1. Elimination Reaction Class : M.Sc. I
Rayat Shikshan Sanstha’s
S. M. Joshi College, Hadapsar, Pune
Post Graduate Department of Chemistry
Elimination Reaction Class : M.Sc. I
Pradeep N. Jadhav
M.Sc , NET

2. Elimination Reactions
(i) Understand E2 and E1 reaction mechanisms
(ii) Understand how experimental evidence from rate equations and stereo chemical outcomes in the product lead to the proposal of reaction mechanisms
(iii) Understand the experimental factors which favour E2 or E1 reaction mechanisms
(vi) Understand the term antiperiplanar in the context of E2 reaction mechanisms
(v) Understand that in assessing the reaction outcome in an elimination reaction, the stereo electronic of the alkyl halide needs to be considered carefully, i.e.. the constitution and conformation
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3. Elimination ReactionsBH
Descriptor Rate Equation Stereochemical
Outcome
E2 rate = k[R-Hal][Nu] Retension
E1 rate = k[R-Hal] Loss of
Stereochemistry
Clearly, two different reaction mechanisms must be in operation.
It is the job of the chemists to fit the experimental data to any proposed mechanism
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4. The E2 Reaction Mechanism
Compare to SN2
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5. Post Graduate Dept. of Chemistry
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6. The E1 Reaction Mechanism
Compare to SN1
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7. Reactive Intermediate
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8. H, C, C and Cl are antiperiplanar
Stereochemistry Compared E2 E1
H, C, C and Cl are antiperiplanar
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9. Alkene Stability Stability Increases Post Graduate Dept. of Chemistry
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10. Statistically favoured!
Constitutionally Different Eliminations
Base
Constitutional
Isomers
Statistically favoured!
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11. Conformational Equilibria
High Energy
Transition State
Conformational Equilibria
Diastereoisomers
Conformers
Low Energy
Transition State
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12. High Energy Transition State Low Energy Transition State
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13. Cyclohexane Rings – E2
Two C-H bonds are antiperiplanar to the C-Cl bond
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14. Cyclohexane Rings – E1
No C-H bonds are antiperiplanar to the C-Cl bond
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15. Elimination Reactions
The difference in electronegativity between the carbon and chlorine atoms in the C-Cl sigma () bond result in a polarised bond, such that there is a partial positive charge (+) on the -carbon atom and a slight negative charge (-) on the halogen atom, which in turn is transmitted to the -carbon atom and the protons associated with it. Thus, the hydrogen atoms on the -carbon atom are slightly acidic. Thus, if we react haloalkanes with bases (chemical species which react with acids), the base will abstract the proton atom, leading to carbon-carbon double bond being formed with cleavage of the C-Cl bond.
The mechanism of this -elimination (or 1,2 elimination) can take two limiting forms described as Bimolecular Elimination (E2) and Unimolecular Elimination (E1).
The E2 mechanism fits with a rate equation which is dependent on both the base and haloalkane, and that the product retains the stereochemical information about the C-C bond. This retension of stereochemical integrity requires an antiperiplanar relationship of the eliminated atoms.
In contrast, The E1 mechanism fits with a rate equation which is dependent on only the haloalkane, and that the product undergoes a loss of the stereochemical information about the C-C bond. Thus, with appropriately substituted haloalkane a pair of diastereomeric alkenes are formed, as result of rotation around the C-C bond upon formation of the carbocationic intermediate.
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16. Exercise 1: Substitution/Elimination Reactions
Rationalise the experimental results that when 1 is reacted with NaOEt in EtOH, two alkenes are formed, whereas 2 under the same conditions affords an inverted substitution product.
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17. Answer 1: Substitution/Elimination Reactions
Rationalise the experimental results that when 1 is reacted with NaOEt in EtOH, two alkenes are formed, whereas 2 under the same conditions affords an inverted substitution product.
As 2 undergoes an inversion of stereochemistry one must assume SN2 mechanism.
As 1 is subject to the same reaction conditions as 2 one must assume that elimination of HCl does not involve the formation of a carbocation, and thus E2 mechanism must operate.
SN2 E2 E2
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18. Exercise 2: Elimination Reactions
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19. Answer2: Elimination ReactionsH C l E t O
TS2
TS1
The energy to attain this transition state TS2 geometry is much higher that TS1, because the largest substituent (t-Bu) and the Cl are both in the axial positions, which leads to large steric clashes. Thus, more energy, i.e. higher reaction temperatures, are required to attain TS2 relative to TS1.
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20. Thank You…
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