DR. SK MOHAMMAD AZIZ

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CHEMISTRY: SEM-II, PAPER- C4T: ORGANIC CHEMISTRY-II, ELIMINATION REACTION

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COMPILED AND CIRCULATED BY DR. SK MOHAMMAD AZIZ,

ASSISTANT PROFESSOR, DEPARTMENT OF CHEMISTRY, NARAJOLE RAJ COLLEGE

Introduction

In this module, different types of elimination reactions are described. From

a practical standpoint, elimination reactions widely used for the generation

of double and triple bonds in compounds from a saturated precursor

molecule. The presence of a good leaving group is a prerequisite in most

elimination reactions. Traditional classification of elimination reactions, in

terms of the molecularity of the reaction is employed. How the changes in

the nature of the substrate as well as reaction conditions affect the

mechanism of elimination are subsequently discussed. The stereochemical

requirements for elimination in a given substrate and its consequence in the

product stereochemistry is emphasized.

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CHEMISTRY: SEM-II, PAPER- C4T: ORGANIC CHEMISTRY-II, ELIMINATION REACTION 3

ELIMINATION REACTIONS

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Basics

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Basics

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More details on β -eliminations

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E ₂ Eliminations

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Note: Often times, E2elimination competes with SN2reactions. This is due to the inherent propensity of the incoming nucleophile also to attack the carbon atom bearing the leaving group. Such situations will lead to substitution products.

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Suggestions on problem solving skills:

Look at the given reaction and reagents first. Do not worry about the complexity of the given substrates. Focus on the ‘site-of-reaction’

In the above examples, Et3N is obviously the base, and ‘OH-group’should be removed. Now, note that OH is a poor leaving group and conversion to O-mesyl is required here (achieved through the use of mesyl chloride, MsCl).

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E ₁ Eliminations

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E ₁ Eliminations

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It can be readily noticed that the carbocation generated in the first step would be stabilized species due to effective delocalization promoted by the presence of two phenyl groups on the tert-carbon.

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Illustration of E1cB mechanism

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E ₁ cB Eliminations

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Mechanistic Continuum

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Variable transition state theory

for elimination reaction

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Important factors that influence

the reaction mechanism

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The relative positions (energies) of TS-I and TS-II can be explained as follows,

Additional information: The product stability (more substituted olefin generated as a result of the elimination reaction) can be explained using the concept of orbital interaction between the CH3 orbitals and that of the pi-bond

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Note: The origin of why Bredt’s rule can be rationalized by considering the need for lateral overlap between two adjacent p-orbitals. Two p-orbitals can’t remain parallel due to the bridged bicyclic structure.

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Other features of directionality of

elimination reactions

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More details on the stereochemical

aspects of elimination reactions

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DR. SK MOHAMMAD AZIZ - Narajole Raj College