-butene (another disubstituted alkene), but
both are more stable than 1-butene (a monosubstituted
alkene).
Stability of Trans Substituted Alkenes

17. Mechanism of the
Elimination Reaction

18. • There are two mechanisms of elimination—E2 and E1.
• E2 mechanism—bimolecular elimination
• E1 mechanism—unimolecular elimination
• The E2 and E1 mechanisms differ in the timing of bond
cleavage and bond formation, analogous to the SN2 and
SN1 mechanisms.
• E2 and SN2 reactions have some features in common, as
do E1 and SN1 reactions.
Mechanisms of Elimination

19. • The most common mechanism for dehydrohalogenation
is the E2 mechanism.
• It exhibits second-order kinetics, and both the alkyl
halide and the base appear in the rate equation i.e.
E2 Mechanism
rate = k[(CH3)3CBr][¯OH]
• The reaction is concerted - all bonds are broken and
formed in a single step.

20. Writing the E2 Mechanism

21. Transition State for an E2 Reaction

22. • There are close parallels between E2 and SN2 mechanisms in
how the identity of the base, the leaving group and the
solvent affect the rate.
• The base appears in the rate equation, so the rate of the E2
reaction increases as the strength of the base increases.
• E2 reactions are generally run with strong, negatively charged
bases like¯OH and ¯OR. Two strong sterically hindered
nitrogen bases called DBN and DBU are also sometimes used.
Strong, Bulky Bases Favor E2 Mechanism

23. DBN Used to Promote an E2 Reaction

24. Nature of the Leaving Group in E2
Reactions

26. • The SN2 and E2 mechanisms differ in how the R group
affects the reaction rate.
Differences Between E2 and SN2 Reactions

27. • The increase in E2 reaction rate with increasing alkyl
substitution can be rationalized in terms of transition state
stability.
• In the transition state, the double bond is partially formed.
Thus, increasing the stability of the double bond with alkyl
substituents stabilizes the transition state (i.e. lowers Ea,
which increases the rate of the reaction according to the
Hammond postulate).
Increasing Substitution Increases rate of E2 Reaction

28. • Increasing the number of R groups on the carbon with the
leaving group forms more highly substituted, more stable
alkenes in E2 reactions.
• In the reactions below, since the disubstituted alkene is more
stable, the 30
alkyl halide reacts faster than the 10
alkyl halide.
Comparing E2 Reactions

29. Overview of the E2 Mechanism

30. Examples of E2 Reactions with Bulky Bases

32. • Recall that when alkyl halides have two or more different β
carbons, more than one alkene product is formed.
• The major product is the more stable product—the one with
the more substituted double bond.
• This phenomenon is called the Zaitsev rule.
The Zaitsev (Saytzeff) Rule

33. • A reaction is regioselective when it yields predominantly or
exclusively one constitutional isomer when more than one is
possible. Thus, the E2 reaction is regioselective.
The Zaitsev (Saytzeff) Rule

34. • When a mixture of stereoisomers is possible from a
dehydrohalogenation, the major product is the more stable
stereoisomer.
• A reaction is stereoselective when it forms predominantly or
exclusively one stereoisomer when two or more are possible.
• The E2 reaction is stereoselective because one stereoisomer
is formed preferentially.
Stereoselective E2 Reactions

36. • The dehydrohalogenation of (CH3)3CI with H2O to form
(CH3)C=CH2 can be used to illustrate the second general
mechanism of elimination, the E1 mechanism.
• An E1 reaction exhibits first-order kinetics:
The E1 Mechanisms
rate = k[(CH3)3CI]
• The E1 reaction proceed 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 and E2 mechanisms both involve the same number of
bonds broken and formed. The only difference is timing. In an
E1, the leaving group comes off before the β proton is
removed, and the reaction occurs in two steps. In an E2
reaction, the leaving group comes off as the β proton is
removed, and the reaction occurs in one step.

38. Energy Diagram for an E1 Reaction

40. • The rate of an E1 reaction increases as the number of R groups
on the carbon with the leaving group increases.
Increasing the Rate of an E1 Reaction
• The strength of the base usually determines whether a reaction
follows the E1 or E2 mechanism. Strong bases like ¯OH and ¯OR
favor E2 reactions, whereas weaker bases like H2O and ROH
favor E1 reactions.

