Unimolecular Elimination Reaction

Unimolecular Elimination Reaction

Contents

• Unimolecular elimination reaction

• First order kinetics

• Mechanism

• Orientation and reactivity

• Compare and contrast E2 and E1 reactions

Learning Objectives

At the end of this lecture, student will be able to

• Explain kinetics and mechanism of E1 reaction

• Outline reactivity and orientation in E1 reaction

• Compare and contrast E2 and E1 reactions

E1 Reaction

• Definition

– Unimolecular elimination reaction

• Example

(CH3)3CCl       +      H2O   ——→  CH2 = C(CH3)2    +  H3O+     + Cl‾

3°butyl chloride                                  2-methyl propene

• Kinetics

– 1st order kinetics

– Rate = k [RX]

• Step 1

– Formation of carbocation

– Rate determining step

• Step 2

– Formation of alkene

• Orientation

– Saytzeff rule

• Reactivity

– Order of reactivity of alkyl halides

• Rearrangement in E1 reaction

E2 vs E1

• Effect of the substrate

– Reactivity of alkyl halides in elimination reactions

– Relative reactivity of alkyl halides in E2 reaction

• Effect of the substrate

– Relative reactivity of alkyl halides in E1 reaction

• Effect of the substrate

– Relative reactivity of alkyl halides in E2 and E1 reactions

• Role of the base

– High concentration of strong base favours E2 reaction

– Weak base favors E1 reaction

Summary

• E1 reaction is unimolecular elimination reaction

• E1 reaction takes place in 2 steps that involves slow ionisation to give carbocation, removal of a proton by base

• Tertiary alkyl halides readily undergoes E1 reaction

• Major product is the more stable alkene as given by Saytzeff rule

• E2 reaction is favoured by strong base

• E1 reaction is favoured by weak base

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