Organic ReactionsElimination
Dr. Sapna Gupta
Elimination Reaction
• There are two mechanisms for elimination. E1 and E2.
• Product of this reaction is an alkene
• Elimination is called b-elimination (a carbon is the one that has the leaving group and b is the one next to it.)
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Conditions of Elimination
Condition E1 E2
Mechanism 1st order 2nd order
Rate Dependent on only one chemical – the substrate
dependent on two chemicals – the substrate and Nu-
Substrate should form a stable carbocation
should be 1o for better results
Nucleophile Not dependent of Nu should be strong base to cause elimination
Solvent polar Non polar
Competes With SN1 With SN2
Stereochemistry Not stereospecific(gives Zaitsev’s product)
Stereospecific (needs the H and leaving group to be in anti periplanar)
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E2 Mechanism
• Bimolecular elimination
• Requires a strong base
• Halide leaving and proton abstraction happens simultaneously - no intermediate.
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E1 Mechanism
• Unimolecular elimination
• Two groups lost (usually X- and H+)
• Nucleophile will also be the base
• Also have SN1 products (mixture)
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E2 Mechanism -Energy Diagram
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E1 Mechanism –Energy Diagram
Zaitsev’s Rule for Elimination
In the elimination of HX from an alkyl halide, the more highly
substituted alkene product predominates
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Zaitsev’s product
Zaitsev’s product
Hoffman’s product
Hoffman’s product
Regioselectivity and Stereoslectivity of E1/E2
• Regioselectivity:
• E1: major product is the more stable alkene (Zaitsev’s product)
• E2: with strong base, the major product is the more stable (more substituted) alkene (Zaitsev’s product).
• E2: with a strong, sterically hindered base such as tert-butoxide, the major product is often the less stable (less substituted) alkene.
• Stereoselectivity:
• E2 is most favorable (lowest activation energy) when H and LG are oriented anti and coplanar.
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CH3 O:-
C C
H
Lv
CH3 O
H
C C
Lv-H and -Lv are anti and coplanar
(dihedral angle 180°)
Stereoselectivity
• In the examples below for anti periplanar elimination in cyclohexane, the equatorial groups are not in proper alignment for elimination.
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Problem
• Draw the conformers for the following cyclohexane stereoisomers to understand how the products are formed.
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Cl
CH3 O-Na
+
CH3OH
CH3 O-Na
+
CH3OHCl
+
(R)-3-Isopropyl-
cyclohexene
1-Isopropyl-
cyclohexene
(major product)
cis-1-Chloro-2-
isopropyl-
cyclohexane
t rans-1-Chloro-2-
isopropyl-
cyclohexane
(R)-3-Isopropyl-
cyclohexene
Substitution or Elimination?
Substitution Elimination
Nucleophile strength Strong Nu but weak base – SN2
Strong Nu – E2
Nucleophile size Small Bulky
Substrate Primary – SN2 Tertiary - SN1, E1 or E2
Temperature Higher temp - > more E
E2 vs SN23o halide and bulky base promote E2
E1 vs SN1High heat promotes E1
Types of Alkyl Halides
1o – will almost always give SN22o – primarily SN2 except when
using a bulky base3o – SN1, E1 or E2. Strong base
and high temperature promotes E
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Substitution or Elimination?
Look at the conditions and then decide.
• Primary substrate
• If the base is small, SN2 competes strongly because approach at carbon is unhindered
• Secondary substrate
• Approach to carbon is sterically hindered and E2 elimination is favored
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Key Words/Concepts
•Elimination Reaction•Nucleophile•Electrophile•Leaving group•1st order reaction (unimolecular)•2nd order reaction (bimolecular)•Transition state•Rate determining step•Carbocation
•Polar protic solvent•Polar aprotic solvent•Non polar solvent•Solvolysis