41. • E1 reactions are regioselective, favoring formation of the
more substituted, more stable alkene.
• Zaitsev’s rule applies to E1 reactions also.
Zaitsev’s rule for E1 Reactions

42. Overview of the E1 Reaction

43. • SN1 and E1 reactions have exactly the same first step—formation
of a carbocation. They differ in what happens to the carbocation.
SN1 vs E1 Reactions
• Because E1 reactions often occur with a competing SN1 reaction,
E1 reactions of alkyl halides are much less useful than E2
reactions.

45. • The transition state of an E2 reaction consists of four atoms from
an alkyl halide—one hydrogen atom, two carbon atoms, and the
leaving group (X)—all aligned in a plane. There are two ways for
the C—H and C—X bonds to be coplanar.
Stereochemistry of the E2 Reaction

46. Anti Conformation Preferred for the E2 Reaction
• E2 elimination occurs most often in the anti periplanar geometry.
This arrangement allows the molecule to react in the lower
energy staggered conformation, and allows the incoming base
and leaving group to be further away from each other.

47. • The stereochemical requirement of an anti periplanar geometry
in an E2 reaction has important consequences for compounds
containing six-membered rings.
• Consider chlorocyclohexane which exists as two chair
conformers. Conformer A is preferred since the bulkier Cl group
is in the equatorial position.
Elimination in Cyclic Systems
• For E2 elimination, the C-Cl bond must be anti periplanar to the
C—H bond on a β carbon, and this occurs only when the H and
Cl atoms are both in the axial position. The requirement for trans
diaxial geometry means that elimination must occur from the
less stable conformer, B.

48. For Elimination, the Cl group must be in the in the
axial position of Cyclohexane Ring

49. Consider the E2 dehydrohalogenation of cis- and
trans-1-chloro-2-methylcyclohexane
• This cis isomer exists as two conformers, A and B, each of
which as one group axial and one group equatorial. E2 reaction
must occur from conformer B, which contains an axial Cl atom.

50. • Because conformer B has two different axial β
hydrogens, labeled Ha and Hb, E2 reaction occurs in two
different directions to afford two alkenes.
• The major product contains the more stable
trisubstituted double bond, as predicted by the Zaitsev
rule.
E2 Reaction with the Cis Isomer

51. • The trans isomer of 1-chloro-2-methylcyclohexane exists
as two conformers: C, having two equatorial substituents,
and D, having two axial substituents.
E2 Reaction with the Trans Isomer
• E2 reaction must occur from D, since it contains an axial Cl
atom.

52. • Because conformer D has only one axial β H, E2 reaction
occurs only in one direction to afford a single product. This
is not predicted by the Zaitzev rule.
Result of the E2 Elimination

54. Alkyne Synthesis

55. • Two elimination reactions are needed to remove two
moles of HX from a dihalide substrate.
• Two different starting materials can be used—a vicinal
dihalide or a geminal dihalide.
E2 Reactions and Alkyne Synthesis

56. • Stronger bases are needed to synthesize alkynes by
dehydrohalogenation than are needed to synthesize
alkenes.
• The typical base used is ¯NH2 (amide), used as the
sodium salt of NaNH2. KOC(CH3)3 can also be used with
DMSO as solvent.
Types of Bases needed for Alkyne Synthesis

57. • The reason that stronger bases are needed for this
dehydrohalogenation is that the transition state for the
second elimination reaction includes partial cleavage of
the C—H bond. In this case however, the carbon atom is
sp2
hybridized and sp2
hybridized C—H bonds are
stronger than sp3
hybridized C—H bonds. As a result, a
stronger base is needed to cleave this bond.
E2 Reactions of Alkenes

59. Predicting the Mechanism
from the Reactants—SN1,
SN2, E1 or E2.

60. • The strength of the base is the most important factor in
determining the mechanism for elimination. Strong
bases favor the E2 mechanism. Weak bases favor the E1
mechanism.
When is the Mechanism E1 or E2

61. • Certain anions always give products of substitution
because they are good nucleophiles but weak bases.
These include I¯, Br¯, HS¯, and CH3COO¯.
Good nucleophiles/weak bases favor
substitution over elimination
Favoring SN2 Reaction

62. • KOC(CH3)3, DBU, and DBN are too sterically hindered to
attack tetravalent carbon, but are able to remove a small
proton, favoring elimination over substitution.
Bulky nonnucleophilic bases favor
elimination over substitution
Favoring E2 Reaction

63. Overview - (SN1, SN2, E1 or E2 Reactions)

64. Overview - (SN1, SN2, E1 or E2 Reactions)

65. Overview - (SN1, SN2, E1 or E2 Reactions)