E1 and E2 reactions – kinetics, order of reactivity of alkyl halides, rearrangement of carbocations, Saytzeffs orientation and evidence.
Kinetics:
E1 Reactions:
These follow first-order kinetics, meaning the reaction rate is directly proportional to the concentration of only the substrate.
This is because the rate-determining step is the loss of the leaving group to form a carbocation, which does not involve the base in this step.
E2 Reactions:
These follow second-order kinetics, where the reaction rate depends on the concentrations of both the substrate and the base.
This is due to the concerted mechanism of E2, where the base abstracts a proton and the leaving group departs simultaneously in the rate-determining step.
Order of Reactivity of Alkyl Halides:
E1:
The reactivity order is 3° > 2° >> 1°. Tertiary alkyl halides are most reactive due to their ability to form stable carbocations, a crucial intermediate in E1.
E2:
The order is slightly different, 3° > 2° > 1°, with tertiary alkyl halides still being the most reactive.
However, the difference between primary and secondary is less pronounced in E2 than in E1 due to the direct departure of the leaving group and proton abstraction.
Rearrangement of Carbocations:
E1 Reactions:
Carbocation rearrangements are possible and common in E1 reactions.
If a more stable carbocation can be formed through hydride or alkyl shifts, the reaction may proceed through this rearranged carbocation, potentially leading to unexpected products.
E2 Reactions:
Since E2 does not involve a carbocation intermediate, there are no rearrangements.
Saytzeff's (Zaitsev's) Rule and Orientation:
E1 and E2 Reactions:
Saytzeff's rule applies, suggesting the most substituted alkene, hence the most stable, will be the major product. However, E1 reactions can yield unexpected products due to carbocation rearrangements, while E2 reactions generally follow Saytzeff's rule more strictly due to the concerted elimination mechanism.
Evidences:
Kinetic studies show E1 is first-order while E2 is second-order.
Stereochemistry in E2 reactions requires the leaving group and the proton being abstracted to be in an anti-periplanar arrangement, a requirement not present in E1.
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 |