E1 and E2 Reactions: Kinetics, Reactivity, Carbocation Rearrangement, Saytzeff's Rule, and Evidence
- S-2-pharmaceutical-organic-chemistry
- Feb 23
- 2 min read
Updated: Feb 25
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Kinetics
E1 Reactions
These follow first-order kinetics, meaning the reaction rate is directly proportional to the concentration of only the substrate.
The rate-determining step is the formation of the carbocation, which does not involve the base.
E2 Reactions
These follow second-order kinetics, where the reaction rate depends on the concentrations of both the substrate and the base.
The reaction is concerted, with the base abstracting a proton as the leaving group departs simultaneously.
Order of Reactivity of Alkyl Halides
E1
Reactivity: 3° > 2° >> 1°
Tertiary alkyl halides are most reactive due to the formation of more stable carbocations.
E2
Reactivity: 3° > 2° > 1°
Tertiary alkyl halides are also more reactive, but the distinction between primary and secondary halides is less significant than in E1 reactions.Rearrangement of Carbocations:
Rearrangement of Carbocations
E1 Reactions
Carbocation rearrangements (hydride or alkyl shifts) are common, leading to more stable intermediates and sometimes unexpected products.
E2 Reactions
No carbocation intermediate, so no rearrangements occur.
Saytzeff's (Zaitsev's) Rule and Orientation
Both E1 and E2 Reactions
Follow Saytzeff’s rule, where the most substituted (and stable) alkene is the major product.
E1 can show deviations due to carbocation rearrangements, while E2 generally follows Saytzeff’s rule strictly due to its concerted mechanism.
Evidence
Kinetic studies
E1 is first-order while E2 is second-order.
Stereochemistry
E2 requires an anti-periplanar arrangement between the leaving group and the proton being abstracted.
Carbocation rearrangements
provide evidence for E1 mechanisms but are absent in E2.
Comparison Table: E1 vs E2 Reactions
Feature | E1 Reactions | E2 Reactions | |
Kinetics | First-order (depends only on substrate) | Second order (depends on substrate and base) | |
Reactivity of Alkyl Halides | 3° > 2° >> 1° | 3° > 2° > 1° | |
Carbocation Rearrangement | Possible and common | Not applicable | |
Saytzev's Rule | Applies, but rearrangements can lead to unexpected products | Strictly applies, leading to the most substituted alkene | |
Stereochemistry | Not specific | Requires anti-periplanar geometry for proton abstraction and leaving group departure | |
Mechanism | Stepwise, with carbocation intermediate | Concerted, with simultaneous proton abstraction and leaving group departure | |
Evidence | Rate depends only on substrate, possible rearrangements | Rate depends on substrate and base, specific stereochemical requirements, no rearrangements |
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