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