Comparison of E1 and E2 reactions
Property | E1 Reaction | E2 Reaction |
Mechanism | Two-step mechanism | One-step concerted mechanism |
Formation of carbocation | Proton abstraction and | |
intermediate | leaving group departure | |
Proton abstraction by a base | occur simultaneously | |
to form double bond | ||
Kinetics | First-order kinetics | Second-order kinetics |
Rate = k[Substrate] | Rate = k[Substrate][Base] | |
Substrate | 3° > 2° >> 1° | 3° > 2° > 1° |
Reactivity | (stability of carbocations) | (stability of transition |
states) | ||
Carbocation | Possible in E1 reactions | Not observed in E2 reactions |
Rearrangement | ||
Stereochemistry | No specific requirement | Anti-periplanar geometry |
(usually) | ||
Major Product | Saytzeff's rule may not | Saytzeff's rule typically |
(Alkene) | always apply due to | applies, forming the most |
carbocation rearrangements | highly substituted alkene |
This table summarizes the main differences between E1 and E2 reactions in terms of mechanism, kinetics, substrate reactivity, carbocation rearrangement, stereochemistry, and the major product (alkene) formed.
Factors Affecting E1 and E2 Reactions
When considering elimination reactions, both E1 (first-order elimination) and E2 (second-order elimination) mechanisms play pivotal roles in organic synthesis.
The choice between these pathways and their efficiencies are influenced by several key factors:
Substrate Structure
E1: Prefers tertiary alkyl halides due to the stability of the resulting carbocation.
E2: More versatile, but the reaction rate can be influenced by the substrate’s ability to adopt an anti-periplanar geometry necessary for the concerted mechanism.
Leaving Group
Affects both E1 and E2 reactions. Better leaving groups (those that can stabilize a negative charge upon departure, such as iodide or bromide) facilitate both types of reactions.
Nucleophile/Base Strength
E1: Involves a weak base or nucleophile since it primarily deals with the departure of the leaving group and carbocation formation.
E2: Requires a strong base to abstract a proton in the concerted step that leads to double bond formation.
Solvent
E1: Favored by polar protic solvents, which stabilize carbocations and the leaving group, thus facilitating the reaction.
E2: Polar aprotic solvents are preferred as they enhance the strength of the base, promoting the concerted elimination process.
Temperature
Generally, higher temperatures favor E2 reactions due to their higher activation energy, which aligns with the endothermic nature of breaking bonds simultaneously during the elimination.