top of page
Search

E1 and E2 Reactions: Kinetics, Reactivity, Carbocation Rearrangement, Saytzeff's Rule, and Evidence

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.

Evidences:

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


Related Posts

See All

Hi! Your clicks on ads help us keep this blog going strong. If you like what you see, please consider clicking on any ads. Thanks for your support!

Hi! Your clicks on ads help us keep this blog going strong. If you like what you see, please consider clicking on any ads. Thanks for your support!

bottom of page