E2 VERSUS E1, ELIMINATION VERSUS SUBSTITUTION – Introduction
In many organic reactions, substitution and elimination can happen at the same time. To get the correct product in synthesis, we must understand how molecules choose between:
- SN2 (Substitution)
- E2 (Elimination)
- SN1 (Substitution)
- E1 (Elimination)
This chapter explains how E2 and E1 mechanisms compete with substitution reactions, and the factors that decide which pathway wins.
Factors Affecting Substitution vs Elimination
Both substitution and elimination depend on:
- Structure of substrate
- Strength of attacking reagent (base/nucleophile)
- Nature of solvent
- Temperature
Effect of Substrate Structure
As branching increases at the carbon attached to the leaving group (α-carbon), the molecule becomes more crowded. This reduces the rate of SN2 but increases the rate of E2.
Example trend (increasing tendency for E2):
CH₃CH₂–X < (CH₃)₂CH–X < (CH₃)₃C–X
Thus, tertiary alkyl halides strongly prefer E2 over SN2. For example:
- Bromoethane → mainly substitution (ether), only 1% alkene
- Tert-butyl bromide → almost entirely E2 product (alkene)
- Isopropyl bromide → mixture of both (approx. 3:1 elimination:substitution)
Effect of the Attacking Reagent
In E2, the reagent must remove a proton → so strong bases favour elimination. In SN2, the reagent must attack carbon → strong nucleophiles favour substitution.
Strong bases (favour E2)
- OH⁻
- OEt⁻
- NH₂⁻
Weak bases but strong nucleophiles (favour SN2)
- Br⁻
- I⁻
- CH₃COO⁻
To suppress elimination and favour substitution, chemists sometimes use a weak base like potassium acetate instead of strong bases like KOH.
Effect of Solvent
Both SN2 and E2 reactions involve charge dispersion in the transition state. Less polar solvents stabilise E2 transition states more than SN2.
Therefore, less polar solvents favour E2 over SN2.
Examples:
- Ethanol–ethoxide gives better alkene yields than water–hydroxide.
- 2-methyl-2-propanol (a less polar solvent) increases E2 even more.
E1 vs SN1 Reactions
In unimolecular pathways, the deciding factor is the structure of the carbocation formed.
More branching at the β-carbon increases E1 elimination.
Examples:
- Tert-butyl chloride gives ~16% alkene
- Highly branched chlorides can give >90% alkene
Effect of Temperature
Elimination reactions generally have higher activation energy than substitution. Therefore, increasing temperature favours elimination.
Summary Table: E2 vs E1 and Substitution
| Condition | Favours Substitution | Favours Elimination |
|---|---|---|
| Strong nucleophile but weak base | SN2 | – |
| Strong base | – | E2 |
| Tertiary substrate | SN1 is possible | E1/E2 favoured strongly |
| High temperature | – | E1/E2 |
| Less polar solvents | SN2 decreases | E2 increases |
Understanding these factors helps students predict whether a reaction will undergo substitution or elimination and how to control the outcome in synthesis.
Detailed Notes:
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