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

E1 and E2 Reactions Definition

• E1 (unimolecular) and E2 (bimolecular) are elimination reactions in organic chemistry.
• E1 occurs in two steps formation of a carbocation followed by proton loss and follows first-order kinetics, favored by weak bases and polar protic solvents.
• E2 is a one-step reaction where a strong base removes a proton as the leaving group departs, following second-order kinetics.
• E2 requires anti-periplanar geometry.
• Both depend on substrate, base strength, and solvent.

Kinetics:

1. E1 Reactions:

   • Follow first-order kinetics: Rate depends on the concentration of the substrate only.
   • The rate-determining step is the formation of the carbocation, which does not involve the base.

2. E2 Reactions:

   • Follow second-order kinetics: Rate depends on both substrate and base concentrations.
   • The reaction is concerted, with the base abstracting a proton as the leaving group departs simultaneously.

Order of Reactivity of Alkyl Halides:

1. E1:

   • Reactivity: 3° > 2° >> 1°
   • Tertiary alkyl halides are most reactive due to the formation of more stable carbocations.

2. 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:

1. E1 Reactions:

   • Carbocation rearrangements (hydride or alkyl shifts) are common, leading to more stable intermediates and sometimes unexpected products.

2. E2 Reactions:

   • No carbocation intermediate, so no rearrangements

Saytzeff’s Rule (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:

1. Kinetic studies:

   • E1 is first-order, E2 is second-order.

2. Stereochemistry:

   • E2 requires an anti-periplanar arrangement between the leaving group and the proton being abstracted.

3. Carbocation rearrangements:

   • Provide evidence for the E1 mechanism but do not occur in E2.

Comparison Table: E1 vs E2 Reactions

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